Physics Part I Chapter 5 Laws Of Motion
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    NCERT Solution For Class 11 Physics Physics Part I

    Laws Of Motion Here is the CBSE Physics Chapter 5 for Class 11 students. Summary and detailed explanation of the lesson, including the definitions of difficult words. All of the exercises and questions and answers from the lesson's back end have been completed. NCERT Solutions for Class 11 Physics Laws Of Motion Chapter 5 NCERT Solutions for Class 11 Physics Laws Of Motion Chapter 5 The following is a summary in Hindi and English for the academic year 2021-2022. You can save these solutions to your computer or use the Class 11 Physics.

    Question 1
    CBSEENPH110024916

    Under what condition is the relation s=ut is correct?

    Solution

    This condition is only true if body is moving in uniform velocity, as there will be no acceleration and distance covered per unit time will be constant.

    Question 2
    CBSEENPH110024917

    State relation between impulse and momentum?

    Solution

    Momentum is mass in motion, and any moving object can have momentum. An object's change in momentum is equal to its impulse. Impulse is a quantity of force times the time interval.

    Question 3
    CBSEENPH110024920

    Action and reaction forces do not balance each other why?

    Solution

    Action and reaction exist in pair and are always equal in magnitude and opposite in direction. But they act on different bodies, hence they cannot balance each other.

    Question 4
    CBSEENPH110024926

    Two billiard balls each of mass 0.05kg moving in opposite directions with speed 6m/s collide and rebound with the same speed. What is the impulse imparted to each ball due to other

    Solution

    Impulse (I) = Change in linear momentum
    Initial momentum of ball ( both) = mv =
    0.05×6 = 0.30 Kg m/s
    Final momentum of both balls = -(0.05×6)Kg-m/s .
    here minus sign is because the direction of final momentum is just opposite to the direction of initial momentum (linear momentum is a vector )
    Now impulse imparted to each ball ;
    I = -0.30 - ( 0.30) = -0.60 Kg-m/s

    Question 5
    CBSEENPH110024931

    How is centripetal force provided in case of the following? 

    1. Motion of planet around the sun, 
    2. Motion of moon around the earth. 
    3. Motion of an electron around the nucleus in an atom

    Solution
    1. Gravitational force acting on the planet and the sun provides the necessary centripetal force.  
    2. Force of gravity due to earth on the moon provides centripetal force. 
    3. Electrostatic force attraction between the electron and the proton provides the necessary centripetal force
    Question 7
    CBSEENPH11016280

    What is Aristotle’s law of motion?

    Solution
    Aristotle’s law of motion states that an external force is required to keep the body in motion.
    Question 8
    CBSEENPH11016282

    Is Aristotle’s law of motion now correct?

    Solution
    No. Aristotl'e law of motion is false. 
    Question 9
    CBSEENPH11016285

    State Galileo’s law of motion.

    Solution
    Galileo’s law of motion states that, a body continues to move in the same direction with constant speed, if no force is acting on the body. 
    Question 10
    CBSEENPH11016288

    Define inertia.

    Solution
    The property by virtue of which the body cannot change its state of rest or uniform motion in a straight line, unless an external force is acting on the body is called as Inertia. 
    Question 11
    CBSEENPH11016292

    When the branches of an apple tree are shaken, the apples fall down. Why?

    Solution
    The apple fall from an apple tree when it shaken because of inertia of rest. Apple is in a state of rest and when the tree is suddenly shaken, apples still tends to remain in it's same state of rest whereas branches move. 
    So, the apples fall down. 
    Question 12
    CBSEENPH11016294

    A person jumping out from a moving car or bus falls forward. Why?

    Solution
    The man falls forward because of inertia of motion. The person is in a state of motion inside the moving bus. When he steps out of the bus, his lower part of the body suddenly comes to rest. But, the upper part of the body still remains in a state of motion. Hence, the person falls forward. 
    Question 13
    CBSEENPH11016297

    Due to what type of inertia, the spark leaves the grind stone tangentially while sharpening the knife?

    Solution
    When sharpening is done, very tiny pieces on the surface of the grinding wheel get broken down from the wheel. These small stone pieces have a linear velocity in the tangential direction just before breaking off from the stone. After breaking off, they fly off in the tangential direction, as they continue to move in the state of uniform motion with a constant velocity. 
    Therefore, inertia of direction is responsible for this phenomenon. 
    Question 14
    CBSEENPH11016299

    Which law of motion is also called law of inertia?

    Solution
    Newton’s first law of motion is the law of inertia.
    Question 15
    CBSEENPH11016302

    What is measure of inertia of body?

    Solution
    Mass of body is measure of its inertia.
    Question 16
    CBSEENPH11016304

    Which law of motion defines the force?

    Solution
    Force is defined by Newton’s first law of motion. 
    Question 17
    CBSEENPH11016312

    What happens to a stone tied to the end of string and whirled in a circle if the string suddenly breaks?

    Solution
    If the string suddenly breaks, the stone will fly off tangentially along the straight line due to inertia of direction. This is because, the velocity at any point is directed along the tangential at that point. 

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    Question 18
    CBSEENPH11016314

    Which law of motion gives the quantitative knowledge of force?

    Solution
    Newton’s second law of motion quantitatively defines force.  
    Question 19
    CBSEENPH11016316

    Which law of motion is the real law of motion?

    Solution
    Newton’s second law of motion is the real law of motion.
    Question 20
    CBSEENPH11016317

    Which law of motion gives the quantitative definition of force?

    Solution
    Newton’s second law of motion gives the quantitative definition of force.
    Question 21
    CBSEENPH11016318

    Why do we pull the rope in downward direction for climbing up?

    Solution
    When a person is is climbing up he will push the rope downwards. Gravity is pulling the body downwards and when a person pulls the rope down, the reaction force will help that to person to climb upwards. 
    Question 22
    CBSEENPH11016320

    What can set the body at rest into motion?

    Solution
    An unbalanced force has to set the body into motion which is at rest. 
    Question 23
    CBSEENPH11016323

    What do you mean by mechanically isolated system?

    Solution
    When there is no external force acting on the system, it is said to be mechanically isolated.
    Question 24
    CBSEENPH11016324

    What are absolute units of force?

    Solution
    Newton and dyne.
    Question 25
    CBSEENPH11016326

    What are gravitational units of force?

    Solution
    1 Kilogram force was the force of gravity, which is pushing a mass of 1 kg at one place. 

    Question 26
    CBSEENPH11016327

    How many newtons are there in 1 kgf?

    Solution
    9.8N
    Question 27
    CBSEENPH11016329

    How many dyne is equal to one gf?

    Solution
    980 dyne.
    Question 28
    CBSEENPH11016337

    A body is placed on a frictionless surface in gravity free space. Do you need any force for body to move?

    Solution
    Yes, a force is required for the body to move because inertia of body opposes its change of state.
    Question 29
    CBSEENPH11016348

    What is momentum?

    Solution
    Momentum is the product of mass of a body and its velocity.
    Question 30
    CBSEENPH11016349

    What does the momentum measure?

    Solution
    Momentum measures the mass of an object. It also measures the quantity of motion of a body.  
    Question 31
    CBSEENPH11016350

    Can a horse pull a cart and run in empty space?

    Solution
    In an empty space, there is no reaction force. Hence, the motion of the system is not possible. That is a horse cannot pull a cart and run in an empty space. 
    Question 32
    CBSEENPH11016351

    Can a rocket operate in free space?

    Solution
    Yes, the rocket can move in an empty space. As the rocket is moving forwards, the exhaust gas is coming out of the engine. Thrust from the exhaust is pushing the rocket forward. Hence, rockets propels through space.  
    Question 33
    CBSEENPH11016353

    Where it is easy to run fast: on ploughed sand or hard road?

    Solution
    The force exerted by our feet on the sand is less. Hence, the reaction force is less, disabling us from running fast. 
    So, it is easier to run on the hard road than on the sand. 
     
    Question 34
    CBSEENPH11016355

    What is impulsive force?

    Solution
    A variable force, which acts for a short interval of time, is known as impulsive force
    e.g. A person kicking a football. 
    Question 35
    CBSEENPH11016356

    Comment on the statement: The direction of force is always in the direction of motion.

    Solution
    The statement is wrong.
    The direction of force is not always in the direction of motion. there is both negative and positive force. 
    Question 36
    CBSEENPH11016358

    Is the force always parallel to the direction of acceleration?

    Solution
    Yes, force acts parallel to the direction of acceleration enabling the body to move.
     
    The acceleration component along a given axis is caused only by the sum of the force components along the same axis. 
    Question 37
    CBSEENPH11016361
    Question 38
    CBSEENPH11016362

    How much acceleration can 1N force produce in 1kg body?

    Solution

    Force is directly proportional to the acceleration of the body. 
    Hence, acceleration produced on the bod is 1 m/s2.

    Question 39
    CBSEENPH11016363

    98N =.......kgf.

    Solution
    10

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    Question 41
    CBSEENPH11016366

    Can you think of an isolated force?

    Solution
    Isolated forces do not occur in nature. The forces in the nature always occur in pairs.
    Question 42
    CBSEENPH11016368

    What does the momentum measure?

    Solution
    The momentum of the body measures the quantity of motion contained in the body.
    Question 43
    CBSEENPH11016369

    A body of mass m is moving with constant velocity v on a frictionless table. How much force is required to keep the body moving with constant velocity?

    Solution
    Since the body is moving on a frictionless table, the force required to move the particle with a constant velocity is zero. 
    Question 44
    CBSEENPH11016370

    A body of mass 10 kg is acted upon by two perpendicular forces of 8N and 6N. What is the magnitude of acceleration of body?

    Solution
    Resultant of the two forces is 10N.
    Mass of the body, m = 10 kg
    Therefore the body will accelerate at an acceleration of, a = straight F over straight m space equals space 10 over 10 space equals space 1 space m divided by s squared 
    Question 45
    CBSEENPH11016371

    What is the magnitude and direction of the net force acting on a drop of rain falling down with a constant speed?

    Solution
    Since the raindrop is falling with a constant velocity, acceleration of the raindrop is equal to zero.
    So, the magnitude and direction of the net force acting on  a drop is zero. 
    Question 46
    CBSEENPH11016372

    What is the magnitude and direction of the net force acting on a cork of mass 10 g floating on water?

    Solution
    Mass of the cork =  10g
    Given that, the cork is floating on the water. Therefore, net force acting on the cork is zero. 

    Question 47
    CBSEENPH11016373

    Give the magnitude and direction of the net force acting on a high-speed electron in space far from all gravitating objects, and free of electric and magnetic field?

    Solution
    The net force acting on the high speed electron is zero because there is not field (gravitational, electric or magnetic) acting on the electron. 
    Question 48
    CBSEENPH11016374

    A body is moving with momentum 12 Ns in a free space. How much force is required to keep the body moving with constant momentum?

    Solution
    The body is moving with a constant momentum. Force is required only if the momentum has to be changed. So, here the force is equal to zero. 
    Question 49
    CBSEENPH11016375

    A man is at rest in the middle of a pond on perfectly smooth ice. By making the use of which law of motion he can get himself to the shore?

    Solution
    By using Newton’s third law of motion, he can get himself to shore. When the person pushes forward, the reaction force offered by the ice is very little. But, this reaction force will help him to move forward. Hence, Newton's Third Law will help him to reach the shore. 
    Question 50
    CBSEENPH11016376

    A body of mass M is placed on the weighing machine inside a lift moving upward with constant velocity. What is the reading of weighing machine?

    Solution
    Mass of the body = M
    Acceleration due to gravity = g 
    Weight of the person = Mg
    Question 51
    CBSEENPH11016377
    Question 52
    CBSEENPH11016379

    A man of mass M is standing in a lift accelerating down with acceleration a. What is the apparent weight of the man in the lift?

    Solution
    Mass of the man = M
    Rate of acceleration of the lift = a m/s2
    Acceleration due to gravity = g m/s2 which is acting downwards. 
    Weight of the man when the lift is moving downwards = M(g-a) kg 
    Question 53
    CBSEENPH11016381

    A man of mass M is standing in a lift accelerating down with acceleration a. If the lift starts accelerating in upward direction with acceleration a, then what will be the reading of weighing machine?

    Solution
    Given, mass of the man = M
    Acceleration of the man standing on the lift = a m/sacting downwards
    Acceleration of the lift = a m/s2 acting upwards 
    Reading of the weighing macine is given by, 
            R' = straight m left parenthesis straight g plus straight a right parenthesis 
    i.e,   R = fraction numerator straight R apostrophe over denominator straight g end fraction kg, is the reading of the weighing machine. 
    Question 54
    CBSEENPH11016382

    Give one example of weightlessness.

    Solution
    Any freely falling body is in the state of weightlessness.
    Question 55
    CBSEENPH11016383

    What is the weight of freely falling body?

    Solution
    For any freely falling body, weight = 0 
    Question 56
    CBSEENPH11016384

    What is the apparent weight of body of mass ‘m’ accelerating upward with acceleration ‘a’?

    Solution
    When an elevator is accelerating upwards,
    Apparent weight of the body is, m(g + a).
    Question 57
    CBSEENPH11016385

    State the law of conservation of momentum.

    Solution
    The law of conservation of momentum states that in the absence of external force, the vector sum of the linear momenta of all the bodies of the system is conserved, and is not affected due to their mutual action and reaction. 
    Question 58
    CBSEENPH11016386

    Name the law of motion which forms basis of law of conservation of momentum.

    Solution
    Newton’s third law of motion describes the conservation of momentum. 
    Question 61
    CBSEENPH11016390

    A meteorite burns in the atmosphere before it reaches the earth’s surface. What happens to its momentum?

    Solution
    According to the Law of conservation of momentum, momentum has to remain conserved. 
    The momentum of meteorite is transferred to air molecules and ultimately to the earth. Hence, the momentum is transferred. 
    Question 62
    CBSEENPH11016391

    Does the law of conservation of linear momentum mean that the momentum of each body of the system individually conserves?

    Solution
    The momentum of a system remains conserved. The momentum of each body of the system individually is not conserved. 
    Question 63
    CBSEENPH11016392

    A body is moving such that its linear momentum remains constant. Is the body in equilibrium?

    Solution
    Here, linear momentum of system remains conserved. Therefore, external force is zero and hence body will be in equilibrium.
    Question 64
    CBSEENPH11016393

    A body explodes into more than two fragments. Is law of conservation of linear momentum valid?

    Solution
    Yes, the linear momentum of the body will remain conserved. It is a general law and applicable irrespective of number of fragments in which the body breaks.
    Question 65
    CBSEENPH11016394

    State impulse momentum theorem.

    Solution
    Impulse momentum theorem states that the force acting on the body is equal to the change in the momentum of body. 
    Question 66
    CBSEENPH11016395

    A body explodes into three fragments of equal masses. Is it necessary that kinetic energy of all the three fragments be equal?

    Solution
    If the fragments fly off symetrically, the kinetic energy of all the three fragments will be equal. So, it is not necessary that K.E  of all the three fragments are equal. 
    Question 67
    CBSEENPH11016397

    A machine gun has a mass 5 kg. It fires 50 gram bullets at the rate of 30 bullets per minute at a speed of 400 m/s. What force is required to keep the machine gun in position?

    Solution
    Mass of the gun = 5 kg
    Mass of bullets =50 g
    Speed at which the bullet is fired = 400 m/s
    Therefore, force required to keep the machine gun in position = fraction numerator Change space in space momentum space of space bullets over denominator time space taken end fraction
                    equals space fraction numerator left parenthesis 50 space straight x space 10 to the power of negative 3 end exponent space straight x space 30 space right parenthesis space straight x space 400 over denominator 60 end fraction

equals space 10 space straight N 
    Question 68
    CBSEENPH11016398

    What is the impulse of bullet gun system, when the bullet is fired?

    Solution
    Impulse is the force acting on the body for a short interval of time. When a bullet is fired from the gun, force acting on the bullet-gun system is zero. 
    Question 69
    CBSEENPH11016400

    What is dimension formula of impulse?

    Solution
    Impulse is defined as force cross times time. 
    Dimensional formula is [M1L1T–1].
    Question 70
    CBSEENPH11016402

    Name the quantity which has same dimensional formula as that of impulse.

    Solution
    The dimensional formula of linear momentum is same as that of impulse. 
    i.e., [M1L1T-1] is the dimensional formula of linear momentum. 
    Question 71
    CBSEENPH11016403

    Does a given force always produce the same impulse in the body?

    Solution
    No. Impulse produced in the body by a given force depends on the time for which the force is acting. 
    Question 72
    CBSEENPH11016405

    Same forces are applied on the two different bodies of unequal masses for same interval of time. In which body the impulse produced by the force will be more?

    Solution
    Same force is applied on two different bodies of unequal masses for the same interval of time. 
    The impulse produced in both the bodies will be the same. 
    Question 73
    CBSEENPH11016406

    Does the batsman impart any impulse to the ball while deflecting it without changing its speed?

    Solution
    Yes. The batsman imaparts a small amount of impulse to the ball while deflecting it ithout changing it's speed. 
    Question 74
    CBSEENPH11016407

    What breaks the chinawares when colloide – force or impulse?

    Solution
    The force exerted for a short duration of time breaks the chinaware. Hence, it is the impulse that is acting on the chinaware. 
    Therefore, the chinaware or glasswares are wrapped in paper or straw pieces before packing. So that it takes longer for the force to reach the chinaware. Hence, the probability of their breakage is reduced.  
    Question 75
    CBSEENPH11016408

    How the impact of a given impulse can be decreased?

    Solution
    Increasingt he time will decrease the impact of impulse. 
    Question 76
    CBSEENPH11016409

    Can a hammer crack a nut in the conditions of weightlessness?

    Solution
    Yes. Hammer can crack the nut by applying force in the condition of weightlessness also. Mass of the hammer remains the same in space also. 
    Question 77
    CBSEENPH11016410
    Question 78
    CBSEENPH11016411

    Name the basic force which is by virtue of mass.

    Solution
    Gravitational force. 
    Question 79
    CBSEENPH11016412

    Name the basic force which by virtue of charge.

    Solution
    Electrostatic force.

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    Question 80
    CBSEENPH11016413

    Name the basic forces which follow inverse square law.

    Solution
    Gravitational and electrostatic forces follow inverse square law. 
    Question 81
    CBSEENPH11016414

    Are the nuclear forces charge dependent?

    Solution
    Nuclear force is nearly independent of charge. Nuclear force depends on whether the nucleaons are parallel or antiparallel. 
    Question 82
    CBSEENPH11016415
    Question 83
    CBSEENPH11016416

    Is nuclear force a central force?

    Solution
    The magnitude of the central force does not depend on the distance of the object to the centre. Hence, it is not a central force. 
    Question 84
    CBSEENPH11016417

    Which force among all the basic forces is the weakest force?

    Solution
    Gravitational force.
    Question 85
    CBSEENPH11016418
    Question 86
    CBSEENPH11016419

    Name the forces among all the basic forces that show conservative nature?

    Solution
    Gravitational and electrostatic forces show conservative nature.
    Question 87
    CBSEENPH11016420

    straight pi meson is the field particle of which interaction? 

    Solution
    Nuclear interaction.
    Question 88
    CBSEENPH11016421

    What is the range of nuclear forces?

    Solution
    The range of nuclear forces is of the order of 10–15m. 
    Question 89
    CBSEENPH11016422
    Question 90
    CBSEENPH11016423

    A passenger train has 21 bogies. Will the tension in the coupling between different bogies be the same?

    Solution
    The tension in coupling will not be the same for all the bogies. Tension is maximum in first coupling and minimum in the last coupling. 
    If the tension in the coupling for different bogies is the same, then there is a possibility that the bogies will collide. 
    Question 91
    CBSEENPH11016424

    Is the earth non-inertial frame of reference?

    Solution
    The surface of the earth is a non-inertial frame of reference because of the gravitational field of the earth. The direction of motion of the earth is continuously changing. Thus, it's velocity is also changing. 
    Rate of change of velocity is acceleration. Hence, the earth is non-inertial frame of reference. 
    Question 92
    CBSEENPH11016425

    Is a bus moving along a circular track, an inertial frame of reference?

    Solution
    No, the bus is a non-inertial frame of reference. This is because the direction of bus is continuosly changing and hence, it's velocity is also changing.
    Hence acceleration is changing. 
    Question 93
    CBSEENPH11016426

    Is Newton’s second law always valid?

    Solution
    Newton's second law is valid only in inertial frames of reference. The frame of reference where the velocity is constant. 
    Question 94
    CBSEENPH11016427

    What is the nature of reference frame attached with body revolving about its own axis?

    Solution
    A body revolving about it's own axis is the non-inertial frame of reference.
    Question 95
    CBSEENPH11016428

    A compressed air is filled in a balloon. If compressed force exerts action force, then discuss about reaction force.

    Solution
    In this case, the balloon exerts reaction force on the gas in inward direction when the compressed force exerts action force. 
    Question 96
    CBSEENPH11016429

    Can a body in equilibrium have linear momentum?

    Solution
    When a body is moving with constant velocity, it is in equilibrium and possesses the linear momentum.
    Question 97
    CBSEENPH11016430

    What is the principle on which the rocket works?

    Solution
    The working of the rocket is based on the principle of conservation of linear momentum. 
    Question 98
    CBSEENPH11016431

    Does the acceleration of rocket during the burning of fuel remain constant? 

    Solution
    The acceleration of rocket increases with time. As the rocket loses mass due to burning of the propellant, it's acceleration increases. 
    Question 99
    CBSEENPH11016433

    Give an expression for the velocity acquired by a rocket in terms of exhaust velocity of burnt gases, initial mass and mass of rocket at any instant.

    Solution

    The velocity acquired by the rocket in terms of exhaust velocity of burnt gases, initial mass of rocket at any instant is given by, 
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    Question 100
    CBSEENPH11016435

    Give an expression for the acceleration of the rocket during the burning of fuel in terms of exhaust velocity of burnt gases, rate of combustion of fuel and mass of rocket at any instant.

    Solution

    Acceleration of the rocket during the burning of fuel is given by, 
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    Question 101
    CBSEENPH11016436

    If in the rocket the fuel burns at the rate of m kg/s and burnt gases leave the rocket with velocity u m/s, then how much force the rocket will experience due to exhaust of gases?

    Solution

    Rate of burning of fuel = m kg/s
    Velocity at which the gas leaves the rocket = u m/s
    So, force experienced by rocket due to exhaust of gases = mu

    Question 102
    CBSEENPH11016438

    Define the terms force, inertia and momentum.

    Solution

    (i) Any external imbalance that changes or tends to change the state of rest or uniform motion in a straight line or the direction of motion is called as Force. 
    Force always produces acceleration in the body. 
    (ii) Inertia is the ability of the body to change the state of rest or uniform motion, unless acted upon by an external force. 
    Inertia is categorized into three types:
    i) Inertia of rest,
    ii) Inertia of motion, and
    iii) Inertia of direction.
    Inertia of rest: It is the property of body by virtue of which it cannot change the state of rest by itself. 
    Inertia of motion: It is the tendency of the body to remain in the state of uniform motion in a straight line. 
    Inertia of direction: It is the property of body by virtue of which it cannot change the direction of motion by itself.
    (iii) Momentum is a measure of the quantity of motion contained in the body and is equal to the product of mass and velocity.
    Momentum is a vector quantity and it's direction is same as that of velocity.
    Mathematically, momentum is given by,
                        straight P with rightwards arrow on top space equals space straight m straight v with rightwards arrow on top

    Question 103
    CBSEENPH11016439

    Bodies of larger mass need larger effort for putting them into motion. Why? 

    Solution
    Mass is a measure of inertia. So, heavier the mass, more is inertia. Thus, large initial effort is required to put the heavy body into motion. 
    Question 104
    CBSEENPH11016442

    Why do we beat our coat to remove dust?

    Solution
    The coat is initally in a state of rest. When we beat the coat, it is set into motion. But, due to inertia of rest, dust particles tends to remain at rest and hence they fall down. 
    Question 105
    CBSEENPH11016443

    If we jerk a piece of paper from under a book quick enough, the book will not move, why?

    Solution
    Both the book and the puece of paper are initially at rest. When sudden jerk is given to a piece of paper which is under the book, the motion of piece of paper is not imparted to the book. Hence, due to inertia of rest the book does not move. 
    Question 106
    CBSEENPH11016444

    Why does an apple fall down from a tree when its branch is shaken?

    Solution
    Initially the tree as at rest. When the branch of a tree is shaken, it is set into motion whereas the apple tends to remain at rest due to inertia of rest.
    Therefore, apple gets separated from the branch of the tree.
    Question 107
    CBSEENPH11016445

    When the bus or train suddenly starts moving, the passengers tend to fall backward. Explain why.

    Solution
    When the bus is standing still, the person is at rest. But, when the bus suddenly moves, the upper part of the passenger's body is still at rest due to inertia of rest. The lower part of passenger’s body starts moving along with the bus or train. But, the lower part of the body tends to remain at the same state of rest. Hence, a person falls backwards.
    Question 108
    CBSEENPH11016446

    A bullet forms a narrow hole in the windowpane when fired from the gun, while windowpane smashes if bullet is thrown with hand. Explain why?

    Solution
    The speed of the bullet is high, when it is fired from the gun. And, the time of contact with glass is very small. Therefore the bullet is not able to impart the motion to whole of the glass but only to a small portion where it makes the contact. Hence, a narrow hole is made. 
    The speed if the bullet is comparitively low when the bullet is thrown with hand. So, the time of contact between the bullet and glass is very large. Thus, there is sufficient time to impart the motion to whole of the glass due to which glass smashes. 
    Question 109
    CBSEENPH11016447

    If the moving bus suddenly stops, why do passengers fall in the forward direction? 

    Solution
    A passenger sitting in a moving bus is in motion. When the bus suddenly stops, the lower part of our body is brought to rest along with the bus whereas upper part continues to be in motion due to inertia of motion and hence falls forward.
    Question 110
    CBSEENPH11016448

    A ball thrown vertically upward by a person in a uniformly moving train comes back to his hands. Why?

    Solution
    All the bodies inside the train have the same velocity, as the train is moving with a uniform velocity. 
    Ball has same horizontal velocity as that of the train. When the ball is thrown upwards, there is no horizontal force. Therefore, the ball has the same horizontal velocity as that of the train due to inertia of motion. 
    So, both the ball and hand travel the same distance in both horizontal direction and hence, ball will fall back into hands. 
    Question 111
    CBSEENPH11016451

    If you have to remove the thick ketchup from sauce bottle, you should turn the bottle upside down, then move it downward with high speed and stop suddenly. Explain. 

    Solution
    On moving the bottle down, both bottle and ketchup move downward with high speed. When the bottle is put upright in a quick manner, the bottle comes to rest while ketchup continues to move due to inertia and come out of bottle. 
    Question 112
    CBSEENPH11016452

    Why does the athlete run through a certain distance before taking a long jump? 

    Solution
    By running a certain distance , the athlete gains the inertia of motion, which helps him in taking the long jump. 
    Question 113
    CBSEENPH11016453

    A man jumping out from a moving train falls forward. Why?

    Solution
    The man jumping out from moving train possesses the inertia of motion. As the man lands on the ground, feet come to rest immediately while upper part of body continue to move due to inertia of motion. Hence, the person falls forward. 
    Question 114
    CBSEENPH11016454

    If the head of the hammer is loose at the top, the carpenter hits the bottom of handle against hard surface to tighten it. Explain why?

    Solution
    The handle comes to rest, when handle-hammer system is hit against the hard surface. While the loose hammer continues to move in the handle due to inertia of motion and gets fit into it. 
    Question 115
    CBSEENPH11016456

    Why mudguards are used in vehicles?

    Solution
    The mudguards are used in vehicles to remove the muds sticking onto the wheels of the vehicles. Due to inertia of direction, the mud sticking to the wheels of a vehicle flies off tangentially when the wheels rotate.
    Question 116
    CBSEENPH11016469

    A stone tied to a string is whirling in a horizontal circle. Why does the stone fly off tangentially if the string suddenly breaks?

    Solution
    Tension in the string provides the necessary force required to change the direction of motion while whirling in horizontal circle. When the string breaks, the force on the stone ceases to act. Hence, stone cannot change its direction of motion and flies off tangentially. 
    Question 117
    CBSEENPH11016487

    If a bull chases you, how will you move to save yourself - along straight path or zig zag path. Explain.

    Solution
    The person should follow a zig-zag path for saving himself from the bull. Mass of the bull is much greater than a man. Thus, the bull possess  more inertia of direction than a man and finds difficulty to change the direction. Therefore, the bull will have a difficulty in following the zig-zag path. 
    Question 118
    CBSEENPH11016500

    State Newton’s laws of motion.

    Solution

    Newton's law of motion are as follows: 
    First law states that every body in the universe continues to be in the state of rest or uniform motion in a straight line until and unless an external force compels it.
    Second law states that the rate of change of momentum of body is directly proportional to the external force applied.
    Third law states that too every action there is an equal and opposite reaction.

    Question 119
    CBSEENPH11016502

    Explain how does Newton’s first law of motion give us the definition of force?

    Solution
    Newton’s first law of motion states that an external force must be applied to change the state of rest or uniform motion in a straight line. This means that force applied on a body alone, can change it's state of rest or uniform motion. 
    For e.g., force is not able to change the state e.g. if we push the wall it will not change the state of rest.
    Hence force may be defined as the external agency that changes or tends to change the state of rest or uniform motion of a body in a straight line. 

    Sponsor Area

    Question 120
    CBSEENPH11016503

    If action and reaction are equal and opposite, then how can the motion take place?

    Solution
    Action and reaction are equal and opposite. Action and reaction act on two different bodies.
    If action and reaction act on the same body they will cancel each other and cannot cause the motion.
    If action and reaction are on different bodies then they will not cancel each other and hence cause the motion.
    Question 121
    CBSEENPH11016504

    Define different units of force.

    Solution

    Absolute unit of force:
    In SI system, the unit of force is newton.
    One Newton of force is the force which produces 1 m/sec2acceleration in 1 kg mass.
    In cgs system, the unit of force is dyne.
    1 dyne is the force which produces 1cm/secacceleration in 1 gm of mass.
    Gravitational unit of force: 
    In SI system, gravitational unit of force is kgf (kilogram force).

    It is that force which accelerates 1 kg mass with acceleration 9.8 m/s2.
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#6 {main}</pre> 
            equals space 9.8 space kg space straight m divided by straight s squared space equals space 9.8 space straight N. 
    In cgs system, unit of force is g-f (gram force).

    1 g-f is the force that accelerates 1 gm of mass  with acceleration 980 cm/s2
    1 space gf space equals space 1 space gm space cross times space 980 space cm divided by straight s squared 
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#6 {main}</pre>

    Question 122
    CBSEENPH11016505

    Show that 1N = 105 dyne.

    Solution
    1 straight N space equals space 1 fraction numerator kg space straight m over denominator straight s squared end fraction
space space space space space space space equals 1 fraction numerator 1000 space gm space cross times space 100 cm over denominator straight s squared end fraction space 
         equals 1000 cross times 100 space gm space cm divided by straight s squared space

equals space 10 to the power of 5 space dyne

    i.e., 1 N = 105 Dyne
    Question 123
    CBSEENPH11016512

    Show that Newton’s second law is the real law of motion.

    Solution

    Second law is the real law of motion:
    This can be proved by showing that first law and third law are contained in second law of motion.
    Let's prove that first law is contained in the second alw of motion. 
    According to the second law of motion,

                        straight F with rightwards arrow on top space equals space straight m straight a with rightwards arrow on top 
    space space therefore           straight F with rightwards arrow on top space equals space 0 space space space space space space rightwards double arrow space space space space straight a with rightwards arrow on top space equals space 0 
    That is, if no force is acting on the body then its acceleration is zero. This means if a body is at rest, it remains at rest and if the body is moving in straight line with constant velocity, it continues to do so. This is what the first law states. Hence first law is contained in the second law.
    (ii) Let's prove that Third law is contained in second law.
    According to the second law of motion, 
    space space space space space straight F with rightwards harpoon with barb upwards on top space equals space fraction numerator d p with rightwards harpoon with barb upwards on top over denominator d t end fraction
i. e. comma space F with rightwards harpoon with barb upwards on top space equals space 0 space

rightwards double arrow space p with rightwards harpoon with barb upwards on top space equals space c o n s tan t
    So, in th absence of external force, total momentum of the system remains constant. 
    Let, us consider an isolated system of two bodies of masses m1 and m2. Let, straight F with rightwards harpoon with barb upwards on top subscript 12 be the force exerted by mass m1 on m2 which is the reaction. 
    Let, straight F with rightwards harpoon with barb upwards on top subscript 21 be the force exerted by mass m1 on m2, which is the reaction. 
    Let, due to   straight F with rightwards harpoon with barb upwards on top subscript 21 ,
    Rate of change of momentum of m1dp subscript 1 over dt
    Rate of change of momentum of m2dp subscript 2 over dt
    Therefore, 
    space space space space space space space space space space straight F with rightwards harpoon with barb upwards on top subscript 12 space equals space fraction numerator d p subscript 1 over denominator d t end fraction space a n d space F with rightwards harpoon with barb upwards on top subscript 21 space equals space fraction numerator d p subscript 2 over denominator d t end fraction

N o w comma space straight F with rightwards harpoon with barb upwards on top subscript 12 space plus space space F with rightwards harpoon with barb upwards on top subscript 21 space equals space fraction numerator d p subscript 1 over denominator d t end fraction plus fraction numerator d p subscript 2 over denominator d t end fraction

space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space equals space fraction numerator d over denominator d t end fraction open parentheses p subscript 1 space plus space p subscript 2 close parentheses space
space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space equals space fraction numerator d p with rightwards harpoon with barb upwards on top over denominator d t end fraction
 
    Here, straight p with rightwards harpoon with barb upwards on top is the total momentum of the system. 
    According to the second law of motion, in the absence of external force, momentum remains conserved. 
    Therefore, 
    therefore space straight F with rightwards harpoon with barb upwards on top subscript 12 space plus space straight F with rightwards harpoon with barb upwards on top subscript 21 space equals space fraction numerator straight d straight p with rightwards harpoon with barb upwards on top over denominator dt end fraction space equals space 0

rightwards double arrow space space straight F with rightwards harpoon with barb upwards on top subscript 12 space equals space minus straight F with rightwards harpoon with barb upwards on top subscript 21
    Hence, the third law of motion is proved. 
    Therefore, Newtons' second law of motion is the real law of motion. 
            

    Question 124
    CBSEENPH11016514

    Why do aeroplanes having wings fly at low altitudes?

    Solution
    The wings of an aeroplane push the external air backward. This pushed air gives equal and opposite reaction to aeroplane in forward direction as per Newton’s third law of motion. Hence the aeroplane moves forward.
    At low altitude the air is denser than at higher altitude, therefore aeroplane gets sufficient force to move forward.
    Question 125
    CBSEENPH11016515

    Air is thrown on a sail attached to a boat from an electric fan placed on the same boat. Will the boat start moving?

    Solution
    Action and reaction are equal and opposite according  to Newton’s third law of motion. Action and reaction are acting on two different bodies.   
    When air is thrown by fan placed on the same boat on which sail is attached, then action and reaction are on the same boat. Hence motion of the boat will not take place. 
    Question 126
    CBSEENPH11016516

    Air is thrown on a sail attached to a boat from an electric fan placed on the second boat nearby the sailboat. Will the sailboat start moving?

    Solution

    According to Newton's third law, action and reaction are equal and opposite and acts on two different bodies. 
    Here, in this case fan and sail are on two different boats. Therefore, air thrown by fan on the sail will exert external force. So, reaction will be on the second boat on which the fan is placed. Thus action and reaction are on two different boats. Hence, the boats will be set into motion. 

    Question 127
    CBSEENPH11016517

    While swimming, the swimmer pushes the water in backward direction. Explain why?

    Solution
    The swimmer pushes the water in backward direction with some force, while swimming. The water exerts equal and opposite force on the swimmer in forward direction and hence the swimmer moves forward. 
    Question 128
    CBSEENPH11016518

    Why does a rubber ball bounce back on throwing against the wall?

    Solution
    The ball when thrown against the wall exerts force on the wall. Now, according to Newton's third law of motion, the wall exerts equal and opposite force on the ball. Hence, the ball bounces back. 
    Question 129
    CBSEENPH11016519

    Which force accelerates the high jumper upwards when leaving the ground?

    Solution
    The jumper pushes the ground in downward direction before taking the jump. As a result, the ground exerts equal and opposite force on the jumper in upward direction. this is according to Newton's third law. This reaction force on the jumper accelerates him upward.
    Question 130
    CBSEENPH11016521

    Why do we pull the handle of bicycle in upward direction while pushing the paddles hard?

    Solution
    When the handle is pulled upwards, an equal and opposite reaction acts in the downward direction. This reaction force acts on the paddle. This increases the push on paddles and hence a cyclist is able to move smoothly. 
    Question 131
    CBSEENPH11016522

    It is difficult to drive the nail into a wooden block without holding the block. Explain why.

    Solution
    The block should be motionless w.r.t. nail, to drive the nail into the block.
    When a person hammer the nail without holding the block, the nail and block both move forward as a single system and hence nail does not get into the block.
    But, if we support the block with something, then the reaction of the support keeps the block motionless and the nail gets into block on hammering easily. 
    Question 132
    CBSEENPH11016523

    There is some water in the beaker placed on the pan of a spring balance. If we dip our finger in the water without touching the sides and bottom of beaker, then how does the reading of spring balance change?

    Solution
    According to Archimedes principle, the water exerts upward thrust on the finger, when we dip the finger in water. Now, as per Newton's third law of motion, the finger will exert equal and opposite force on the water in downward direction. This will result in increase of the weight of water.
    Thus, the reading of spring balance will increase. 
    Question 133
    CBSEENPH11016524

    A one kg weight kept on the glass sheet does not break it, but a small piece of stone dropped from certain height over it breaks it. Why?

    Solution
    There is no momentum assosciated with 1 kg weight placed over glass sheet.
    But, the falling stone possesses momentum and when strikes with the glass sheet, it transfers its momentum to the glass particles. Hence, the glass sheet breaks. 
    Question 134
    CBSEENPH11016526

    Give the magnitude and direction of the net force acting on a drop of rain falling down with constant speed.

    Solution

    As the raindrop is falling with a constant speed, acceleration = 0. Hence, net force ma= 0. 

    Question 135
    CBSEENPH11016527

    Give the magnitude and direction of the net force acting on a cork of mass 10 gm floating on water.

    Solution

    As the cork is floating on water, it's weight is being balanced by the upthrust which is equal to the weight of the water displaced. Hence, net force acting on the cork is zero. 

    Question 136
    CBSEENPH11016528

    Give the magnitude and direction of a car moving with constant speed on rough road.

    Solution

    Given, the velocity with which the car is moving is constant. So, acceleration=0. Hence, net force on the car is zero. 

    Question 137
    CBSEENPH11016530

    Give the magnitude and direction of the net force action on a high-speed electron in space far from all material objects, and free of electric and magnetic fields.

    Solution

    Given that, no field is acting on the electron. Therefore, net force on the particle is equal to zero. 

    Question 138
    CBSEENPH11016531

    What are balanced and unbalanced forces?

    Solution
    When a number of forces are acting simultaneously on a body, then their resultant is given by the vector sum of all the forces. If the resultant of all the forces is a null vector, then the forces are called balanced forces. 
    If the net resultant is non-zero force, then the forces are called unbalanced forces. 
    Question 139
    CBSEENPH11016532

    In the figure shown, what is the force exerted by D on C?


    Solution
    The sum of masses of A, B and C is 17 kg.
    Therefore the force exerted by C on D is 17 g N.
    Now, according to Newton’s third law action and reaction are equal and opposit. Thus, forces exerted by D on C is also 17 g N. 
    Question 140
    CBSEENPH11016534

    In the figure shown what is ratio of reaction R1 to reaction R2?



    Solution

    Since, space straight R subscript 1 reaction is due to weight 5 kg
    space space therefore           space space space straight R subscript 1 space equals space 5 straight g space straight N              and
    straight R subscript 2 reaction is due to two weights 5 kg and 10 kg.
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    Thus,         straight R subscript 1 over straight R subscript 2 equals fraction numerator 5 straight g over denominator 15 straight g end fraction equals 1 third 
    1:3 is the required ratio of the reaction R1 and R2.

    Question 141
    CBSEENPH11016535

    An electron is subjected to a force 1.8 cross times 10 to the power of negative 23 end exponent straight N. How long it will travel in 1.2 ms?

    Solution

    Given,
    Force acting on the electron, F = 1.82 space cross times space 10 to the power of negative 23 end exponent straight N
    Mass of electron, 
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    The acceleration of electron is given by, 
    space space space straight a equals straight F over straight m equals fraction numerator 1.82 cross times 10 to the power of negative 23 end exponent over denominator 9.1 cross times 10 to the power of negative 31 end exponent end fraction equals 2 cross times 10 to the power of 7 straight m divided by straight s squared
    Now, using the second equation of motion, 
                     space space space straight s equals ut plus 1 half at squared
    We have, 
    sequals space 0 cross times straight t plus 1 half left parenthesis 2 cross times 10 to the power of 7 right parenthesis space left parenthesis 1.2 cross times 10 to the power of negative 3 end exponent right parenthesis squared
      equals space 0.144 space straight m 

    Question 142
    CBSEENPH11016537

    A machine gun fires the bullets of mass 30 gm at the rate of 20 bullets per second with speed 400m/s. Find the force required to keep the gun in position.

    Solution

    Mass of bullet is, m = 30 gm = 0.03 kg
    Velocity of bullet, v = 400 m/s
    Frequency of firing the bullets is, n = 20 bullets/second
    Change in momentum of one bullet is,
           mv = 0.03 x 400 = 12 Ns
    Change in momentum of 20 bullets is, 
                 12 x 20 = 240 Ns
    As 20 bullets are fired in one second, therefore 240 Ns is change in momentum of bullets in 1 second.
    Since change in momentum of body in one second is equal to force acting on the body, therefore force exerted by gun on the bullets is 240N.  
    Now according to Newton’s third law, the bullets exert equal and opposite force on the gun.
    Thus the force required to hold the gun is 240N.

    Question 143
    CBSEENPH11016538

    A balloon with mass M is descending down with an acceleration a where a < g. What mass m of its contents must be removed so that it starts moving up with acceleration a?

    Solution
    The balloon descends with acceleration a < g.
    Thus, the force experienced by the balloon is in the upward direction and is less than the weight Mg, which is acting downwards. 

                    
    Therefore the equation of motion of the balloon is,
                       Mg - F = Ma                        ...(1) 
    If some of the mass of the balloon is removed, the downward force i.e. weight of the balloon decreases.
    As more and more mass is removed, the downward force on the balloon decreases more and more, and becomes equal to upward force F.
    On further removal of mass from balloon, the upward force F exceeds the weight of balloon and balloon starts accelerating in upward direction.
    Let, mass 'm' be removed from the balloon. 
    Upward acceleration acquired by balloon = a 
    Now the equation of motion of balloon is, 
    F - (M - m)g = (M - m)a                       ...(2) 
    Adding (1) and (2), we get
                  mg = (2M - m)a
    rightwards double arrow            straight m equals fraction numerator 2 straight a over denominator straight g plus straight a end fraction straight M
    Mass 'm' given in the above expression should be removed so that the balloon starts accelerating upwards. 
    Question 144
    CBSEENPH11016540

    Two bodies of mass 12kg and 5 kg are moving with velocities 5 m/s and 12 m/s. Which of the two bodies has more quantity of motion?

    Solution

    Given, 
    Mass of first body, M1 = 12 kg
    Velocity of first body, v1 = 5 m/s
    Mass of second body, M2 = 5 kg
    Velocity of second body, v2 = 12 m/s
    Therefore, 
    The momentum of Mass straight M subscript 1 is,
                   straight M subscript 1 straight v subscript 1 space equals space 12 cross times 5 space equals space 60 space straight m divided by straight s
     The momentum of mass straight M subscript 2 is,
                  straight M subscript 2 straight v subscript 2 space equals space 5 cross times 12 space equals space 60 space straight m divided by straight s
    Since the momentum of both the bodies is same, thus both the bodies have same quantity of motion. 

    Question 145
    CBSEENPH11016542

    Force-time graph for a body is shown in figure. Locate the regions of time if the body has uniform motion, uniformly accelerated motion, non-uniformly accelerated motion, uniformly retarded motion, non-uniformly retarded motion.


    Solution

    Ans.
    (i) For <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>,
    The force acting on the body is constant. Thus,  the body has uniform accelerated motion.
    (ii) Forspace space space 5 less than straight t less or equal than 8,
    Force on the body is zero, therefore body has uniform motion. 
    (iii) For 8 less than straight t less or equal than 13
    The force is negative and increases linearly with time in magnitude, thus the motion is non-uniformly retarded motion.
    (iv) For 13 less than straight t less or equal than 17,
    The force is positive and increases linearly with time, thus the motion is non-uniformly accelerated motion.
    (v) For <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>,
    The force acting on the body is constant, thus the body has uniform accelerated motion.
    (vi) For 20 less than straight t less or equal than 24,
    The force acting on the body is constant and negative, thus the body has uniformly retarded motion.

    Question 146
    CBSEENPH11016543

    Driver of a car is driving at the speed 9-8 m/s. He saw a child standing in the middle of road and applied the brakes when the car was at a distance 4.9m away from the child. Show that the minimum retarding force of the brakes on the car to just save the child must be half the weight of the car.

    Solution
    The car must stop by travelling the maximum distance of 4.9m, in order to save the child.
    Let M be mass of the car.
    Here,      
    Initial velocity of the car, u = 9.8 m/s
    Final velocity, v = 0
    MAximum distance travelled,  straight S subscript max space equals space 4.9 space straight m
    Therefore minimum retardation of the car is,
                 straight r subscript min space equals space minus fraction numerator straight v squared minus straight u squared over denominator 2 straight S subscript max end fraction
                       equals negative fraction numerator 0 squared minus left parenthesis 9.8 right parenthesis squared over denominator 2 cross times 9.8 end fraction space equals space 4.9 space straight m divided by straight s squared
    The minimum retarding force on the car is,
                          straight M cross times straight r subscript min space equals space 4.9 straight M
    Now,
          fraction numerator Retarding space force over denominator Weight end fraction space equals space fraction numerator 4.9 straight M over denominator 9.8 straight M end fraction equals 1 half 
    Thus,
           Retarding force = 1 half Weight
          
    Question 147
    CBSEENPH11016545

    A 15N force accelerates the mass mwith acceleration 3m/s2, and m2 mass with acceleration 5 m/s2. What would be the acceleration, when 30N force is applied on tying the two masses together?

    Solution

    According to Newton's first law of motion,
                        F = ma
    i.e.,              m = F/a 
    Force acting on two bodies, F = 15 N

    Mass of first body, <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>
    Mass of second body, straight m subscript 2 space equals space straight F over straight a subscript 2 space equals space 15 over 5 space equals space 3 space kg
    Now the acceleration produced by 30N force when both masses are tied together is,
    straight a equals fraction numerator 30 over denominator straight m subscript 1 plus straight m subscript 2 end fraction equals fraction numerator 30 over denominator 3 plus 5 end fraction
     
      equals space 3.75 space straight m divided by straight s squared

    Question 148
    CBSEENPH11016546

    A soda water bottle is falling freely. Will the bubbles of gas rise in the soda water in the bottle?

    Solution
    No, the bubbles of gas will not rise in the soda water bottle.
    This is because the freely falling bodies are in the state of weightlessness. Therefore, the bubbles do not experience any upward thrust in freely falling bottle and hence do not rise in the bottle. 
    Question 149
    CBSEENPH11016549

    A man weighing M kg, stands on a weighing machine inside a lift. What will be the reading of the machine if it:
    (a) descends with acceleration a,
    (b) ascends with an acceleration a and
    (c) moves with constant velocity in upward direction?

    Solution

    (a)  
    The true weight of body = Mg
    Reaction of floor = R
    Net force acting on the body is,
                         F = Mg  - R
    If a is the acceleration with which lift moves downward, then
                         F = M a
    ∴                Ma = Mg - R
    i.e.,               R = M(g - a)
    Thus the apparent weight of the body is,
                     straight W subscript straight a space equals space straight R space equals space straight M left parenthesis straight g minus straight a right parenthesis
    (b)
    If lift is ascending in upward direction with acceleration a then
                     Ma = R - Mg
    or               straight R space equals space straight W subscript straight a space equals space straight M left parenthesis straight g plus straight a right parenthesis 
    (c)  
    If lift moves with constant velocity then acceleration, a = 0. 
    ∴                straight W subscript straight a space equals space Mg
    So, reading on the machine = Mg

    Question 150
    CBSEENPH11016550

    A monkey weighing 14kg is hanging from rope, which can withstand a maximum tension of 168N. Find the maximum acceleration with which the monkey can climb up along the rope?

    Solution
    Let, the monkey climb up with an acceleration a.
    The tension in the string is, 
                              T = m(g + a)     ... (1)
    Here,
    Tension in the string, T = 168 N
    Mass of the monkey, m = 14 kg
    ∴ Putting the values in equation (1), we have
                      168 = 14 (9.8 + a)
                          a = 2.2 m/s2, is the maximum acceleration with which the monkey can climb along the rope. 
    Question 151
    CBSEENPH11016551

    An elevator of mass 400kg supported by a cable which can withstand a maximum tension of 4400N, is going down with constant velocity 2m/s. Will it be wise to stop the elevator in l.6m. Take g = 9.8m/s2.

    Solution

    Given, 
    Initial velocity, u = 2m/s
    Final velocity, v = 0 m/s
    Distance, s = 1.6m
    The acceleration of elevator is,
             straight a equals fraction numerator straight v squared minus straight u squared over denominator 2 straight s end fraction equals fraction numerator 0 minus 4 over denominator 2 cross times 1.6 end fraction space equals negative 1.25 straight m divided by straight s squared.
    The tension in the cable is,
              T = m(g - a) 
                 = 400[9.8 - (-1.25)]
                 = 4420 N
    The tension in the cable is greater than the strength of string, therefore the cable will break.
    Thus it will not be wise to stop the elevator within 1.6m.

    Question 152
    CBSEENPH11016553

    The motion of a particle of mass 50gm is given by x = 10t + 100t2. Find the force acting on the particle.

    Solution
    Given, 
    Mass of the particle, m = 50 gm

    The equation of motion of particle is,
                     x = 10t + 100t2

    Therefore acceleration of the particle is,
                    straight a equals fraction numerator straight d squared straight x over denominator dt squared end fraction equals 200 straight m divided by straight s squared 
     
    The force acting on the particle is,
    F = ma = 0.05 x 200 = 10 N
    Question 153
    CBSEENPH11016556

    State and prove law of conservation of momentum.

    Solution

    Law of conservation of momentum states that total momentum of system remains conserved in the absence of external force.
    Proof:
    Consider a body of mass m1 moving with veocity straight u with rightwards harpoon with barb upwards on top subscript 1 striking against another body of mass m2 moving with velocity straight u with rightwards harpoon with barb upwards on top subscript 2.
    Let, the two bodies remain in contact with each other for a small interval increment straight t .
    Let, straight F with rightwards harpoon with barb upwards on top subscript 12 space be the average force exerted by mass m1 on m2 and let straight F with rightwards harpoon with barb upwards on top subscript 21 be the force on m2 due to m1. 
    Let, v1 and v2 be the velcoities of two bodies after collision. 

    Momentum of mass m1 before collision = m1u1
    Momentum of mass m2 after collision = m2stack straight u subscript 2 with rightwards harpoon with barb upwards on top
    Momentum of mass m1 after collision = m1v1
    By using the defintion of impulse, change in momentum of mass m1 is, 
    straight F with rightwards harpoon with barb upwards on top subscript 12 increment t space equals space m subscript 1 stack v subscript 1 with rightwards harpoon with barb upwards on top space minus space m subscript 1 u with rightwards harpoon with barb upwards on top subscript 1 space space space space space space space space space space... left parenthesis 1 right parenthesis space

C h a n g e space i n space m o m e n t u m space o f space m a s s space m subscript 2 subscript space end subscript space i s comma space
straight F with rightwards harpoon with barb upwards on top subscript 21 increment t space equals space space m subscript 2 stack v subscript 2 with rightwards harpoon with barb upwards on top space minus space m subscript 2 u with rightwards harpoon with barb upwards on top subscript 2 space space space space end subscript space space space space space... left parenthesis 2 right parenthesis space

A d d i n g space e q u a t i o n s space left parenthesis 1 right parenthesis space a n d space left parenthesis 2 right parenthesis comma space w e space h a v e

space space space space space space open parentheses straight F with rightwards harpoon with barb upwards on top subscript 12 increment t space plus straight F with rightwards harpoon with barb upwards on top subscript 21 increment t space close parentheses space equals space left parenthesis m subscript 1 stack v subscript 1 with rightwards harpoon with barb upwards on top space minus space m subscript 1 u with rightwards harpoon with barb upwards on top subscript 1 right parenthesis space plus space left parenthesis space m subscript 2 stack v subscript 2 with rightwards harpoon with barb upwards on top space minus space m subscript 2 u with rightwards harpoon with barb upwards on top subscript 2 space space space space end subscript right parenthesis
rightwards double arrow space left parenthesis straight F with rightwards harpoon with barb upwards on top subscript 12 plus straight F with rightwards harpoon with barb upwards on top subscript 21 right parenthesis increment t space space equals space left parenthesis m subscript 1 stack v subscript 1 with rightwards harpoon with barb upwards on top plus m subscript 2 stack v subscript 2 with rightwards harpoon with barb upwards on top right parenthesis space minus space left parenthesis space m subscript 1 u with rightwards harpoon with barb upwards on top subscript 1 space minus m subscript 2 u with rightwards harpoon with barb upwards on top subscript 2 space space space space end subscript right parenthesis

S i n c e comma space F with rightwards harpoon with barb upwards on top space subscript 12 space a n d space F with rightwards harpoon with barb upwards on top subscript 21 space a r e space e q u a l space a n d space o p p o s i t e comma space

space space space space space space space space space space space space space space space space space space space space space space space space space space space F with rightwards harpoon with barb upwards on top space subscript 12 space plus F with rightwards harpoon with barb upwards on top subscript 21 space equals space 0

T h u s comma space left parenthesis m subscript 1 stack v subscript 1 with rightwards harpoon with barb upwards on top plus m subscript 2 stack v subscript 2 with rightwards harpoon with barb upwards on top right parenthesis space equals space left parenthesis m subscript 1 u with rightwards harpoon with barb upwards on top subscript 1 space minus m subscript 2 u with rightwards harpoon with barb upwards on top subscript 2 space space space space end subscript right parenthesis space

M o m e n t u m space a f t e r space c o l l i s i o n equals space M o m e n t u m space b e f o r e space c o l l i s i o n

H e n c e comma space m o m e n t u m space o f space i s o l a t e d space s y s t e m space i s space c o n s e r v e d. space

    Question 154
    CBSEENPH11016557

    What should an astronaut do in an open space if he wants to return to spaceship?

    Solution
    If an astronaut wants to come back from an open space into the spaceship, then he should throw some object in direction opposite to the direction of motion of spaceship. As a result of this, the astronaut will acquire some velocity towards the spaceship in accordance with the law of conservation of momentum and thus he can return to the spaceship.
    Question 155
    CBSEENPH11016558

    On firing the bullet, the rifle always gives a backward kick. Why?

    Solution
    Both gun as well as bullet are at rest, before firing. Therefore, momentum of system is zero.
    A bullet moves in the forward direction, as soon as it is fired. According to the law of conservation of momentum, total momentum of system remains conserved in the absence of external force. Therefore, momentum of the gun and bullet after fire should remain zero. This is possible, if and only if gun moves in backward direction.
    Question 156
    CBSEENPH11016559

    Heavier the rifle, lesser the kick. Why?

    Solution
    Let M and m be the mass of the gun and bullet respectively.
    Initially both the gun and bullet are at rest.
    Therefore, initial momentum before fire of the system is zero. 
    after firing, let straight v with rightwards arrow on top space and space straight V with rightwards arrow on top be the velocity of the bullet and gun respectively.
    Momentum of bullet after fire = straight m straight v with rightwards arrow on top 
    Momentum of gun after fire = straight M straight V with rightwards arrow on top
    Total momentum of system after fire = straight m straight v with rightwards arrow on top space plus space straight M straight V with rightwards arrow on top 
    According to the law of conservation of momentum,
                          straight m straight v with rightwards arrow on top space plus space straight M straight V with rightwards arrow on top space equals space 0 
    i.e.,                  straight V with rightwards arrow on top space equals space minus straight m over straight M straight v with rightwards arrow on top 
    Negative sign indicates that gun will recoil.
    It is clear from above equation that for a given momentum of bullet, the velocity of gun is inversely proportional to the mass of gun.
    Hence heavier the gun, lesser is the kick. 
    Question 157
    CBSEENPH11016560

    Why is it advised to hold the gun tightly with shoulder when it is being fired?

    Solution
    The velocity of kick is inversely proportional to the mass of the gun.
    When the gun is held tightly to the shoulder, the effective mass of gun increases and hence recoil velocity of the gun decreases. thus, injury of the shoulder is avoided. 
    Question 158
    CBSEENPH11016561

    How does the man get himself on the shore, standing at rest in the middle of an ice-pond?

    Solution
    By throwing some object or cloth the man can get himself to shore by throwing some object. By doing so, the man will be set into motion in opposite to the direction in which he throws the object. Since the ice is frictionless, the man once set into motion remains in motion and reaches the shore. 
    Question 159
    CBSEENPH11016562

    When a man jumps from a boat to the shore, the boat moves away from the shore. Why? 

    Solution
    When a man jumps from a boat to the shore, the boat slightly moves away from the shore.
    This is due to the reason that when man jumps from boat, the man gets the momentum in forward direction and to conserve the momentum of system (man+boat) the momentum of boat should be equal and opposite to that of man.  
    Question 160
    CBSEENPH11016563

    If a ball is dropped from a certain height, its momentum increases. Does it violate the law of conservation of linear momentum?

    Solution
    No, law of conservation of linear momentum is not violated.
    Law of conservation of linear momentum states that in the absence of any external force the linear momentum of the system remains conserved.
    Here on the ball, an external force due to gravity is acting and this force on the ball increases the momentum of the ball. 
    Question 161
    CBSEENPH11016564

    Two boys on ice skates hold a rope between them. One is heavier than the other. Do both the boys move when only one boy pulls the rope?

    Solution
    Initially,  both the boys are at rest. Therefore, momentum of boys is zero.
    When one of the two boys pulls the other, both the boys will be set into motion and move towards each other to conserve the momentum.
    Let 'm' and straight v with rightwards harpoon with barb upwards on top be the mass and velocity of light boy, and 
    'M' and straight V with rightwards harpoon with barb upwards on top the mass and velocity of heavy boy, respectively. 
    According to law of conservation of momentum,
                              straight m straight v with rightwards arrow on top space plus space straight M straight V with rightwards arrow on top space equals space 0 
    rightwards double arrow                        straight V with rightwards arrow on top space equals space minus straight m over straight M straight v with rightwards arrow on top 
    Since velocity of heavier boy is less than the other, therefore, heavier boy will travel less distance than the lighter one. 
    Question 162
    CBSEENPH11016565

    A body falls freely. Does the momentum of body remain conserved?
    If not, does this example contradict the principle of conservation of momentum?

    Solution

    When a body is falling, it's momentum increases in the downward direction. But, this will not violate the law of conervation of momentum because there is external gravitational force acting on body, which increases the momentum of body. 
    But if we consider body and the earth as a system then total momentum of body and the earth remains constant because then gravitational force will be internal force. 

    Question 163
    CBSEENPH11016566

    A bullet of mass m is fired with velocity u into a block of mass M at rest. What is the final velocity of system supposing bullet remain embedded into the block?

    Solution

    Initial momentum of block = M x 0 = 0
    Initial momentum of bullet = mu 
    Therefore,
    Initial momentum of system (bullet + block)
    before the bullet is embedded into block = mu
    Let be the velocity of system (bullet + block)
    after bullet gets embedded into block.
    ∴ Final momentum of system (bullet + block)
    after bullet gets embedded into block = (m + M)V


    According to law of conservation of momentum, 
                (m+M)V = mu 
                  V italic space italic equals italic space fraction numerator m over denominator m italic plus M end fraction u 
    This is the final velocity of the system. 

    Question 164
    CBSEENPH11016568

    A gun of mass 100 kg fires a shell of mass 0.020 kg. If the muzzle speed of the shell is 80 m/s, then find the recoil speed of the gun.

    Solution

    Here, 
    Mass of shell = m = 0.020 kg
    Mass of gun = M = 100 kg
    Velocity of shell = u = 80 m/s
    Let be the velocity of gun.
    According to the law of conservation of linear momentum,
                      mu + MV = 0
                     straight V equals negative mu over straight M space equals space minus fraction numerator 0.02 cross times 80 over denominator 100 end fraction 
                        = -0.016 m/s
    Negative sign means the gun recoils.

    Question 165
    CBSEENPH11016570

    A bomb at rest explodes into three parts of same mass. The momentum of two parts is italic minus italic 2 p i with italic hat on top italic space a n d italic space p j with italic hat on top italic. Find the magnitude of momentum of third point.

    Solution

    Let straight P with rightwards arrow on top be the momentum of third point.
    According to law of conservation of linear momentum,
     straight P with rightwards arrow on top space minus space 2 straight p straight i with hat on top space plus space straight p space straight j with hat on top space equals space 0
    rightwards double arrow             P with italic rightwards arrow on top italic space italic equals italic space italic 2 p italic space i with italic hat on top italic space italic minus italic space p j with italic hat on top
     i.e.,            straight P with rightwards arrow on top space equals space square root of left parenthesis 2 straight p right parenthesis squared plus left parenthesis straight p right parenthesis squared end root space equals space left parenthesis square root of 5 right parenthesis straight p, is the required magnitude of momentum of the third point. 

    Question 166
    CBSEENPH11016572

    A body of mass 10 kg explodes into three masses in the ratio 2 : 3 : 5. Two light fragment fly off at right angle with velocity 12m/ s and 8m/s. Find the velocity of third fragment.

    Solution

    After explosion,
    Let 2 kg mass after explosion fly off along X-axis and 3 kg mass fly off along Y-axis perpendicular to direction of motion of 2 kg mass.
    Suppose the velocity of third fragment after explosion is <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>
    ∴ Momentum of 2 kg mass is, 
                 stack straight P subscript 1 with rightwards arrow on top space equals space straight m subscript 1 stack straight v subscript 1 with rightwards arrow on top space equals space 2 cross times 12 straight i with hat on top space equals space 24 straight i with hat on top space space Ns 
    Momentum of 3 kg mass is, 
                stack straight P subscript 2 with rightwards arrow on top space equals space straight m subscript 2 stack straight v subscript 2 with rightwards arrow on top space equals space 3 cross times 8 straight j with hat on top space equals space 24 space straight j with hat on top space space Ns 
    Momentum of 5 kg mass is, 
              stack straight P subscript 3 with rightwards arrow on top space equals space straight m subscript 3 space stack straight v subscript 3 with rightwards arrow on top space equals space 3 cross times straight v with rightwards arrow on top space equals space 3 straight v with rightwards arrow on top space space Ns 
    According to the law of conservation of linear momentum, 
                  stack straight P subscript 1 with rightwards arrow on top plus stack straight P subscript 2 with rightwards arrow on top plus stack straight P subscript 3 with rightwards arrow on top space equals space 0 
    rightwards double arrow         24 straight i with hat on top space plus space 24 straight j with hat on top space plus space 5 straight v with rightwards arrow on top space equals space 0 
    rightwards double arrow         <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre> 
    ∴          space space space open vertical bar straight v with rightwards arrow on top close vertical bar space equals space square root of left parenthesis 4.8 right parenthesis squared plus left parenthesis 4.8 right parenthesis squared end root                       

                      equals space 4.8 square root of 2 space straight m divided by straight s   
    This is the required velocity of the third fragment. 

    Question 167
    CBSEENPH11016573

    A boy weighing 50kg is running with velocity 24km/hr jumps onto a cart of mass 100kg moving with velocity 12km/hr in opposite direction. What is the velocity of boy–cart system after the boy lands over the cart?

    Solution

    Given,
    Mass of the boy, m1 = 50 kg 
    Mass of the cart, m2 = 100 kg 
    Velocity of boy,space u subscript italic 1 space equals space plus space 24 space km divided by hr
    Velocity of cart,    u subscript italic 2 space equals space minus 12 space km divided by hr 
    Let be the velocity of boy - cart system. 
    Using law of conservation of linear momentum, 
         50 x 24 - 100 x 12 = (50 + 100) x V

    rightwards double arrow                           = 0
    Therefore, the velocity of the boy-cart system becomes zero after the boy lands on the cart. 

    Question 168
    CBSEENPH11016574

    A body of mass 2 m is projected with velocity 100 m/s at angle 60° with horizontal. At highest point, it explodes into two fragments of equal masses. If after explosion one fragment retraces its path, then find the velocity of second fragment.

    Solution

    Given,
    Initial velocity, u = 100 m/s
    angle of projection of the particle,  straight theta space equals space 60 degree
    Therefore the velocity of body at highest point is,
     U italic space italic equals italic space u italic space cosθ space equals space 100 space cross times space cos space 60 degree space equals space 50 space straight m divided by straight s
    At the highest point one of the fragment retraces its path, therefore after explosion the direction of the velocity of fragment reverses without change in the magnitude of velocity.
    Therefore the velocity of fragment that retraces its path after explosion becomes 50 m/s.
    Let V be the velocity of second fragment.
    Now according to the law of conservation of linear momentum,
    space space space 2 straight m space straight x space 50 space equals space straight m space cross times space left parenthesis negative 50 right parenthesis space plus space mV
    rightwards double arrow   V = 150 m/s, is the velocity of the second fragment.

    Question 169
    CBSEENPH11016575

    A shell of mass 40 kg moving with velocity 20m/s explodes and burst into two portions of masses 32kg and 8 kg. Heavier mass comes to rest after explosion. What is the energy released during the explosion?

    Solution
    Mass of the shell, m = 40 kg
    Velocity with which the shell is moving, v = 20 m/s
    Let V be the velocity of 8 kg mass after explosion.
    Now using law of conservation of linear momentum.
    We have, 
                     40 x 20 = 32 x 0 + 8 x V
    rightwards double arrow              V = 100 m/s
    Kinetic energy of shell before explosion is,
       straight K subscript 1 space equals space 1 half cross times 40 cross times left parenthesis 20 right parenthesis squared space equals space 8000 straight J       
    Kinetic energy of shell after explosion is, 
        straight K subscript 2 space equals space 1 half cross times 32 cross times left parenthesis 0 right parenthesis squared plus 1 half cross times 8 cross times left parenthesis 100 right parenthesis squared      
             = 0 + 40000
             = 40000J

    Thus, amount of energy released is,
               straight K subscript 2 minus straight K subscript 1 space equals space 40000 minus 8000 space equals space 32000 straight J 
    Question 170
    CBSEENPH11016576

    Is the relation F = ma applicable to motion of rocket?

    Solution
    The relation F = ma is applicable only if the mass of the body is constant.
    But in case of rocket, the mass of the rocket is continuously decreasing, therefore the relation F = ma is not applicable. 
    Question 171
    CBSEENPH11016577

    What are the factors on which the velocity of a rocket depends?

    Solution

    The velocity of a rocket depends on: 
    (i) Initial mass of rocket.
    (ii) Velocity of ejecting gases.
    (iii) Rate of ejection of burnt gases.

    Question 172
    CBSEENPH11016578

    What are the factors on which thrust on the rocket depends?

    Solution

    The thrust on a rocket depends on:
    (i) Velocity of ejecting gases w.r.t. rocket.
    (ii) Rate of ejection of burnt gases.

    Question 173
    CBSEENPH11016579

    Derive the expression for the velocity of rocket at any instant in the case of rocket propulsion in the presence of gravity.

    Solution

    Consider, a rocket of mass m0 take off with velocity v0 from ground.

    Let the fuel burn at the rate of dm/dt and burnt gases eject with velocity u w.r.t. rocket.
    Let at any instant t, v be the velocity of rocket and m be the mass of rocket.
    Therefore velocity of burnt gas ejected w.r.t. ground is (v – u).
    Here, fuel is burning at the rate of dm/dt.
    Therefore in time dt, dm mass of the fuel will burn and velocity of rocket increases by dv.
    M o m e n t u m space o f space r o c k e t space a t space a n y space i n s t a n t space i s comma space

space space space space space space space space space space space space space space space space space straight p subscript 1 space equals space m u space

M o m e n t u m space o f space left parenthesis r o c k e t plus b u r n t space f u e l right parenthesis space a t space straight t plus d t space i s comma space

straight p subscript 2 space equals space left parenthesis straight m minus d m right parenthesis space left parenthesis straight v plus d v right parenthesis space plus space d m left parenthesis straight v plus d v minus straight u right parenthesis

space space space space space equals space m v space plus space m d v space minus space u d m space

therefore space C h a n g e space i n space m o m e n t u m space i n space t i m e space d t space i s comma space

space space space space space space space space space space space space space space space d p equals straight p subscript 2 space minus space straight p subscript 1

space space space space space space space space space space space space space space space space space space space space equals space left parenthesis m v space plus space m d v space minus space u d m right parenthesis space minus m v

space space space space space space space space space space space space space space space space space space space space space equals space m d v space minus space u d m space

W e space k n o w comma space

I m p u l s e space equals space C h a n g e space i n space m o m e n t u m

H e r e comma space i m p u l s e space i s space d u e space t o space g r a v i t a i o n a l space f o r c e.

therefore space space space minus space m g space d t space equals space straight m space d v space minus space u d m space

rightwards double arrow space space space space d v space equals space straight u fraction numerator d m over denominator straight m end fraction space minus space straight g space d t
    Here, we have
    'm' is the mass of rocket, and
    dm is the mass of gas ejected.
    As the fuel burns the mass of rocket decreases.
    Therefore, to calculate the velocity of rocket at any instant in terms of mass of rocket, m must be replaced by ‘dm’. 
    therefore space dv space equals space minus straight u space dm over straight m space minus space straight g space dt space

Integrating space both space sides comma space

space space space space space integral subscript straight v subscript straight o end subscript superscript straight v dv space equals space minus space straight u space integral subscript straight m subscript straight o end subscript superscript straight m dm over straight m space minus space straight g space integral subscript 0 superscript straight t dt space

rightwards double arrow space space space space right enclose space straight v end enclose subscript straight v subscript straight o end subscript superscript straight v space equals space minus space straight u space right enclose left parenthesis Ln space straight m right parenthesis end enclose subscript straight m subscript straight o end subscript superscript straight m space minus space straight g space right enclose straight t subscript 0 superscript straight t space

rightwards double arrow space space space space space straight v space minus space straight v subscript 0 space equals space minus space straight u space left square bracket Ln space straight m space minus space Ln space straight m subscript straight o right square bracket space minus space gt space

rightwards double arrow space space space space space space space space space space space space space space space straight v space equals space straight v subscript 0 space plus space Ln space open parentheses straight m subscript straight o over straight m close parentheses space minus space gt

    Question 174
    CBSEENPH11016580

    A rocket takes off from rest and leaves the burnt gases with speed u w.r.t. rocket. Find the maximum velocity of rocket if 50% of the total mass of rocket is fuel.

    Solution
    Let m0 be the total mass of rocket.
    The velocity of the rocket when m gram of fuel gets burnt is, 
                         straight v space equals space straight u space ln space open parentheses fraction numerator straight m subscript straight o over denominator straight m subscript straight o space minus space straight m end fraction close parentheses
    Maximum velocity is acquired by the rocket when the whole fuel will burn out. 
    Since mass of the fuel is 50% of m0, thus
    M a x i m u m space v e l o c i t y comma space v subscript m a x end subscript space equals space u space ln space 2

space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space equals space u space cross times space 0.623 
    Question 175
    CBSEENPH11016581

    A rocket of initial mass 5000kg consumes the fuel at the rate of 20kg/s. The burnt gases leave the rocket with speed 1000m/s. Find the initial thrust on the rocket?

    Solution

    Here,
    Initial mass of rocket = 5000 kg
    Velocity with the burnt gas is leaving the rocket, u = 1000m/s
    Rate of consumption of fuel, dm over dt equals 20 space kg divided by straight s
    The thrust on the rocket is,
    space italic space F italic equals u fraction numerator d m over denominator d t end fraction equals 1000 cross times 20 space equals space 20000 straight N 

    Question 176
    CBSEENPH11016583

    The fuel in the rocket of mass 8000kg burns at the rate of 16kg/s. What should be minimum velocity of ejecting gases w.r.t. rocket to overcome the gravity?

    Solution
    Here,
    Mass of fuel in rocket, m0 = 8000 kg
    Rate of consumtion of fuel, dm/dt = 16 kg/s
    Let u be the minimum required velocity of ejecting gases to lift the rocket against gravity.
    ∴           m g equals u fraction numerator d m over denominator d t end fraction
    rightwards double arrow     8000 x 9.8 = u x 16
                            u = 4900 m/s
    Therefore, a velocity of 4900 m/s is required for ejecting the gases. 
    Question 177
    CBSEENPH11016584

    A rocket has mass 6000kg. 25% of the initial mass of the rocket is fuel. The fuel burns at the rate of 25kg/s. If the gases leave the rocket with speed 2000m/s w.r.t. rocket, then find the velocity of rocket at t = 40s. Assume the rocket takes off from rest.

    Solution
    The velocity of rocket at any instant
    is given by,
                    straight v equals straight u space Ln open parentheses fraction numerator straight m subscript straight o over denominator straight m subscript straight o minus begin display style dm over dt end style straight t end fraction close parentheses      ... (1)
    Given,
    Mass of rocket, straight m subscript straight o space equals space 6000
    Rate of burning of fuel,  dm/dt = 25 kg/s
    Speed with which the gas leaves rocket, u = 2000m/s
    Time, t = 40 s
    Putting the values in eqn. (1), we have 
           straight v equals 2000 cross times Ln fraction numerator 6000 over denominator 6000 minus 25 cross times 40 end fraction
             = space space 2000 cross times Ln left parenthesis 1.2 right parenthesis
             = 364.64 m/s 
    Question 178
    CBSEENPH11016585

    A cracker rocket is ejecting 80gm of gases per minute at the speed of 420m/s. Find the accelerating force on the rocket.

    Solution

    Given, 
    Rate of ejection of gases, 
    dm over dt equals 80 gm divided by min
space space space space space space space space space equals fraction numerator 80 cross times 10 to the power of negative 3 end exponent over denominator 60 end fraction kg divided by straight s 
         space space equals 4 over 3 cross times 10 to the power of negative 3 end exponent kg divided by straight s 
    Initial speed of ejection of gases from rocket, space space u space equals space 2000 straight m divided by straight s
    ∴  Accelerating force on the rocket is, 
     straight F equals straight u dm over dt equals 420 cross times 4 over 3 cross times 10 to the power of negative 3 end exponent space equals space 0.56 space straight N 

    Question 179
    CBSEENPH11016586

    A stream of water flowing horizontally with a speed of 15 m/s gushes out of a tube of cross-sectional area 10–2 m2. What is the force exerted by stream on the tube?

    Solution
    The force exerted by the stream on the tube is, 
                          space space F equals a v squared straight rho 
    Given,
    Area of tube, a = 10-2 m2

    Speed of water gushing out, v = 15 m/s
    Density of water,  straight rho space equals space 1000 space kg divided by straight m cubed 
    ∴ Force exerted by a stream on the tube is, 
         
           F equals 10 to the power of negative 2 end exponent cross times left parenthesis 15 right parenthesis squared cross times 1000
              = 2250 N 
    Question 180
    CBSEENPH11016587

    Define impulse. Derive an expression for impulse in terms of momentum.

    Solution
    The measure of the total effect produced by force is called as Impulse.
    Momentum of the body changes as a result of the force acting on the body.
    Therefore, impulse of force is defined as the change in momentum produced by the force and is equal to the product of force and time for which force acts. 
    I f space straight a space f o r c e space straight F with rightwards harpoon with barb upwards on top space a c t s space o n space straight a space b o d y space f o r space s h o r t space i n t e r v a l space o f space t i m e space d t comma space t h e n space i m p u l s e space i s comma space

space space space space space space space space space space space space space space space space space space space space space space stack d I space with rightwards harpoon with barb upwards on top space equals space straight F with rightwards harpoon with barb upwards on top space. space d t space

T o t a l space i m p u l s e space p r o d u c e d space b y space f o r c e space i n space i n t e r v a l space b e t w e e n space straight t subscript 1 space t o space straight t subscript 2 space i s comma space

stack I italic space with italic rightwards harpoon with barb upwards on top space equals space stack d I space with rightwards harpoon with barb upwards on top space equals space integral subscript straight t subscript 1 end subscript superscript straight t subscript 2 end superscript straight F with rightwards harpoon with barb upwards on top. space stack d t with rightwards harpoon with barb upwards on top space
space space space space space space space space space space space space space space equals space integral subscript straight p subscript 1 end subscript superscript straight p subscript 2 end superscript space stack d p with rightwards harpoon with barb upwards on top space
space space space space space space space space space space space space space space equals space right enclose straight p with rightwards harpoon with barb upwards on top end enclose subscript straight p subscript 1 end subscript superscript straight p subscript 2 end superscript space
space space space space space space space space space space space space space space equals space stack straight p subscript 2 with rightwards harpoon with barb upwards on top space minus space stack straight p subscript 1 with rightwards harpoon with barb upwards on top
    Question 181
    CBSEENPH11016588

    Are impulse and momentum similar type of quantities?

    Solution
    Impulse and momentum are different types of quantities.
    Impulse is measure of the change in the momentum when an external force is applied while momentum is measure of the quantity of motion contained in the body. 
    Question 182
    CBSEENPH11016589

    Why buffers are provided between the bogies of a train?

    Solution
    Buffers are provided between the bogies of a railway train to avoid the severe jerk to bogies and to the passengers sitting in them.
    Buffers increase the time of impact between the two bogies before they stop and hence jerk is reduced. 
    Question 183
    CBSEENPH11016590

    Why are chinawares wrapped in paper cutting or straw?

    Solution
    China and glasswares are wrapped in paper or straw pieces before packing.  In the event of fall, impact will take a longer time to reach the glass/chinawares through paper or straw. As a result the average force exerted on the china or glassware is small and chances of their breaking reduces. 
    Question 184
    CBSEENPH11016591

    Why automobiles are provided with springs?

    Solution
    Automobiles are provided with spring system because when an automobile moves on uneven roads, it receives a jerk. Spring system increases the time of impact and hence reduces the jerk and minimises the damages. 
    Question 185
    CBSEENPH11016593

    Why does a cricket player lower his hands while catching a ball? 

    Solution
    While catching a ball, a cricket player lowers his hands, because by doing so, he increases the time of catch. That is, the person increases the time to bring about a given change in momentum, and hence rate of change of momentum decreases.
    Thus, a small force is exerted by ball on the hands. 
    Question 186
    CBSEENPH11016594

    A man falling on hard concrete road receives more injuries than the one falling on ploughed sand floor. Why?

    Solution
    A concrete floor is hard and it does not yield.  Therefore, man falling on concrete floor is stopped abruptly in a short interval of time and suffers a sudden change in momentum. Therefore, concrete floor exerts a large force and man gets more injury.
    On the other hand, when man jumps on ploughed sand, due to depression of sand, the time of impact increases. Therefore, rate of change of momentum decreases and the person experiences less force. Thus, man gets less injury. 
    Question 187
    CBSEENPH11016595

    Same forces are applied on the two different bodies of unequal masses for same interval of time. In which body the impulse produced by the force will be more?

    Solution
    Magnitude of force and the time for which the force is acting determines the impulse of the body. Impulse is  independent of mass of the body. 
    Thus the impulse produced in both the bodies will be same. 
    Question 188
    CBSEENPH11016596

    A slow change in momentum reduces force of action and reaction and causes less injury or damages. Give two practical application of the said fact.

    Solution

    A slow change in momentum reduces force of action and reaction, causing less injury.
    (i) Seat of cycles is provided with springs. The spring slows down the rate of change of momentum and reduces the hurt to the cyclist.
    (ii) The paper cutting used in packing the glasswares decreases the rate of change of  momentum and avoids their damages.

    Question 189
    CBSEENPH11016598

    A block of mass M is placed on frictionless surface and given a jerk by average force of F which acts for time t. What is the velocity acquired by the block?

    Solution
    Average force, F acts on the body for time t. 
    Therefore, impulse of force is,
                                    I = Ft 

    Impulse is equal to change in momentum.
    If V is the velocity acquired by body after jerk, then change in momentum of body will be mv. 
    ∴ Impulse, I = Ft = MV 
    That is,      V = Ft/M, is the velocity acquired by the block. 
    Question 190
    CBSEENPH11016600

    Two billiard balls, each of mass 0.05 kg, moving in opposite directions with speed 6 m/s collide and rebound with the same speed. What is the impulse imparted to each ball due to the other?

    Solution
    Mass of billiard balls = 0.05 kg 
    After collision the two balls rebound with same speed, therefore, the change in the momentum of each ball is, 
                         increment p equals 2 m u 
    Substituting m = 0.05 kg and u = 6 m/s.
    We get,
    Impulse, I italic equals italic increment p space equals space 2 cross times 0.05 cross times 6 equals 0.6 space Ns


     
    Question 191
    CBSEENPH11016603

    A cricket player caught a ball of mass 180 gm moving with a speed of 54 km/hr. If he took 0.08 s to catch the ball, then how much force did the player exert on the ball?

    Solution

    Given,
    Mass of the ball, m = 180gm = 0.18kg 
    Initial velocity of ball is, u = 54 km/hr = 15 m/s
    Final velocity of the ball, v = 0 
    ∴     Impulse of the ball, I = m(u - v)
                                           = 0.18 x 15
                                           = 2.7 Ns 
    Time of catching the ball, Δt space equals space 0.08 space straight s
    Thus,
    Force exerted by player on the ball,       straight F equals straight I over Δt equals fraction numerator 2.7 over denominator 0.08 end fraction equals 33.75 straight N

    Question 192
    CBSEENPH11016605

    A ball of mass 0.125kg moving with speed 4m/s collides the vertical wall and rebounds with same speed. What is the impulse imparted to the ball?

    Solution

    Mass of the ball, m = 0.125 kg
    Velocity of the ball, v = 4 m/s
    Initial momentum of ball is, 
    straight p subscript 1 space equals m v space equals space 0.125 cross times 4 space equals space 0.5 space Ns 
    After collision, the ball rebounds with the same speed. That is it is travelling in an opposite direction. 
    Momentum of ball after collision is,
    space straight p subscript 2 space equals space m left parenthesis negative v right parenthesis space equals 0.125 cross times left parenthesis negative 4 right parenthesis space equals negative 0.5 space Ns 
    Therefore impulse of ball is 
    increment straight p space equals space straight p subscript 2 minus straight p subscript 1 space equals space minus 1 space Ns

    Question 193
    CBSEENPH11016606

    Force – time curve for a body of mass 7.59 kg is shown in the figure. What is the velocity of the body at the end of 10s? Assume the body starts from rest. 


    Solution

    Area under Force–time graph is equal to the impulse or change in the momentum of the body. 

    Now, Area under F–t graph,
    From, t = 0 to t = 10s = Area of trapezium 1+ Area of trapezium 2 + Area of rectangle 3 
            space space space equals left square bracket 1 half left parenthesis 5 plus 12 right parenthesis cross times 4 right square bracket space plus space left square bracket 1 half left parenthesis 12 plus 7 right parenthesis cross times 3 right square bracket plus left square bracket 7 cross times 3 right square bracket 
               = 34 + 28.5 + 21 = 83.5

    ∴ Change in momentum of body = 83.5 Ns 
    i.e.            m(v - 0) = 83.5 
    rightwards double arrow            7.59 x v = 83.5 
    rightwards double arrow                       v = 11 m/s, is the velocity of the body. 

    Question 194
    CBSEENPH11016608

    Figure below shows the position-time graph of a particle of mass 4 kg.



    What is the: 

    (a) force on particle for, 

                t<0;  t>4s; 0<t<4s? 

    (b) impulse at t = 0 and t = 4s? 

    Solution
    (a) From the given graph, for t < 0, and t > 4s, the position of particle does not change.
    Therefore, the particle is at rest and for 0 < t < 4s the position-time graph is a straight line.
    This implies the velocity of particle is constant. 
    Therefore, 
    Acceleration of particle for t < 0, t < 4s and 0 < t < 4s is zero. Hence, no force acts on the particle. 
    b) We know, the slope of the position-time graph is velocity. Therefore, velocity of particle in interval 0 < t < 4 is, 
    straight v space equals space fraction numerator straight x left parenthesis 4 right parenthesis space minus space straight x left parenthesis 0 right parenthesis over denominator 4 minus 0 end fraction space equals space fraction numerator 3 minus 0 over denominator 4 minus 0 end fraction space equals space 3 over 4 space straight m divided by straight s

Momentum space of space particle space at space straight t less than space 0 space is comma space

straight p subscript 1 space equals space straight m space straight X space 0 space equals space 0 space

Momentum space of space partcile space at space straight t greater than space 0 space is comma space

straight p subscript 2 space equals space 4 space straight X space open parentheses 3 over 4 close parentheses space equals space 3 space Ns space

Therefore comma space

Impulse space at space straight t equals space 0 space is comma space

straight I space equals space straight p subscript 2 space minus space straight p subscript 1 space equals space 3 space Ns space

Now comma space momentum space of space particle space at space time space straight t less than space 4 space is comma space

straight p subscript 3 space equals space 3 space Ns space

Momentum space of space particle space at space straight t space greater than space 4 space is comma space

straight p subscript 4 space equals space straight m space straight X space 0 space equals space 0

Therefore comma space impulse space at space straight t equals 4 space sec space is comma space

straight I space equals space left parenthesis straight p subscript 4 minus straight p subscript 3 space right parenthesis space equals space minus space 3 space Ns
    Question 195
    CBSEENPH11016610

    A batsman hits a ball approaching with momentum 12 Ns and deflects by angle 135° as shown in figure. Find the impulse imparted to ball.

    Solution

    Momentum of the ball, m = 12 Ns 
    Initial momentum of ball, stack straight P subscript 1 with rightwards arrow on top space equals space 12 space Ns along AO with rightwards arrow on top
    Final momentum of ball is,  stack straight P subscript 2 with rightwards arrow on top space equals space 12 space Ns along <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
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#6 {main}</pre> 
    ∴  Impulse imparted to ball, I with rightwards arrow on top space equals space stack P subscript 2 with rightwards arrow on top space minus space stack P subscript 1 with rightwards arrow on top 
    From the given figure, the angle between vector stack straight P subscript 2 with rightwards arrow on top and stack straight P subscript 1 with rightwards arrow on top is 45 degree.
    So, impulse imparted to the ball is, 
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#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
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#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
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#6 {main}</pre>

        = 9.18 Ns

    Question 196
    CBSEENPH11016611

    Name the basic interactions in the universe.

    Solution

    The four basic interactions in the universe are:
    (i) Gravitational interaction.
    (ii) Electromagnetic interaction.
    (iii) Strong interaction.
    (iv) Weak interaction.

    Question 197
    CBSEENPH11016612

    What are similarities between gravitational and electro-magnetic forces?

    Solution

    The similarities between gravitaional and electromagnetic forces are: 
    (i) Both the forces follow inverse square law. 
    (ii) Both the forces are central forces.
    (iii) Both the forces are conservative forces.

    Question 198
    CBSEENPH11016613

    If strong interaction is of the order of 1N, then what is the order of electromagnetic, weak and gravitational interaction?

    Solution
    Strong interaction is of the order of 1N.
    Then electromagnetic, weak and gravitational interactions are of the order of 10–2 N, 10–13 N and 10–38 N.
    Question 199
    CBSEENPH11016614

    Name the exchange particles in all types of interactions.

    Solution
    The exchange particles for different interactions are:
    Gravitational interaction - gravitons
    Electrostatic interaction - photons
    Weak interaction - intermediate vector bosons, W and Z bosons. 
    Nuclear interactions - exchange of pions.
    Question 200
    CBSEENPH11016615

    Show that ratio of electric to gravitational interaction between two protons is of the order of 1036.

    Solution
    The gravitational force between two masses is, 
                        straight F subscript straight g space equals space straight G fraction numerator straight m subscript 1 straight m subscript 2 over denominator straight r squared end fraction 
    The electrostatic force between two charges is,
                        straight F subscript straight e space equals space fraction numerator 1 over denominator 4 πε subscript straight o end fraction fraction numerator straight q subscript 1 straight q subscript 2 over denominator straight r squared end fraction 
    Therefore, 
    straight F subscript straight e over straight F subscript straight g equals fraction numerator begin display style fraction numerator 1 over denominator 4 πε subscript 0 end fraction fraction numerator straight q subscript 1 straight q subscript 2 over denominator straight r squared end fraction end style over denominator straight G begin display style fraction numerator straight m subscript 1 straight m subscript 2 over denominator straight r squared end fraction end style end fraction equals fraction numerator 1 over denominator 4 πε subscript straight o straight G end fraction fraction numerator straight q subscript 1 straight q subscript 2 over denominator straight m subscript 1 straight m subscript 2 end fraction 
    For proton: 
    Mass of protons, straight m subscript 1 equals straight m subscript 2 equals 1.67 cross times 10 to the power of negative 27 end exponent space kg
    Charge on proton, straight q subscript 1 equals straight q subscript 1 equals 1.6 cross times 10 to the power of negative 19 end exponent straight C 
    So ratio of electric to gravitational force is, 
    straight F subscript straight e over straight F subscript straight g equals fraction numerator 9 cross times 10 to the power of 9 over denominator 6.67 cross times 10 to the power of negative 11 end exponent end fraction open parentheses 1.67 cross times 10 to the power of negative 27 end exponent close parentheses squared over open parentheses 1.6 cross times 10 to the power of negative 19 end exponent close parentheses squared 
       space space almost equal to space 10 to the power of 36
    Question 201
    CBSEENPH11016617

    A block is supported by a cord A from a rigid support and another cord B is attached to the bottom of block as shown. If we pull the string B steadily then which of the two cords will break? Assume cords A and B are identical.

    Solution
    When string B is pulled with force F, then
    Tension in string B = F
    Tension in string A = F+ mg.
                            
    Since tension in cord A is greater than in B, therefore, cord A will break. 

    Question 202
    CBSEENPH11016619

    Discuss the horse cart problem on level road.

    Solution

    Let the cart be connected to horse through a string, as shown in the figure.
          
    Let space T with italic rightwards arrow on top be the tension in the string.
    For the cart to move, the horse presses the ground and the horse gets equal and opposite reaction space straight R with rightwards arrow on top.
    Let F with rightwards arrow on top be the force of friction between cart and ground, and
    Acceleration of system = straight a with rightwards arrow on top.
    Resolve R with rightwards arrow on top into two rectangular components <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre> 
    From the free body diagram of horse given above
                                V = mg 
    and                 H - T = ma                       ...(1) 
    From the free body diagram of the cart, 
                        T - F = Ma                          ...(2)
    Adding (1) and (2), we have,
                       H - F = (m+M)a
    rightwards double arrow                     a equals fraction numerator H minus F over denominator m plus M end fraction
    The system will move if and only if a>0.
    i.e.                      H>F, for the cart to move. 
                       

    Question 203
    CBSEENPH11016620

    Two bodies of masses m1 and m2 are connected by massless rope and pulled by force F acting on mass m1. What is the acceleration of system and tension in the string?

    Solution
    Let 'a' be the acceleration of system and 'T' be the tension in the string.

    ∴  Equations of dynamic of mass m1 and m2 are
                               F - T = m1a                       ...(1)
                                     T = m2a                      ...(2)
    Solving (1) and (2), we have,
                          space space straight a equals fraction numerator straight F over denominator straight m subscript 1 plus straight m subscript 2 end fraction
    and                  <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>
    Question 204
    CBSEENPH11016621

    A uniform rope of length L resting on frictionless horizontal surface is pulled at one end by a force F. What is the tension in the rope at a distance ℓ from the end where the force is applied?

    Solution
    Length of the rope = L 
    Linera mass density of rope = λ
    Total mass of rope M = λL.
    The acceleration produced by force F in the string is, straight a equals straight F divided by λL
                  

    Let 'T' be tension in the string at given point B which is equal to force required to pull the part AB of the string.

    Question 205
    CBSEENPH11016623

    Three masses 6kg, 2kg and 4kg are kept on a frictionless horizontal surface and pulled by a force 42N. Find the acceleration of system, tension T1 and T2.


    Solution
    The equations of motion for mass m1, m2 and mas under:
    straight F space minus space straight T subscript 1 space equals space straight m subscript 1 straight a space
straight T subscript 1 space minus space straight T subscript 2 space equals space straight m subscript 2 straight a space
straight T subscript 2 space equals space straight m subscript 3 straight a space
    Solving the above equations, we get
    straight a space equals space fraction numerator straight F over denominator straight m subscript 1 plus straight m subscript 2 plus straight m subscript 3 end fraction and, 
    T1fraction numerator left parenthesis straight m subscript 2 plus straight m subscript 3 right parenthesis straight F over denominator straight m subscript 1 plus straight m subscript 2 plus straight m subscript 3 end fraction and
    T2fraction numerator straight m subscript 3 straight F over denominator straight m subscript 1 plus straight m subscript 2 plus straight m subscript 3 end fraction
    Substituting the value of m1, m2 and m3, we get
    Acceleration,  a equals space fraction numerator 42 over denominator 6 plus 2 plus 4 end fraction space space 3.5 space m divided by s squared
    Tension, T1fraction numerator left parenthesis 2 plus 6 right parenthesis 42 over denominator 6 plus 2 plus 4 end fraction space equals space 28 space N
    Tension, T2fraction numerator 6 left parenthesis 42 right parenthesis over denominator 6 plus 2 plus 4 end fraction space equals space 21 space N

    Question 206
    CBSEENPH11016645

    Two masses m1 and m2 are attached with massless, flexible and inextensible string and passed over a pulley (frictionless). Find the acceleration of masses and tension in the string. 

    Solution
    Weight of mass m1 is greater than the mass m2
    Let these two masses m1 and m2 be attached with massless, flexible and inextensible string and passed over the pulley.
                 

    Let 'T' be the tension in the string and 'a' be the acceleration of mass m1 and m2
    Equations of motion of masses m1 and m2 are, 
                  m subscript italic 1 g italic minus T italic equals m subscript italic 1 a                 ...(1)
                <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>                 ...(2)
    Solving (1) and (2), we get
    Acceleration of the masses is given by, 
                     space space a italic equals fraction numerator m subscript italic 1 italic minus m subscript italic 2 over denominator m subscript italic 1 italic plus m subscript italic 2 end fraction g         
            
    and          
    Tension in the string is,
                      T italic equals fraction numerator italic 2 m subscript italic 1 m subscript italic 2 over denominator m subscript italic 1 italic plus m subscript italic 2 end fraction g


    Question 207
    CBSEENPH11016667

    Two bodies of masses M and m are placed in contact with each other on frictionless surface. Does the contact force depend upon the body on which force is applied? Explain.

    Solution
    Yes, contact force depends upon the body on which force is applied.

    Let us apply the force F on system.
    The acceleration of system is given by, 
    straight a space equals space fraction numerator straight F over denominator straight M plus straight m end fraction

    The acceleartion acting on the system is irrespective of whether force is applied on mass M or m. 
    If force F is applied on mass m then contact force F1 is, 
                       F subscript 1 equals M a equals fraction numerator M over denominator M plus m end fraction F 
    If force is applied on Mass M then contact force F2 is, 
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#6 {main}</pre>

    Question 208
    CBSEENPH11016688

    Two masses 5kg and 3kg are attached with massless, flexible and inextensible string and passed over a pulley (frictionless). After 4s of release of system, the string breaks. Find how much higher the 3 kg mass would go after the string breaks?

    Solution

    Given,
    Mass of first body, m subscript italic 1 italic space italic equals italic space italic 5 italic space k g
    Mass of second body, m subscript 2 space equals space 3 space k g 
    The acceleration of system of masses is, 
    a equals fraction numerator m subscript 1 minus m subscript 2 over denominator m subscript 1 plus m subscript 2 end fraction g space equals space fraction numerator 5 minus 3 over denominator 5 plus 3 end fraction cross times 10 space equals space 2.5 space m divided by s squared 
    i.e. The mass 3 kg accelerates upward with acceleration 2.5 m/s2
                 

    Let the motion of 3 kg mass start from A and accelerate for 4s and reach the point B.
    At point B the string breaks and 3kg mass rises to point C.
    Motion from A to B:
    Initial velocity, "<pre
    Rate of acceleration, a subscript 1 equals 2.5 space m divided by s squared
    Time of travel, space straight t equals 4 straight s
    ∴ Velocity attained during the motion is given by first equation of motion, 
                   space space space space space space space space space space space v subscript B equals u subscript A plus a subscript 1 t equals 0 plus 2.5 cross times 4 equals 10 m divided by s 
    Motion from B to C: 
    Velocity, V e l o c i t y comma space v subscript B equals 10 space m divided by s comma
    v subscript c equals 0 comma 
    Acceleration, a subscript 2 equals negative 10 m divided by s squared 
    ∴ Distance travelled by the mass, 
    BC equals fraction numerator straight v subscript straight C superscript 2 minus straight v subscript straight B superscript 2 over denominator 2 straight a subscript 2 end fraction
      space space space equals fraction numerator 0 minus 100 over denominator negative 2 cross times 10 end fraction equals 5 straight m           
    The mass would go 5m high.                  

    Question 209
    CBSEENPH11016691

    In the figure below, each mass is m. Find the tension T1 and T2.


    Solution
    Let a be the acceleration of the system.
    The equation of motion of mass A is, 
    mg space minus space straight T subscript 2 space equals space ma space space space space space space space... left parenthesis 1 right parenthesis space

The space equation space of space motion space of space mass space straight B space is comma space

mg space plus space straight T subscript 2 space minus space straight T subscript 1 equals space ma space space space space space... space left parenthesis 2 right parenthesis space

The space equation space of space moton space space of space mass space straight C space is comma space

straight T subscript 1 minus space mg space equals space ma space space space space space space space space space space space space space... space left parenthesis 3 right parenthesis space

Adding space equations space left parenthesis 1 right parenthesis comma space left parenthesis 2 right parenthesis space and space left parenthesis 3 right parenthesis comma space we space get

acceleration comma space straight a space equals space straight g divided by 3 space

From space equation space left parenthesis 1 right parenthesis comma space we space have

straight T subscript 2 space equals space mg space minus space ma space equals space mg space minus space straight m straight g over 3 space equals space 2 over 3 mg

From space equation space left parenthesis 3 right parenthesis comma space we space have

straight T subscript 1 space equals space mg space plus space ma space equals space mg space plus straight m straight g over 3 space equals space 4 over 3 mg
 
    Question 210
    CBSEENPH11016692

    Two bodies of masses m1 and m2 are connected by a light string passing over a smooth light pulley fixed at the end of a horizontal table. The mass m1 is hanging and mass m2 lies on horizontal surface. Find the acceleration of system and tension in the string.

    Solution
    Let 'a' be acceleration of system and 'T' be the tension in the string. 
     
    Equation of motion of mass m1 is, 
    straight m subscript 1 straight g minus straight T equals straight m subscript 1 straight a                   ...(1)
    Equation of motion of mass m subscript 2 is, 
    T equals m subscript 2 a                        ...(2)
    Adding (1) and (2), 
                   m subscript 1 g equals left parenthesis m subscript 1 plus m subscript 2 right parenthesis a
                    a equals fraction numerator m subscript 1 over denominator m subscript 1 plus m subscript 2 end fraction g , is the acceleration of the system.
    Substituting the value of a in (2), 
    Tension in the string, T equals fraction numerator m subscript 1 m subscript 2 over denominator m subscript 1 plus m subscript 2 end fraction g

    Question 211
    CBSEENPH11016693

    Two bodies of masses m1 and m2 are connected by a light string passing over a smooth light pulley fixed at the end of an inclined plane. The mass m1 is hanging and mass m2 lies on inclined plane in equilibrium. Find the angle of inclination of inclined plane.

    Solution
    The system is in equilibrium, therefore the acceleration of the system of masses is zero. 

    ∴    
    Equation of motion for mass m1
                      m subscript 1 g equals T                           ...(1) 
    Equation of motion for mass m2

                        space space T equals m subscript 2 g sin theta                 ...(2) 
    From (1) and (2), 
                        s i n theta italic space italic equals italic space m subscript italic 1 over m subscript italic 2 
    rightwards double arrow     space space theta equals sin to the power of negative 1 end exponent open parentheses m subscript 1 over m subscript 2 close parentheses , is the required angle of inclination of inclined plane. 
    Question 212
    CBSEENPH11016694

    What are inertial and non-inertial frames of reference?

    Solution

    Inertial reference frames:
    Inertial reference frames are the frames in which Newton’s laws of motion hold good. These are the frames which are at rest or move with constant velocity with respect to one another.
    Non-inertial reference frames:
    The frames in which Newton’s laws of motion do not hold good are called non-inertial reference frames. All accelerated frames are non-inertial reference frames.

    Question 213
    CBSEENPH11016696

    In the system of pulleys shown in figure, P1 is fixed and while P2 is capable of moving freely in vertical direction. Both the pulleys are frictionless and massless. Find the acceleration of m1 and m2 and tensions T1 and T2.


    Solution
    Let 'a1' and 'a2' be the accelerations of mass m1 in downward direction and mass m2 in upward direction respectively.
    Since the displacement of mass m1 is twice that of pulley Pand hence mass m2, thus the acceleration a1 is equal to twice of acceleration a2.
    Hence,
                     space space space straight a subscript 1 equals 2 straight a subscript 2                          ...(1) 
    Equation of motion of mass straight m subscript 1 is, 
                     straight m subscript 1 straight g minus straight T subscript 1 equals straight m subscript 1 straight a subscript 1                 ...(2) 
    Equation of motion of mass straight m subscript 2 is, 
                     straight T subscript 2 minus straight m subscript 2 straight g equals straight m subscript 2 straight a subscript 2                  ...(3) 
    Also,                  straight T subscript 2 equals 2 straight T subscript 1                     ...(4) 
    Solving the equations, we get 
                 straight a subscript 1 equals 2 straight a subscript 2 equals fraction numerator 4 straight m subscript 1 minus 2 straight m subscript 2 over denominator 4 straight m subscript 1 plus straight m subscript 2 end fraction straight g 
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    Now, substituting the values of mass m1 and m2
             straight m subscript 1 equals 15 kg comma      space space space straight m subscript 2 equals 25 kg comma 
    we get
    Acceleration of masses, 
                       straight a subscript 1 equals 1.176 space straight m divided by straight s squared comma space space space space space space space space space space space space space space space space straight a subscript 2 equals 0.588 straight m divided by straight s squared 
    and  
    Tension in the string, 
     straight T subscript 2 equals 264.7 space straight N      straight T subscript 1 equals 132.35 straight N 
    Question 215
    CBSEENPH11016698

    If you are seated in sound proof room without window, can you detect if room is moving with a uniform velocity? Can you detect a uniform acceleration motion of room? 

    Solution

    Any body which is moving with a constant velocity, does not experience any unbalanced force. When the soundproof room is moving with constant velocity, then we cannot detect the motion of room.
    If the room is uniformly accelerated then there is net unbalanced force and in order to keep the bodies stable inside the room one has to exert the force. Hence accelerated motion of room can be detected.

    Question 216
    CBSEENPH11016699

    What is friction?

    Solution
    The force that opposes the relative motion between two bodies in contact, is called friction. 
    Question 217
    CBSEENPH11016700

    What is direction of force of friction?

    Solution
    Force of friction acts tangentially along the surface of contact and is always opposite to the direction in which body slides or tends to slide. 
    Question 218
    CBSEENPH11016701

    If we give impulse to the body, after travelling some distance it comes to rest though we do not apply some force. Is it contradictory to Newton’s first law of motion?

    Solution
    No, it is not contradictory to Newton’s first law of motion. It is the force of friction that destroys the motion of the body. 
    Question 219
    CBSEENPH11016702

    Friction is self-adjusting force. Comment.

    Solution
    Force of friction is self-adjusting force because it adjusts itself, in a direction, equal and opposite to the applied force till there is no motion. 
    Question 220
    CBSEENPH11016703

    What is sliding friction?

    Solution
    Sliding friction is the force of friction that comes into play when a body slides or tends to slide over the surface of another body. 
    Question 221
    CBSEENPH11016704

    What is rolling friction?

    Solution
    Rolling friction is the force of friction that comes into play when a body rolls or tends to roll over the surface of another body. 
    Question 222
    CBSEENPH11016705

    What is static friction?

    Solution
    Static friction is the force of friction that exactly balances the applied force till the body is at rest. 
    Question 223
    CBSEENPH11016706

    What is the maximum value of friction known as?

    Solution
    The maximum value of friction is called limiting friction.
    Question 224
    CBSEENPH11016707

    Define limiting friction.

    Solution
    Limiting friction is the maximum value of static friction that comes into play when a body is just at the point of sliding over the surface of another body. 
    Question 225
    CBSEENPH11016708

    What is dynamic friction?

    Solution
    Dynamic friction is the opposing force that comes into play when one body is actually moving over the surface of another body. 
    Question 227
    CBSEENPH11016710

    On what factors does the force of friction depend?

    Solution
    The force of friction depends on the nature of surfaces in contact and the normal reaction.
    Question 228
    CBSEENPH11016711

    How does the force of friction depend on the area of contact of the body with the surface?

    Solution
    The force of friction is independent of the area of contact of the body. 
    Question 229
    CBSEENPH11016712

    Which is more-sliding friction or rolling friction?

    Solution
    Sliding friction is greater than rolling friction.
    Question 230
    CBSEENPH11016713

    Which is more-static friction or dynamic friction?

    Solution
    Static friction is greater than dynamic friction.
    Question 231
    CBSEENPH11016714

    How does limiting friction depend upon normal reaction?

    Solution
    Limiting friction is directly proportional to the normal reaction.
    That is, 
                                 F proportional to R
    Question 232
    CBSEENPH11016715

    What is angle of friction?

    Solution
    Angle of friction is the angle that resultant of force of friction and normal reaction makes with the direction of normal reaction R.
    Question 233
    CBSEENPH11016717

    What is angle of friction for the rough surface of coefficient of friction μ?

    Solution

    Angle of friction for the rough surface is, 
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    Question 234
    CBSEENPH11016718

    What is angle of repose? 

    Solution
    Angle of repose or angle of sliding is the minimum angle of inclination of the plane with the horizontal, at which the body placed on it, just at the point of sliding. 
    Question 235
    CBSEENPH11016719

    What is the relation between angle of friction and angle of repose?

    Solution
    Angle of friction is equal to angle of repose.
    Question 236
    CBSEENPH11016720

    A body is placed on rough inclined plane having inclination less than the angle of repose. With what acceleration the body will slide down the inclined plane?

    Solution
    The body will not accelerate but remains at rest because angle of inclination is less than the angle of repose. 
    Question 237
    CBSEENPH11016721

    A body is placed on rough inclined plane having inclination greater than angle of repose. With what acceleration body will slide down the inclined plane?

    Solution

    Given that angle of inclination is greater than angle of repose. 
    So, 
    Acceleration of the body is, 
                          space straight a equals straight g left parenthesis sinθ minus μcosθ right parenthesis.

    Question 238
    CBSEENPH11016724

    A body is projected up on a rough inclined plane. What is the retardation of the body?

    Solution

    When a body is moving down the inclined plane, coefficient of friction = straight mu
    When the block is at rest on the plane, frictional force is mg sin straight theta
    When the block is moving down the plane, f = u mg cos straight theta
    Retardation of the body is given by, 
    space space space straight a equals straight g left parenthesis sinθ plus μcosθ right parenthesis

    Question 239
    CBSEENPH11016726

    A body is placed on rough inclined plane having inclination θ less than angle of repose ϕ. What is the force of friction between the body and the inclined plane?

    Solution

    The force of friction between the body and the inclined plane is , 
                           f =space space space space mg space sinθ.

    Question 240
    CBSEENPH11016727

    A body is placed on the rough inclined plane having inclination θ greater than angle of repose ϕ. What is the force of friction between the body and inclined plane?

    Solution

    A body placed on the rough inclined plane has angle of inclination greater than angle of repose. 
    So, 
    Force of friction between the body and the inclined plane is given by, 
    f = space space mg space cosθ space tanϕ.

    Question 241
    CBSEENPH11016728

    A body is placed on a rough inclined plane having inclination θ greater than the angle of repose ϕ. What is the minimum force required to prevent the body from sliding?

    Solution

    Angle of inclination = straight theta
    Angle of repose = straight ϕ
    Here, angle of inclination is greater than angle of repose. 
    Minimum force required to prevent the body from sliding is, 
           F= space space mg left parenthesis sinθ plus cosθtanϕ right parenthesis

    Question 242
    CBSEENPH11016729

    A body is placed on a rough inclined plane having inclination θ greater than angle of repose ϕ. What is the minimum force required to take the body up on inclined plane?

    Solution

    Given angle of inclination straight theta is greater than angle of repose straight ϕ.
    So, minimum force required to take the body upto inclined plane =  space space mg left parenthesis sinθ plus cosθ space tanϕ right parenthesis

    Question 243
    CBSEENPH11016730

    It is difficult to move a cycle along a road with its brakes on. Why?

    Solution
    When brakes are on, the force of friction between brakes and the rim of cycle wheels is too large to move the cycle. Hence, it is difficult to move a cycle along a road with it's brakes on. 
    Question 244
    CBSEENPH11016731

    Carts with rubber wheels are easier to ply than those with iron wheels. Why?

    Solution
    The force of friction between rubber wheels and the road is much smaller than the force of friction between iron wheels and road. Hence, it is easier to ply cart with rubber wheels. 
    Question 245
    CBSEENPH11016732
    Question 246
    CBSEENPH11016733

    Why we lubricate the machines?

    Solution
    Machines are lubricated to reduce the force of friction.
    Question 247
    CBSEENPH11016734

    Name two methods by which the force of friction can be decreased.

    Solution
    Polishing and streamlining the bodies will  decrease friction.
    Question 248
    CBSEENPH11016735

    What happens to limiting dry friction if the speed of the moving body increases?

    Solution
    With increase in the speed of moving body, limiting dry friction decreases.
    Question 249
    CBSEENPH11016736

    What happens to fluid friction if the speed of the moving body increases?

    Solution
    With increase in speed of the body, fluid friction decreases. 
    Question 250
    CBSEENPH11016737

    Can machines perform when the friction is totally absent?

    Solution

    No, when friction is not present, the machine will not perform. 

    Question 251
    CBSEENPH11016738

    A body of mass m is placed on the rough horizontal surface. What is the force of friction?

    Solution

    The force of friction acting on a body of mass m placed on the rough horizontal surface is zero. 

    Question 252
    CBSEENPH11016739

    Is the force of friction a conservative force?

    Solution
    No. The force of friction is a non-conservative force. The work done on the body depends on the path taken by the body. 
    Question 253
    CBSEENPH11016740

    Why is force of friction a non-conservative force?

    Solution
    The force of friction is a non-conservative force because the work done against friction depends upon the total length of path. 
    Question 254
    CBSEENPH11016741

    What are the factors on which coefficient of friction depends?

    Solution
    The coefficient of friction depends on:
    i) nature of surfaces in contact, and
    ii) nature of motion.
    Question 255
    CBSEENPH11016742

    Why do we slip on wet surface? 

    Solution
    We slip on wet surface because force of friction on wet surface is very low.
    Question 256
    CBSEENPH11016743

    Why are wheels made circular?

    Solution
    Wheel are made circular to convert sliding friction into rolling friction. 
    Question 257
    CBSEENPH11016744
    Question 258
    CBSEENPH11016745
    Question 259
    CBSEENPH11016746

    Arrange dynamic friction, limiting friction and rolling friction in increasing order of their magnitude.

    Solution
    Rolling friction< dynamic friction< limiting friction.
    Question 260
    CBSEENPH11016747

    How is it easier to move the body-by pushing or by pulling?

    Solution
    It is easier to move the body by pulling than by pushing it. The frictional force acting on a body when we are pushing it is more as compared to when we are pulling the body. 
    Question 261
    CBSEENPH11016748

    A body exerts a force 200N normal to the rough surface having coefficient of friction 0.3. What is the value of limiting friction?

    Solution

    Normal force, N = 200 N

    Coefficient of friction, straight mu = 0.3 
    So, 
    Limiting friction, FlμN space equals space 0.3 space cross times space 200 space equals space 60 space straight N

    Question 262
    CBSEENPH11016749

    What is friction and what is the cause of friction?

    Solution

    When a body actually moves or tends to move over the surface of another body, frictional force comes into play as the retarding force.
    The surface of a moving body is not perfectly smooth. even though it appears to be smooth, there are irregularities on the surface.
    When a body is placed on the second body, the irregularities of the two surfaces get interlocked. To break the interlocking, we have to apply the force. Therefore, the cause of friction is interlocking of irregularities. 

    Question 263
    CBSEENPH11016750

    What is the modern theory of friction?

    Solution
    When one body is placed on the surface of another body, the actual area of contact is very small as compared to normally visible area as shown in figure.
                 
    At the points of contact, pressure is very high and adhesion or cohesion forces are very strong. At these points a cold welding will take place. A force is required to break this cold welding at points of contact, when the body is moved over another body. This is the cause of friction.
    This friction increases with the increase in smoothness because area of contact increases as the smoothness of the surface increases. 
    Question 264
    CBSEENPH11016751

    Why is limiting friction greater than dynamic friction?

    Solution

    Limiting friction is the maximum static friction which comes into play when the body is just at the point of motion.
    Dynamic friction is the friction that comes into play when the body is in motion.
    When the body is at rest, the cavities of two bodies are fully interlocked and we require large force to overcome the cavities.
    Once the sliding has begun, the two surfaces have not sufficient time to fully settle down and hence cavities will not fully interlock and we require lesser force to keep the body in motion.
    Therefore, dynamic friction is less than static friction.

    Question 265
    CBSEENPH11016753

    State the laws of limiting friction.

    Solution

    Laws of limiting friction:
    (i) The force of limiting friction depends upon the nature of surfaces in contact and-acts tangentially to the interface between the two surfaces.
    (ii) The force of limiting friction between two surfaces in contact is independent of the area of contact.
    (iii) Limiting friction is directly proportional to the magnitude of normal reaction N between two surfaces in contact,
    i.e.
                               F proportional to N 
    rightwards double arrow                    space space F equals mu N 
    where n is constant of proportionality and known as coefficient of friction.
    (iv)The direction of force of friction is always opposite to the direction in which the body moves or tends to move. 

    Question 266
    CBSEENPH11016754

    Define rolling friction. What is the cause of rolling friction?

    Solution
    Roiling friction is the friction which comes into play between two surfaces, when one body rolls over the surface of other the body. 
                     

    (i) When a wheel or rolling object is placed on the surface, it depresses the surface and causes a slight elevation in the surrounding of depression as shown in the fig. above.
    Therefore, body has to continuously climb up the hill and force is required to do so.
    (ii) There are adhesion forces between rolling body and the surface. So, the rolling body has to detach itself from the surface on which it rolls and the force is required to detach the body. 

    Question 267
    CBSEENPH11016755

    Why sliding friction is greater than rolling friction?

    Solution
    In sliding motion, the heights of cavities on the surface are to climb up which introduces the friction. While in rolling motion the heights of cavities are to cross over which avoids the friction. 
    Question 268
    CBSEENPH11016758

    Define angle of friction and show that tan θ =μ, where θ is angle of friction and μ is coefficient of friction.

    Solution

    Angle of friction is the angle which the resultant of force of friction and normal reaction make with the normal reaction.

    Let a body of weight 'W' be placed on the horizontal surface and pulled by force F. 
    Weight of the body is acting in vertically downward direction and there is no component of force F in vertical direction, therefore, normal reaction will be equal to weight.
    Let μ be coefficient of friction.
    Therefore, force of friction f = μW, will act in horizontal direction opposite to the direction of F applied.
    Let the resultant of space space f with rightwards arrow on top space and space N with rightwards arrow on top make an angle straight theta with <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    ∴                        tan theta equals f over N equals fraction numerator mu W over denominator W end fraction 
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#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    Hence, proved. 

    Question 269
    CBSEENPH11016760

    Define angle of repose and find the relation between angle of repose and angle of friction.

    Solution

    Angle of repose is the angle that an inclined plane makes with the horizontal when a body placed on it just starts sliding down.
    Let a body of weight W be placed on rough inclined plane and the body just starts sliding when angle of inclination is ϕ.

    W can be resolved into two components:
    (i) W sinϕ along the plane.
    (ii) W cosϕ perpendicular to the plane.
    When body is just at the point of sliding, then 
                    Wsinϕ equals straight f equals μWcosϕ 
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#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>                      ...(1) 
    We know the relation between angle of friction<pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
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#6 {main}</pre> is,
                             tanθ space equals straight mu                ...(2) 
    So, from (1) and (2), 
                             tan space straight theta space equals space tan space straight ϕ 
    rightwards double arrow                           straight theta equals straight ϕ

    Question 270
    CBSEENPH11016761

    Why are the wheels of automobiles made circular?

    Solution
    Wheels are made circular so as to change the sliding friction into rolling friction. Rolling friction is smaller as compared to sliding friction.
    Question 271
    CBSEENPH11016762

    Why do we easily slip on a rainy day?

    Solution
    Due to water between our feet and ground,  so on a rainy day, the dry friction is converted into fluid friction. 
    Therefore, friction between our feet and ground is very much reduced. That is why we slip easily on a rainy day. 
    Question 272
    CBSEENPH11016763

    Sand is thrown on tracks covered with snow. Why?

    Solution
    When tracks are covered with snow, they have negligible friction and it is impossible to walk or drive the vehicles. When sand is thrown on tracks, force of friction is increased. 
    Question 273
    CBSEENPH11016764

    Proper inflation of tyres of vehicles saves fuel. Why?

    Solution
    When the tyres are not properly inflated, the tyres get flattened at these points the area of contact increases. Hence, the rolling friction increases due to increase in the area of contact.
    When tyres are properly inflated, the area of contact decreases and, therefore, force of rolling friction decreases. 
    Question 274
    CBSEENPH11016765

    Why automobile tyres have generally irregular projections over their surface?

    Solution
    To avoid the slipping of automobiles there should be some minimum friction between the tyres and road. Therefore, to increase the friction, irregular projections are made on the surface of tyres. 
    Question 275
    CBSEENPH11016766

    Why are lubricants used in machines?

    Solution
    Lubricants are used in machines because they  fill the depressions present in the surface of contact and also change the sliding friction into fluid friction. Hence the friction is reduced. 
    Question 276
    CBSEENPH11016767

    Is large brake on a bicycle wheel more effective than a small one? Explain.

    Solution
    Force of friction is independent of the area of contact. Therefore, large brakes or small brakes have the same effect.
    Question 277
    CBSEENPH11016768

    Carts with rubber tyres are easy to ply than those with iron tyres. Why?

    Solution
    The coefficient of friction between rubber and road is smaller than that between iron and road. Hence it is easy to ply with rubber tyres than iron tyres.
    Question 278
    CBSEENPH11016770

    What are the various methods by which force of friction can be reduced?

    Solution
    Force of friction can be reduced by:
    i) polishing,
    ii) lubricating, streamlining and,
    iii) Using ball bearings, we can decrease the friction.
    Question 279
    CBSEENPH11016771

    What is the unit of coefficient of friction?

    Solution
    It has no unit.
    Question 280
    CBSEENPH11016772

    What is the role of balls in the ball bearing?

    Solution
    Balls in the ball bearing convert sliding friction into rolling friction. The value of rolling friction is less than that of sliding friction. 
    Question 281
    CBSEENPH11016773

    Why friction is an evil?

    Solution
    A lot of energy is wasted in overcoming friction in various parts of machines which decrease the efficiency of machines. Energy used in overcoming the friction is converted into heat which increases the wear and tear of machine. Heat so produced may also damage the machine parts. Hence, friction has been called an evil. 
    Question 282
    CBSEENPH11016774

    What is the necessity of friction?

    Solution

    Friction is necessary because it is helpful to us in many ways. For example, 
    (i) Without friction we cannot write.
    (ii) Speed of vehicles can be controlled only due to friction.
    (iii) We cannot rotate the pulley without friction between belt and pulley.
    (iv) It is impossible to climb up with rope without friction between hand and rope. Frictional force helps the person to move forward. 
    (v) Without friction we even cannot walk.

    Question 283
    CBSEENPH11016776

    Why it is easier to pull a lawn mower than to push it?

    Solution

    When we push or pull the lawn mower with force F at angle θ with horizontal, the force of push or pull can be resolved into two rectangular components: 
    (i) F cosθ, the horizontal component which helps it to move in forward direction.
    (ii) F sinθ, the vertical component of force.
     
    In case of pull, Fsinθ acts in vertically upward direction which decreases the normal reaction and hence force of friction decreases.

    Since in case of pull, force of friction is less than that in the case of push. Hence it is easy to pull than to push.


    Question 284
    CBSEENPH11016777

    A block of mass 4 kg lies on a rough horizontal surface. The coefficient of friction between the block and surface is 0.6. What is the minimum horizontal force required to set the block into motion?

    Solution

    Given,

    Mass of the block, m = 4kg
    Coefficient of friction,space space straight mu equals 0.6
    ∴   Force of limiting friction is, 
              straight F subscript straight e space equals space μmg space equals space 0.6 cross times 4 cross times 9.8
                  = 23.52 N
    The minimum force required to set the body into motion is equal to limiting friction. Thus, force required to set the body into motion is 23.52N.  

    Question 285
    CBSEENPH11016778

    A wagon of mass 2000kg is separated from a shunting engine moving at 54km/h. It comes to rest in 12 sec. Find the value of coefficient of friction.

    Solution

    Given,
    Initial velocity, u = 54km/h = 15 m/s
    Final velocity, v = 0
    The wagon comes to rest in, t = 12s
    Mass of the wagon, m = 2000 kg
    The retarding force on the wagon is,
           space space straight F equals fraction numerator straight m left parenthesis straight v minus straight u right parenthesis over denominator straight t end fraction equals fraction numerator 2000 left parenthesis 0 minus 15 right parenthesis over denominator 12 end fraction
              = -2500N
    Therefore coefficient of friction is,
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#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
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#6 {main}</pre>

    Question 286
    CBSEENPH11016780

    When a force of 29.4N applied on body of mass 3 square root of 3 kg comma it just starts moving. Find the angle of friction.

    Solution

    Givem, 
    Effective space force comma space straight F subscript straight e equals 29.4 straight N

Mass space of space the space body comma space straight m equals 3 square root of 3 kg
    Angle of friction, tanθ equals straight F subscript straight e over mg equals fraction numerator 29.4 over denominator 3 square root of 3 cross times 9.8 end fraction equals fraction numerator 1 over denominator square root of 3 end fraction
                                  straight theta equals 30 degree

    Question 287
    CBSEENPH11016782

    A block of mass 4 kg is placed on a rough horizontal surface. When an impulse of 12Ns is given to the body it comes to rest by sliding a distance of 1-5m. Find the coefficient of friction. Take g = 10m/s2v.

    Solution
    Let straight mu be the coefficient of friction between block and horizontal surface.
    The force of friction between block and horizontal surface is, 
                        straight F equals μmg
    The retardation of block is, 
               straight r equals straight F over straight m equals μmg over straight m equals μg
    Mass of block, m = 4 kg
    Impulse given to block, I = 12Ns 
    Initial velocity of block, u = I/m = 3m/s
    Here,
     u = 3 m/s,  v = 0
    Acceleration, straight a equals negative μg comma       
    Distance travelled, s = 1.5m 
    Now, using equation of motion, 
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#6 {main}</pre>
    rightwards double arrow         straight v squared minus straight u squared equals negative 2 μsg
    rightwards double arrow            0 minus 9 equals negative 2 straight mu cross times 10 cross times 1.5
    i.e.,             space space space space straight mu equals 0.3

                         
    Question 288
    CBSEENPH11016783

    The masses shown in the figure accelerate with acceleration 2m/s . If the tension in the coupling is 8N, find the force of friction on each block.



    Solution

    Given,
    Mass,space space straight m subscript 1 equals 2 kg comma     
    Mass, straight m subscript 2 equals 3 kg 
    Force acting on the block, F = 24N
    Tension in the block, T = 8N
    Acceleration, <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>
    Let straight f subscript 1 space and space straight f subscript 2 be the force of frictions on masses space straight m subscript 1 space and space straight m subscript 2 respectively.
    Therefore by using free body diagram,
                          straight F minus straight T minus straight f subscript 1 equals straight m subscript 2 straight a
    and                      straight T minus straight f subscript 2 equals straight m subscript 1 straight a
    Substituting the given values,
                         24 minus 8 minus straight f subscript 1 equals 3 cross times 2
    and                        8 minus straight f subscript 2 equals 2 cross times 2
    On solving we get,
                      straight f subscript 1 equals 10 straight N  and  <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>
    f1 and f2 are the required force of frcition. 
                

    Question 289
    CBSEENPH11016785

    A body of mass m is placed on a rough inclined plane of inclination θ and coefficient of friction μ, such that tan θ > μ. Find the acceleration of body down the inclined plane.

    Solution

    Let a body of mass 'm' be placed on the inclined plane of inclination θ as shown in the figure. Since tan θ > μ.

    Therefore,
    Body will accelerate in downward direction and is in equilibrium perpendicular to inclined plane.
    Now,
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#6 {main}</pre>
    Net force acting on the body in downward direction along the plane is, 
    straight F equals mg space sinθ minus straight mu space mg space cosθ
    Acceleration of body along inclined plane is,
    straight a equals straight F over straight M equals straight g left parenthesis sinθ minus μcosθ right parenthesis

    Question 290
    CBSEENPH11016786

    A body of mass m is placed on a rough inclined plane of inclination θ and coefficient of friction μ, such that tanθ < μ, then what is the maximum and minimum force required to just slide the body along the inclined plane?

    Solution

    If tanθ < μ, then limiting friction μgcos θ will be greater than mgsinθ, the force that tends to slide the body down.
    Thus, the body will not slide down. That is, the force has to be applied to slide the body down or up the inclined plane. 
    Case I:
    Force should be applied parallel to the plane in downward direction, to slide the body down the inclined plane. 
             
    Here, force of friction is acting upwards, therefore the force required to just slide the body down the inclined plane is, 
                         f = mg sinθ

    Case II:
    Force should be applied parallel to the plane in the upward direction in order to slide the body up the inclined plane.

    In this case, force of friction is acting downwards. Therefore, the force required to slide the body up the inclined plane is,
    F' = f + mg sin θ
    i.e., F' = straight mu space mg space cos space straight theta space plus space mg space sin space straight theta space equals space mg space left parenthesis straight mu space cos space straight theta space plus space sin space straight theta right parenthesis 

    Question 291
    CBSEENPH11016788

    A block of mass m is placed on a rough inclined plane. When the plane is gradually inclined and at angle of inclination α, the block just starts moving. The angle of inclination is further increased and fixed at angle θ. Find the acceleration of the block.

    Solution
    The block just starts sliding at angle of inclination 'a'.
    Thus, a is the angle of repose and coefficient of friction is tan a.
    When angle of inclination is increased to angle θ, then the body starts sliding with acceleration a along the inclined plane. 

    Now,
                         R = mg space cosθ 
    and             ma equals mgsinθ minus μR
    rightwards double arrow              ma equals mg space sinθ space minus space straight mu space mg space cosθ
    rightwards double arrow               space space straight a equals straight g left parenthesis sinθ minus μcosθ right parenthesis
    Question 292
    CBSEENPH11016791

    A man is standing stationary on a horizontal conveyor belt which is accelerating with 1 m/s2. What is the pseudo force on the man? If the coefficient of static friction between the man’s shoes and the belt is 0.2, then what is the maximum acceleration of the belt so that the man can continue to be stationary on the belt? (Mass of the man = 65 kg.)

    Solution
    The acceleration of the belt results in pseudo force acting on the man and is equal to mass times the acceleration of the belt opposite to the direction of belt.
    i.e.           F = ma =65 x 1 = 65N
    The man will continue to be stationary on the belt till the pseudo force on the man is less or equal to limiting friction.
    i.e.            space space space space F less-than or slanted equal to f subscript 1
    rightwards double arrow              <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    rightwards double arrow                  straight a less-than or slanted equal to μg
    Here straight mu equals 0.2,
    Therefore, 
                       straight a less-than or slanted equal to 0.2 cross times 9.8 equals 1.96 straight m divided by straight s squared
    Hence,  the maximum acceleration with which the belt can accelerate is <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    Question 293
    CBSEENPH11016793

    A body of mass m is placed on rough horizontal surface and pulled with force F at an angle straight theta as shown. Find the acceleration with which the body will accelerate.

    Let coefficient of friction be μ.

    Solution
    Force is acting on the body as shown in the fig. below.
    Now, breaking the rectangular component of F along horizontal and vertical direction.
    (i) F cosθ in horizontal direction. (ii) F sinθ in vertically upward direction.
                 
    Weight of body mg acts in vertically downward direction.
    Since the body is in equilibrium in vertical direction, 
    ∴              straight F space sinθ space plus space straight R space equals space mg 
    rightwards double arrow                        straight R space equals space mg minus Fsinθ 
    The force of limiting friction is given by, 
                      <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    If straight F space cosθ space less than space straight f then acceleration will be zero, and
    if<pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> then body will accelerate and is given by
                       space space straight a equals fraction numerator Fcosθ minus straight f over denominator straight m end fraction
                           equals fraction numerator Fcosθ space minus straight mu left parenthesis mg minus Fsinθ right parenthesis over denominator straight m end fraction
                           equals fraction numerator straight F left parenthesis cosθ plus μsinθ right parenthesis minus μmg over denominator straight m end fraction
                         space space space space equals fraction numerator straight F left parenthesis cosθ plus μsinθ right parenthesis over denominator straight m end fraction minus μg
    So, 'a' is the acceleration with which the body will accelerate. 
    Question 294
    CBSEENPH11016795

    A 70 kg man stands in contact against the inner wall of a hollow cylindrical drum of radius 3 m rotating about its vertical axis. The coefficient of friction between the wall and his clothing is 0.15. What is the minimum rotational speed of the cylinder to enable the man to remain stuck to the wall (without falling) when the floor is suddenly removed?

    Solution
    The centrifugal force experienced by the man, when the drum rotates =  mrω2 , in outward direction which is normal to the wall of cylinder. 
    When the floor is removed, due to the weight of the man he has tendency to fall from the drum.
    But, the man will not fall down as, 
                     μN = μmrω2, opposes it.
    The man remains stuck to the wall without falling when the floor is suddenly removed if the force of friction is greater or equal to the weight of the man.
    i.e.                  <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    rightwards double arrow                   <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    i.e.,                 space space straight omega equals 4.67 space rad divided by straight s, is the minimum rotational speed. 
    Question 295
    CBSEENPH11016797

    A body of mass m is placed on the rough inclined plane of inclination θ. Coefficient of friction of surface is μ. What is the work done to take the body up the inclined plane against friction by distance S?

    Solution
    Let a body of mass m be placed on a rough inclined plane and displaced from A to B by a distance S, as shown in the fig. 


    Now, breaking force into it's rectangular components, 
    (i) mg sinθ is the component down the inclined plane.

    (ii) mgcosθ is the component perpendicular to plane.
    Since body is in equilibrium perpendicular to the plane,
                         straight R equals mg space cosθ
    Since the body is displaced up the inclined plane, therefore force of friction will be down the inclined plane.
    Force of friction, straight f equals μR equals straight mu space mg space cosθ
    ∴      Work done against friction is,
                  straight W equals straight f with rightwards arrow on top. straight S with rightwards arrow on top space equals space μmg space cosθ

    Question 296
    CBSEENPH11016798

    A block of mass 2 kg rests on a rough inclined plane making an angle 30° with horizontal. The coefficient of friction between the block and the inclined plane is 0.65. Find the force of friction on the block.

    Solution

    Given,
    MAss of the block, m = 2kg
    Angle of inclination between the block and horizontal line is, straight theta equals 30 degree 
    Coefficient of friction, straight mu equals 0.65
    The force that tends to slide the body on the inclined plane is,
               straight F equals mg space sinθ
                <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    The limiting friction between the block and the inclined plane is,
           f subscript e space equals space mu space m g space cos theta
                <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    Here, we can see that the limiting friction is greater than the force that tends to slide the body.
    Therefore the body will be a rest and force of friction on the block is 9.8N

    Question 297
    CBSEENPH11016800

    A block of mass 5 kg is placed on an inclined plane making an angle of 37° with the horizontal. The block is given a velocity of 10m/s up the slope. The coefficient of friction between block and the inclined plane is 0.5. Find how far the block goes up the slope. Take g = 10m/s2.

    Solution
    Given, 
    Mass of the block, m = 5 kg
    Angle of inclination with the horizontal, straight theta = 370
    Velocity with which the block is moving, v = 10 m/s
    Coefficient of friction = 0.5


    See the fig. given above,
    Component of weight acting parallel and down the inclined plane is,
              <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    Component of weight normal to inclined plane is,
             straight N equals mg space cosθ space equals space 5 cross times 10 cross times cos 37 space equals space 40 straight N 
    The force of friction between block and the inclined plane is, 
                f equals mu N space equals space 0.5 cross times 40 space equals space 20 straight N 
    Since the body is projected up the inclined plane, therefore the force of friction will act on the block down the inclined plane.
    Thus the total force acting on the block down the inclined plane is,
                  <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    The retardation of block is,
                        a italic equals F italic divided by m italic equals italic 10 m italic divided by s to the power of italic 2
    Now,              straight v equals negative 10 straight m divided by straight s  and
                          straight a equals negative 10 straight m divided by straight s squared 
    Therefore, the distance by which the block goes up the inclined plane is, 
                 straight s equals fraction numerator straight v squared minus straight u squared over denominator 2 straight a end fraction equals fraction numerator 0 minus 100 over denominator negative 2 cross times 10 end fraction equals 5 straight m
     
    Question 298
    CBSEENPH11016804

    An insect is crawling up a fixed hemisphere bowl of radius 40cm. How high it can crawl up? Coefficient of friction is 0.75.

    Solution
    Let h be the maximum height to which the insect can crawl up the hemisphere.
    Radius of the hemisphere, r = 40 cm
    Coefficient of friction, straight mu space equals space 0.75
    The insect can crawl up the bowl to a height where the tangential component of weight is just equal to limiting friction.

    Thus,
                  <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    rightwards double arrow                      straight mu space equals space tanθ
    i.e.,                 cosθ space equals space fraction numerator 1 over denominator square root of straight mu squared plus 1 end root end fraction
    Putting the value of straight mu in the above equation, 
    ∴           <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    Also from figure,
                  cosθ equals fraction numerator straight R minus straight h over denominator straight h end fraction
    rightwards double arrow             space space straight h equals straight R left parenthesis 1 minus cosθ right parenthesis 
                        <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    Question 299
    CBSEENPH11016805

    The force required to just move the body up the inclined plane is twice the force required to just prevent it from sliding down the inclined plane. Find the coefficient of friction.

    Solution
    The force of friction acts along the slope down the inclined plane, when the body moves up the inclined plane.

    Let Fl be the force required to move the body up the inclined plane. 
    straight F subscript 1 equals mg space sinθ space plus space straight mu space mg space cosθ 
    Now, let F2 be the force required to just prevent the body sliding down.
    When the body is just at the point of sliding down, the force of friction acts up the inclined plane.

    Thus
                   space space space space space straight F subscript 2 equals mg space sinθ minus straight mu space mg space cosθ
    Since,
                           space space space straight F subscript 1 equals 2 straight F subscript 2    [ Given] 
    Therefore,
         mg space sinθ space plus space straight mu space mg space cosθ space equals space 2 left parenthesis mgsinθ minus straight mu space mg space cosθ right parenthesis 
    rightwards double arrow              3 straight mu space mg space cosθ space equals space mg space sinθ
    rightwards double arrow                           3 straight mu equals tanθ
    rightwards double arrow                         space space space space space straight theta equals tan to the power of negative 1 end exponent left parenthesis 3 straight mu right parenthesis
    So, coefficient of friction is given by,
                        straight mu space equals space 1 third space tan space straight theta
    Question 300
    CBSEENPH11016806

    Two blocks of masses m1 = 3kg and m2 = 5kg are connected by a light string passing over a smooth light pulley fixed at the end of a horizontal rough table. The mass m1 is hanging and mass m2 lies on the horizontal surface. Find acceleration of the system and tension in the string. The coefficient of friction between block and the surface of table as 0.4.

    Solution
    Let a be acceleration of system and T be the tension in the string.

    Equation of motion of mass m1 is, 
                <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>                       ...(1) 
    Equation of motion of mass straight m subscript 2 is,
                 straight T minus 0.4 cross times 5 cross times straight g equals 5 straight a
    rightwards double arrow             straight T minus 2 straight g equals 5 straight a                      ...(2)
    Adding equation (1) and (2), we have 
                          g = 8a 
    rightwards double arrow                   <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    Substituting 'a' in equation (2), we have
                  space space straight T minus 20 equals 5 cross times 1.25 
                           <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    T is the required tension in the string. 
    Question 301
    CBSEENPH11016807

    A block slides down a rough inclined plane (making an angle 45° with the horizontal) in the time twice the time it takes in a smooth inclined plane of same inclination. Find the coefficient of friction of rough inclined plane.

    Solution
    Acceleration of block on smooth inclined plane is,space straight a subscript 1 equals gsinθ 
    The acceleration of block on rough inclined plane is,
    straight a subscript 2 equals straight g left parenthesis sinθ minus μcosθ right parenthesis
    where, 
    p is the coefficient of friction of rough inclined plane.
    Let, t1 and t2 be the time taken by the block to slide down the smooth inclined plane and rough inclined plane respectively.
    If s is the distance moved by the block on the inclined plane then, 
                    <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    rightwards double arrow        <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    Here       <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 

    ∴        <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    rightwards double arrow                 1 minus straight mu equals 1 fourth 
    rightwards double arrow                       straight mu equals 0.75, is the coefficient of friction of rough inclined plane. 
    Question 302
    CBSEENPH11016808

    Two bodies A and B are released such that A slides down along the inclined plane and B falls freely as shown in the figure. If time taken by A to reach the ground is twice the time taken by B to fall freely then show that inclined plane is frictionless.

    Solution
    Let h be the height, Error converting from MathML to accessible text. be the length of the inclined plane and μ be the coefficient of friction between the block and inclined plane.
    Therefore, 
    Error converting from MathML to accessible text. 
    If t is the time taken by block B to reach the ground, then equation of motion of B is, 
    space space space straight h equals 1 half gt squared                         ...(1)
    Since, block A slides down the rough inclined plane, therefore acceleration of block A is, 
    a = g ( sin 30ostraight mu space cos space 30 to the power of straight o right parenthesis 
    straight g over 2 left parenthesis 1 space minus space square root of 3 space mu right parenthesis space
    Here, A takes twice the time taken by B. 
    Therefore, equation of motion of block A is, 
    Error converting from MathML to accessible text.
    That is, the inclined plane is frictionless. 
    Question 303
    CBSEENPH11016809

    A chain of length L has some of its portion hanging vertically over the edge of the rough horizontal table of coefficient of friction μ. Find the greatest length of the chain that can hang without slipping.

    Solution
    Let x be the maximum length of the chain that can hang without slipping.
                        
    Therefore weight of hanging part of the chain is, straight W subscript 1 space equals space λxg
    where,
    A, is the linear mass density of the chain.
    Weight of the chain on the table is, straight W subscript 2 space equals space straight lambda left parenthesis straight L space minus space straight x right parenthesis space straight g
    The chain is just at the point of slipping when,
    Error converting from MathML to accessible text.

                  
    Question 304
    CBSEENPH11016810

    A car starts from rest and accelerates uniformly with 2 m/s2. At t = 5s, a stone is dropped out of the window 15m high of the car. What is velocity and acceleration of stone at t = 5.5 s? (g = 10m/s2)

    Solution
    Given the car starts from rest.
    That is initial velocity, u = 0 m/s
    Acceleration of the car = 2 m/s2.
    Therefore,
    Velocity of car at t = 5s is 10 m/s.
    When the stone is dropped, the velocity of the car and hence velocity of stone is 10 m/s in the horizontal direction.
    At t= 5s the stone loses the contact from the car. Therefore, the stone will move uniformly in the horizontal direction and accelerate in vertically downward direction due to gravity.

    After 0.5 seconds, the stone is dropped. 
    On resolving velocity, we get X and Y as it's components. 
    Vx = vx + axt = 10 + 0 X 0.5 = 10 m/s
    Vy = vy + ayt0 + 10 X 0.5 = 5 m/s
    Acceleration of stone at t= 5 sec is the only acceleration due to gravity. 
    That is, a = 10 m/s2
    Question 305
    CBSEENPH11016811

    A man weighs 70 kg. He stands on a weighing scale in a lift which is moving:
    (a) upwards with uniform speed of 10m/s,
    (b) downwards with uniform acceleration of 5 m/s2 and
    (c) upwards with uniform acceleration of 5 m/s2.
         What would be the reading on the scale in each case?
    (d) What would be the reading, if the lift mechanism failed and it hurtled down freely under gravity?

    Solution
    We know the apparent weight of body in the lift is given by,
                          W = m(g+a)
    where the positive sign stands for lift acceleration in vertically upward direction and the negative sign stands, if lift accelerates in vertically downward direction.
    (a) Since lift moves with uniform velocity,
    therefore a = 0
    ∴            W = 70(9.8+0) = 686N
    (b) Since lift accelerates in downward direction with acceleration 5 space straight m divided by straight s squared
    ∴            W = 70 (9.8 - 5) = 336 N
    (c) Since  lift accelerates in vertically upward direction with acceleration 5 space straight m divided by straight s squared
    ∴             W = 70 (9.8 - 5) = 1036 N
    When lift falls freely, the acceleration of lift in verticlly downward direction is <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>
    ∴             W = 70(9.8 - 9.8) = 0 N
    Question 306
    CBSEENPH11016812

    What is angular displacement?

    Solution
    Angular displacement is the angle by which the radius vector revolves. 
    Any rotating body goes through an angular displacement. 
    Question 307
    CBSEENPH11016813

    What are the units of angular displacement?

    Solution
    Angular displacement is measured in radians. 
    2 straight pi space radian space equals 360 degree
    Question 308
    CBSEENPH11016814

    What is the relation between revolution and radian?

    Solution
    1 revolution = 2π radian
    Question 309
    CBSEENPH11016815

    Define angular velocity.

    Solution
    The rate of change of angular position of a rotating body is called angular velocity.
    Question 310
    CBSEENPH11016816

    Define uniform angular velocity.

    Solution
    When a body undergoes equal angular displacement in equal interval of time, the particle is said to be moving with constant angular velocity. 
    Question 311
    CBSEENPH11016817

    When a particle is said to be revolving with variable angular velocity?

    Solution
    When a body undergoes unequal angular displacement in equal interval of time, it is said to be moving with a variable angular velocity. 
    Question 312
    CBSEENPH11016818

    Define average angular velocity.

    Solution
    When the particle undergoes same angular displacement in same time as it is undergone by variable velocity, then the particle has a constant average velocity. 
    Question 313
    CBSEENPH11016819

    What are the units of angular velocity?

    Solution
    Unit of angular velocity,
    Radian/s and rps (revolution per second).
    Question 314
    CBSEENPH11016821

    What are the units of angular velocity?

    Solution
    Radian/s and rps (revolution per second).
    Question 315
    CBSEENPH11016822

    What is rpm?

    Solution
    Rpm means revolution per minute and it is the unit of angular velocity. 
    Question 316
    CBSEENPH11016823
    Question 317
    CBSEENPH11016824

    Define angular acceleration.

    Solution
    Angular acceleration is the rate at which the angular velocity changes. 
    Question 318
    CBSEENPH11016825

    What are the units of angular acceleration?

    Solution
     Rad/s2 and rps2 is the unit of angular acceleration. 
    Question 319
    CBSEENPH11016826
    Question 320
    CBSEENPH11016827

    What is the angular velocity of second hand of a watch, in rpm?

    Solution
    The angular velocity of second hand of a watch is 1 rpm.
    Question 321
    CBSEENPH11016828

    Which is greater - angular velocity of the earth about its own axis or angular velocity of hour hand of a watch?

    Solution
    Angular velocity of hour hand of watch is greater than the angular veocity of the earth about its own axis.
    Question 322
    CBSEENPH11016829

    Are angular velocity and angular acceleration vector quantities?

    Solution
    Yes. Both angular velocity and angular acceleration are vector quantities and directed along the axis of rotation. 
    Question 323
    CBSEENPH11016830
    Question 324
    CBSEENPH11016831

    What is the distance travelled by particle when it completes one revolution in circular orbit of radius r? Also find the displacement.

    Solution
    The distance travelled by the particle is 2 πr and displacement is zero. 
    Question 325
    CBSEENPH11016832

    What is the relation between instantaneous speed and angular speed of a particle revolving in a circular track of radius r?

    Solution

    Relation between instantaneous speed and angular speed of a particle, 
                               straight nu equals rω

    Question 326
    CBSEENPH11016833

    What is the relation between instantaneous linear velocity and angular velocity?

    Solution

    Relation between instantaneous linear velocity and angular velocity is, 
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#6 {main}</pre>

    Question 327
    CBSEENPH11016834

    What is the relation between instantaneous tangential acceleration and angular acceleration?

    Solution

    Relation between instantaneous tangential acceleration and angular acceleration is, 
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#6 {main}</pre>

    Question 328
    CBSEENPH11016835
    Question 329
    CBSEENPH11016836
    Question 330
    CBSEENPH11016837

    Is there any force required to revolve the body in a circular orbit with constant angular velocity?

    Solution
    Yes, the force that is required to revolve the body in a circular orbit with constant angular velocity is centripetal force. 
    Question 331
    CBSEENPH11016838

    Define centripetal force.

    Solution
    Centripetal force is the force which is essential to keep the body moving in a circular path. This force is directed along radius towards the centre of the circle. 
    Question 332
    CBSEENPH11016839

    How much force is required to revolve the body of mass m in circular orbit of radius r with speed space nu?

    Solution

    The force required to revolve the body of mass 'm' in circular orbit of radius 'r' with speed 'v' 
                                     fraction numerator m nu squared over denominator r end fraction

    Question 333
    CBSEENPH11016840

    What is the direction of centripetal acceleration possessed by a body revolving in circular orbit? 

    Solution
    The centripetal acceleration possessed by a revolving body is directed towards the centre of circle. 
    Question 334
    CBSEENPH11016841

    Is the magnitude of centripetal acceleration always constant?

    Solution
    No.
    When the particle revolves with uniform angular velocity, the magnitude of centripetal acceleration is constant. 
    Question 335
    CBSEENPH11016842

    What is centrifuge?

    Solution
    Centrifuge is a device used to separate the contents of different densities in a mixture.
    Question 336
    CBSEENPH11016843

    If both the speed of a body and radius of its circular path are doubled, what will happen to the centripetal force?

    Solution
    Centripetal force = mv squared over straight r 
    The centripetal force gets doubled, when the speed of the body and radius of it's circular path gets doubled. 

    Question 337
    CBSEENPH11016844

    Is circular motion possible at constant speed?

    Solution
    Yes, when the particle revolves with  constant angular velocity in a circular orbit, circular motion is possible at constant speed. 
    Question 338
    CBSEENPH11016845

    Why does a cyclist lean to one side while going along a curve? In which direction does he lean?

    Solution
    In order to provide the necessary centripetal force, the cyclist leans towards inward direction of circular path or curved path. 
    Question 339
    CBSEENPH11016846

    Does the angle at which the cyclist leans towards inner edge depend on his mass?

    Solution
    No, the angle of bending is independent of mass of the cyclist.
    Question 340
    CBSEENPH11016847

    What furnishes the centripetal force for the earth to revolve around the sun?

    Solution
    The necessary centripetal force for the earth to revolve around the sun is provided by the gravitational force between the sun and the earth.
    Question 341
    CBSEENPH11016848

    What furnishes the centripetal force for electron to revolve round the nucleus?

    Solution
    Electrostatic force of attraction between electron and nucleus provides the necessary centripetal force to electron to revolve round the nucleus.
    Question 342
    CBSEENPH11016849

    What is the work done by centripetal force?

    Solution

    the work done by the centripetal force is zero because the initial and the final points are the same. 

    Question 343
    CBSEENPH11016850

    At what point on the earth, the centripetal force is maximum?

    Solution
    Centripetal force is maximum, at the equator. 
    Question 344
    CBSEENPH11016851

    What is the application of centrifugal force?

    Solution
    Applications of centrifugal force in daily life are: 
    1. We use washing machines daily. There wet cloths are moving circular path and force act on water particle in cloths and this force pull water to outer side.This water is removed from cloths and cloths are dried in this machine. Basic principle is centrifugal force acting on water.
    2. Designing of centrifuges,  and 
    3. Buses moving in a circular path. 

    Question 345
    CBSEENPH11016852
    Question 346
    CBSEENPH11016853

    Is it possible for a body to negotiate a curve on a horizontal road?

    Solution
    No, it is not possible for a body to negotiate curve on a horizontal road. 
    Question 347
    CBSEENPH11016854

    What will be the work done by centripetal force when a body completes half a revolution?

    Solution
    When the body completes half a revolution, the work done will be zero. 
    Question 348
    CBSEENPH11016855

    What is banked road?

    Solution
    The road, whose outer edge is raised above the inner edge, such that the road is sloped perpendicular to the length of road, is called a  banked road.
    Question 349
    CBSEENPH11016856

    What is the optimum speed on banked road?

    Solution

    The optimum speed = square root of rgtanθ
    where,
    r is radius of track and 
    straight theta is the angle of banking.

    Question 350
    CBSEENPH11016858

    What is the optimum speed at flat rough road?

    Solution

    The optimum speed = <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre>
    where,
    r is radius of track and 
    space straight mu is coefficient of friction.

    Question 351
    CBSEENPH11016859

    What provides the centripetal force to a car taking turn on a level road?

    Solution
    The centripetal force is provided by the force of friction between the tyres and road, to a car taking turn on a level road.  
    Question 352
    CBSEENPH11016860

    A bob is suspended from a string of length ℓ What minimum speed must be given to the bob at lowest point so that bob may complete the loop in vertical plane?

    Solution

    Length of the string = l 
    Minimum speed given to the bob=space space space space space square root of 5 g ell end root 
    With this speed, the bob may complete the loop in the vertical plane. 

    Question 353
    CBSEENPH11016861

    Does the kinetic energy of body remain conserved while it is going in a vertical circle?

    Solution
    No, the kinetic energy of the body does not remain conserved. When the mass moves downward, gravitaonal potential energy is converted into kinetic energy. 
    Question 354
    CBSEENPH11016862

    When is the tension maximum in a thread while the bob is going in a vertical circle?

    Solution
    When the bob is at the lowest point, the tension in the thread is maximum. 
    Question 355
    CBSEENPH11016863

    An automobile overturns when going too fast around a curve. Which wheels leave the ground first?

    Solution
    The inner wheels leave the ground first, when an automobile overturns when going too fast around a curve. 
    Question 357
    CBSEENPH11016871

    Define angular displacement displacement, angular velocity and angular acceleration.

    Solution

    Angular displacement is defined as the angle described by particle about the axis of rotation. It is represented by straight theta.
    Angular velocity is defined as the rate of change of angular position.
    It is represented by straight omega comma
    Mathematically it is given by, straight omega equals dθ over dt
    Angular acceleration is the rate of change of angular velocity.
    It is represented by a.
    Mathematically it is, straight alpha equals dω over dt

    Question 358
    CBSEENPH11016873

    A body is moving with uniform angular velocity. Is it moving with uniform velocity also?

    Solution
    Velocity is a vector quantity and it will change if the direction of motion changes, even if the speed is constant.
    When the body moves with uniform angular velocity, the direction of motion continuously changes. Therefore, the body is not moving with a uniform velocity. 
    Question 359
    CBSEENPH11016877

    Is circular motion possible at constant velocity? Explain. 

    Solution
    The direction of velocity at any instant is given by the tangent to the path followed by particle. 

                               
    Therefore, even if the magnitude of velocity is constant, the direction is continuously changing. Therefore, circular motion at constant velocity is not possible. Velocity being a vector quantity is dependent on direction of motion. 
    Question 360
    CBSEENPH11016879

    Which is greater - angular velocity of the earth around its own axis or angular velocity of the earth around the sun?

    Solution
    Angular velocity of a body is given by, 
                             straight omega equals 2 straight pi divided by straight capital tau
    where T is the time period of revolution.
    Time period for earth to complete one rotation about its own axis is 24 hours (1 day).
    And inorder to roatet about the sun, Earth takes 365 days. Therefore, angular velocity of the earth about its own axis is greater than that about the sun.
    Question 361
    CBSEENPH11016881

    Give an example of a body moving with zero angular acceleration but non-zero radial acceleration.

    Solution
    Uniform circular motion is an example of a body moving with zero angular acceleration, but non-zero radial acceleration.
    Angular acceleration is the rate of change of angular velocity, which is zero for uniform circular motion.
    Centripetal acceleration, when directed along the radius gives us the radial acceleration. 
    Radial acceleration = straight v squared over straight r, is non-zero.
    Question 362
    CBSEENPH11016882

    The minute hand of  a watch, which points towards 6, is turned by fraction numerator 2 straight pi over denominator 3 end fraction space radian. Where will it come? 

    Solution

    When the minute hand turns by straight pi over 6 radian comma the minute hand advances by one unit.
    Therefore, on turning through <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre> the minute hand advances by 4 units.
    Thus the minute hand will come at 10.

    Question 363
    CBSEENPH11016886

    What is the angular velocity of hour’s hand and the second’s hand of a clock and what is the ratio of the angular velocity?

    Solution

    Hour's hand completes one revolution in 12 hours.
    Angular velocity of hour's hand is,
    space space straight omega subscript 1 equals fraction numerator 2 straight pi over denominator straight T end fraction equals fraction numerator 2 straight pi over denominator 43200 end fraction rad divided by sec
    Second’s hand completes one revolution in 60 seconds.
    Angular velocity of second’s hand is,
    straight omega subscript 2 equals fraction numerator 2 straight pi over denominator 60 end fraction rad divided by sec
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#6 {main}</pre> , is the required ratio. 

    Question 364
    CBSEENPH11016891

    Derive the relation ω = ω0 + at for uniform angular accelerated motion.

    Solution
    Let a particle revolve in a circular orbit with constant angular velocity straight omega.
    Let the particle start from ω0 and start accelerating with angular acceleration straight alpha.
    Let straight omega be the angular velocity at any instant.
    We know,
    space space space space space space space straight alpha space equals space dω over dt
rightwards double arrow space dω space equals space space straight alpha space dt space

Integrating space both space sides comma space

space space space space space space space space integral subscript straight omega subscript straight o end subscript superscript straight omega d straight omega space space equals space integral subscript 0 superscript straight t straight alpha d straight t space equals space straight alpha integral subscript 0 superscript straight t dt space

rightwards double arrow space space space right enclose straight omega subscript straight omega subscript straight o end subscript superscript straight omega space equals space right enclose αt subscript 0 superscript straight t space

rightwards double arrow space space space straight omega space minus space straight omega subscript straight o space equals space αt

rightwards double arrow space space space straight omega space equals space space straight omega subscript straight o space plus space αt space 
    Hence, the result. 
    Question 365
    CBSEENPH11016903

    Derive the relation, 

    straight theta equals straight theta subscript 0 plus straight omega subscript 0 straight t plus 1 half αt squared

    for uniform angular accelerated motion.

    Solution

    We know, 
    Angular velocity, straight omega equals dθ over dt
    or                     dθ equals straight omega space dt
    For uniform angular accelerated motion, 
    space space space space space space space space space straight omega space equals space straight omega subscript straight o space plus space αt space
therefore space space space dθ space equals space left parenthesis straight omega subscript straight o space plus space αt space right parenthesis space dt space

Integrating space both space sides comma space we space get

integral subscript straight theta subscript straight o end subscript superscript straight theta dθ space equals space integral subscript 0 superscript straight t left parenthesis straight omega subscript straight o space plus space αt space right parenthesis space dt space equals straight omega subscript straight o space integral subscript 0 superscript straight t dt space plus space straight alpha space integral subscript 0 superscript straight t straight t space dt space

rightwards double arrow space space space right enclose straight theta space subscript straight theta subscript straight o end subscript superscript straight theta space equals right enclose space straight omega subscript straight o straight t end enclose subscript 0 superscript straight t space plus space 1 half right enclose αt squared end enclose subscript 0 superscript straight t space

rightwards double arrow space space straight theta space minus space straight theta subscript 0 space equals ωt space plus space 1 half αt squared
rightwards double arrow space straight theta space equals space space straight theta subscript 0 space plus ωt space plus space 1 half αt squared
    Hence, proved. 




    Question 366
    CBSEENPH11016907

    Derive the relation straight omega squared minus straight omega subscript straight o superscript 2 equals 2 straight alpha left parenthesis straight theta minus straight theta subscript straight o right parenthesis for uniform angular accelerated motion.

    Solution

    We know, 
    Angular acceleration,
    space space straight alpha equals dω over dt equals dω over dθ dθ over dt equals dω over dθ straight omega
    rightwards double arrow space straight omega space dω space equals space straight alpha space dθ
    Now, on integrating both sides, we get
    space space space space space space integral subscript straight omega subscript 0 end subscript superscript straight omega omega space d omega space equals space integral subscript theta subscript o end subscript superscript theta alpha space d theta

rightwards double arrow space 1 half right enclose omega to the power of 2 space end exponent end enclose subscript omega subscript 0 end subscript superscript omega space equals space alpha space right enclose theta subscript theta subscript o end subscript superscript theta space

rightwards double arrow space 1 half left parenthesis omega squared space minus space omega subscript o squared right parenthesis space equals space alpha space left parenthesis theta space minus space theta subscript o right parenthesis

rightwards double arrow space space space space space omega squared space minus space omega subscript o squared space equals space 2 space alpha space left parenthesis theta space minus space theta subscript o right parenthesis

    Question 367
    CBSEENPH11016912

    A particle starts from rest and accelerates with constant angular acceleration alpha= 0.6 rad/s2. Find the angular speed of the particle after it turns through 24 revolutions.

    Solution

    Given, a particle starts from rest.
    That is, initial angular velocity, straight omega subscript straight o = 0 rad/s
    Angular acceleration, space space straight alpha equals 0.6 space rad divided by straight s squared 
     
    straight theta equals 24 space rev space equals space 48 straight pi space rad
    We know, straight omega squared minus straight omega subscript straight o superscript 2 equals 2 αθ
    i.e.,                  straight omega squared equals straight omega subscript straight o superscript 2 plus 2 αθ
                               equals 0 plus 2 cross times 0.6 cross times 48 straight pi
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#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    That is, angular speed of the particle is,  <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>

    Question 368
    CBSEENPH11016914

    A uniformly accelerated wheel reaches the angular velocity ω = 20 rad/s in 10 revolutions after rotation begins. Determine the angular acceleration of the wheel.

    Solution

    Here, we have
    Initial angular velocity, straight omega subscript straight o space equals space 0 space rad divided by straight s 
    Velocity attained after some time, space straight omega equals 20 space rad divided by straight s
    No. space of space revolutions space taken comma space straight theta space equals space 10 space rev space equals space 20 straight pi space rad
    Now, using the formula,
                      straight omega squared minus straight omega subscript straight o superscript 2 equals 2 αθ
    rightwards double arrow                       straight alpha equals fraction numerator straight omega squared minus straight omega subscript straight o superscript 2 over denominator 2 straight theta end fraction
     ∴                     space space space space space straight alpha equals fraction numerator left parenthesis 20 right parenthesis squared minus left parenthesis 0 right parenthesis squared over denominator 2 cross times 20 straight pi end fraction
                                 equals 10 over straight pi equals 3.18 space rad divided by straight s squared, is the angular acceleration of the wheel.       

    Question 369
    CBSEENPH11016924

    In one minute after it begins to rotate, a flywheel acquires a velocity corresponding to ω = 720 rpm. Find the angular acceleration of the wheel and the number of its revolutions in one minute. The motion is uniformly accelerated.

    Solution

    We have,
     Initial angular velocity, <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    V e l o c i t y space a t t a i n e d comma space space straight omega equals 720 space r p m space equals space 24 straight pi space r a d divided by straight s

T i m e space o f space t r a v e l comma space straight t space equals space 1 space m i n space equals space 60 space straight s
    Now, using the formula,
                    straight omega minus straight omega subscript straight o equals αt
                      straight alpha equals fraction numerator straight omega minus straight omega subscript straight o over denominator straight t end fraction 
    ∴                straight alpha equals fraction numerator 24 straight pi minus 0 over denominator 60 end fraction equals 0.4 straight pi space space rad divided by straight s squared 
                                         equals 1.26 space rad divided by straight s squared 
    Also,
     <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
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#6 {main}</pre> 
    rightwards double arrow  space space space space space space space straight theta equals fraction numerator straight omega squared minus straight omega subscript straight o superscript 2 over denominator 2 straight alpha end fraction 
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#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
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#6 {main}</pre> 
                  equals 720 straight pi space rad space equals space 360 space rev                  

    Question 370
    CBSEENPH11016938

    A particle moves in a circle of radius r with constant angular velocity straight omega. What is the change in velocity when the particle describes an angle of 90°?

    Solution
    Let the particle revolve in circular orbit of radius r with constant angular velocity straight omega.
    The speed of particle will be, 
                          nu equals omega r
    Let  stack straight v subscript 1 with rightwards arrow on top be the velocity of particle at P and space space space stack straight v subscript 2 with rightwards arrow on top  be the velocity of particle at Q after revolving by angle 90 degree.
    Therefore, angle between stack straight v subscript 1 with rightwards arrow on top space and space stack straight v subscript 2 with rightwards arrow on top is 90 degree.
    ∴           Change in velocity equals left parenthesis stack straight v subscript 2 with rightwards arrow on top minus stack straight v subscript 1 with rightwards arrow on top right parenthesis
    i.e.         space space open vertical bar stack straight v subscript 2 with rightwards arrow on top minus stack straight v subscript 1 with rightwards arrow on top close vertical bar space equals space square root of straight v subscript 2 superscript 2 plus straight v subscript 1 superscript 2 minus 2 straight v subscript 1 straight v subscript 2 cos 90 degree end root 
                           space space space space space space equals square root of straight omega squared straight r squared plus straight omega squared straight r squared minus 0 end root         
                                equals square root of 2 space space ωr        
    Question 371
    CBSEENPH11016939

    A flywheel starting from rest acquires the speed of 300 rpm in 20s. What is the angular acceleration of the flywheel? Also find the angular displacement of the flywheel in first 10s. 

    Solution

    Here,    
    Angular speed, straight omega space equals space 300 space rpm space equals space 10 space rad divided by straight s
    Initial speed,  straight omega subscript straight o space equals space 0
    Time of travel, t = 20s
    By using the relation space space straight omega equals straight omega subscript straight o plus at comma we have
                   straight alpha equals fraction numerator straight omega minus straight omega subscript straight o over denominator straight t end fraction
    rightwards double arrow           straight alpha equals fraction numerator 10 straight pi minus 0 over denominator 20 end fraction equals straight pi over 2 space rad divided by straight s
    The angular displacement of flywheel is,
                      straight theta equals straight omega subscript straight o straight t plus 1 half αt squared
    Here, we have
     straight omega subscript straight o space equals space 0 comma space space space space straight alpha equals straight pi divided by 2   and   t = 10s 
    ∴              straight theta equals 1 half straight pi over 2 left parenthesis 10 right parenthesis squared equals 25 straight pi space rad
    rightwards double arrow         space space straight theta space equals space 12.5 space rev

    Question 372
    CBSEENPH11016943

    The angular velocity of the flywheel increases from 180 rpm to 240 rpm in 4 revolutions. Find the angular acceleration of the flywheel.

    Solution

    Given, 
    Attained angular velocity, straight omega equals 240 space rpm space equals space 8 straight pi space rad divided by straight s

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#6 {main}</pre>
    Using the formula,

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#6 {main}</pre>

    or                straight alpha equals fraction numerator straight omega squared minus straight omega subscript straight o superscript 2 over denominator 2 straight theta end fraction
     ∴          <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
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#6 {main}</pre> 

    Question 373
    CBSEENPH11016946

    When brake is applied, a uniformly retarded wheel reduces its velocity from 300 rpm to 180 rpm during one minute. Find the angular acceleration of the wheel and the number of revolutions it completes in this time.

    Solution

    Given,
    Initial speed, straight omega subscript straight o equals 300 space rpm space equals space 10 space straight pi space rad divided by straight s
    Attained speed, straight omega equals 180 space rpm space equals space 6 straight pi space rad divided by straight s
    Time of travel, straight t space equals space 1 space min space equals space 60 straight s
    Now, using the formula, 
                      straight omega minus straight omega subscript straight o equals alpha t
    or               straight alpha equals fraction numerator straight omega minus straight omega subscript straight o over denominator straight t end fraction
    ∴                straight alpha equals fraction numerator 6 straight pi minus 10 straight pi over denominator 60 end fraction equals negative fraction numerator 4 straight pi over denominator 60 end fraction space rad divided by straight s squared
                        equals negative straight pi over 15 rad divided by straight s squared
    Also, we have  straight omega squared minus straight omega subscript straight o superscript 2 equals 2 αθ
    That is, 
                  straight theta equals fraction numerator straight omega squared minus straight omega subscript straight o superscript 2 over denominator 2 straight alpha end fraction
space space equals fraction numerator left parenthesis 6 straight pi right parenthesis squared minus left parenthesis 10 straight pi right parenthesis squared over denominator 2 open parentheses begin display style fraction numerator negative straight pi over denominator 15 end fraction end style close parentheses end fraction
                    equals fraction numerator negative 64 straight pi squared over denominator negative 2 straight pi divided by 15 end fraction equals 480 straight pi space rad              

                     = 240 rev 
                                      
                                         

    Question 374
    CBSEENPH11016959

    A fan rotates with a velocity corresponding to a frequency of 900 rev/min. When its motor is switched off, the fan uniformly slows down and performs 75 revolutions before it comes to a stop. How much time elapsed from the moment the fan was switched off to the moment it stopped?

    Solution

    Here,
    straight omega subscript straight o space equals space 900 space rpm space equals space 30 straight pi space rad divided by straight s
    straight omega space equals space 0 space rad divided by straight s comma
     straight theta equals 75 space rev space equals space 150 straight pi space rad 
    We have the formula,      
                            straight omega squared minus straight omega subscript straight o superscript 2 equals 2 αθ
    That is, 
                          straight alpha equals fraction numerator straight omega squared minus straight omega subscript straight o superscript 2 over denominator 2 straight theta end fraction
    ∴                   straight alpha equals fraction numerator left parenthesis 0 right parenthesis squared minus left parenthesis 30 straight pi right parenthesis squared over denominator 2 cross times 150 straight pi end fraction equals negative 3 straight pi space rad divided by straight s squared
    Now              straight omega minus straight omega subscript straight o space equals space αt
    rightwards double arrow              space space space space space space space space space space space space space space space space straight t equals fraction numerator straight omega minus straight omega subscript straight o over denominator straight alpha end fraction 
    ∴                          straight t equals fraction numerator 0 minus 30 straight pi over denominator negative 3 straight pi end fraction equals 10 straight s
    So, 10 seconds elapsed from the moment the fan was switched off to the moment it stopeed. 

    Question 375
    CBSEENPH11016964

    A wheel mounted on a stationary axle starts from rest and accelerates with angular acceleration tarannm = 2 + 0.4t rad/s2. Find the angular velocity of wheel after 4 seconds of its start.

    Solution
    The angular acceleration of the wheel depends on time.
    Therefore, equation of kinematics of uniform accelerated motion is not applicable. 
    We have here, 
    straight alpha space equals space 0 space plus space 0.4 space straight t space rad divided by straight s squared space

therefore space dω over dt space equals space 2 space plus space 0.4 space straight t space

rightwards double arrow space dω space equals space left parenthesis 2 space plus space 0.4 space straight t right parenthesis space dt space

On space integrating space both space sides comma space

integral subscript 0 superscript straight omega dω space equals space integral subscript 0 superscript 4 left parenthesis 2 space plus space 0.4 space straight t right parenthesis space dt space

rightwards double arrow space straight omega vertical line subscript 0 superscript straight omega space equals space left parenthesis 2 straight t space plus space 0.2 space straight t squared right parenthesis vertical line subscript 0 superscript 4 space

rightwards double arrow space straight omega space equals space 11.2 space rad divided by sec
    This is the required angular velocity of the wheel after t = 4 seconds. 
    Question 376
    CBSEENPH11016968

    Derive the relation between linear displacement and angular displacement.

    Solution
    Consider, a particle is undergoing uniform circular motion in the plane of the paper in an anticlockwise direction as shown in the figure. 
                        
    Let, at any instant t, the particle is at A.
    At t + dt, the particle is at B.
    In going from A to B, the particle undergoes linear displacement space AB with rightwards arrow on top and angular displacement dθ with rightwards arrow on top. 
    From figure,
                         dθ equals fraction numerator AB with overparenthesis on top over denominator straight r end fraction 
     rightwards double arrow               AB with overparenthesis on top space equals space rdθ 
    Since AB with overparenthesis on top is very small, therefore AB with overparenthesis on top space equals space AB. 
    Thus,           space space AB space equals space rdθ 
    or                   dr equals rdθ                ...(1) 
    In vector form:
    Here, angular displacement is a vector quantity and according to the right-hand thumb rule, its direction is along the axis of rotation.
    In the figure, the angular displacement space stack d theta with italic rightwards arrow on top    is perpendicular to plane of paper in outward direction, <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre> is along OA with rightwards arrow on top in the plane of paper and space stack d r with italic rightwards arrow on top is in the plane of paper perpendicular to both r with rightwards arrow on top space a n d space stack d theta. with rightwards arrow on top 
    Thus, using cross product rule, equation (1) in vector form can be expressed as 
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#6 {main}</pre>, is the required relation. 
    Question 377
    CBSEENPH11016969

    Derive the relation between linear speed and angular speed.

    Solution
    Let a particle be moving in a circular orbit of radius r.
    Let at any instant 't' the particle be at A and in time ∆t the particle undergoes angular displacement ∆θ and reach point B. 

    AB with overparenthesis on top space equals space straight r space triangle straight theta space space space space space space space space space space space space space space space space space space... space left parenthesis 1 right parenthesis

Dividing space both space sides space by space increment straight t comma space we space have

fraction numerator AB with overparenthesis on top over denominator increment straight t end fraction space equals space straight r space fraction numerator space triangle straight theta over denominator increment straight t end fraction space space space space space space space space space... space left parenthesis 2 right parenthesis

Taking space the space limit space increment straight t space rightwards arrow 0 space on space both space sides comma space

limit as increment straight t space rightwards arrow space 0 of space fraction numerator AB with overparenthesis on top over denominator increment straight t end fraction space equals stack space lim with increment straight t space rightwards arrow space 0 below space straight r space fraction numerator space triangle straight theta over denominator increment straight t end fraction

space space space space space space space space space space space space space space space space space space space space space space equals space straight r stack space lim with increment straight t space rightwards arrow space 0 below space fraction numerator space triangle straight theta over denominator increment straight t end fraction space space space space space space space space space... space left parenthesis 3 right parenthesis

If space increment straight t space rightwards arrow space 0 comma space arc space AB space becomes space equal space to space segment space
AB space which space is space equal space to space the space linear space displacement.

Therefore comma space

stack lim space with increment straight t space rightwards arrow 0 below space fraction numerator AB with overparenthesis on top over denominator increment straight t end fraction space equals space stack lim space with increment straight t space rightwards arrow 0 below space fraction numerator AB over denominator increment straight t end fraction space equals space straight v space

and

straight r space stack lim space with increment straight t space rightwards arrow 0 below space fraction numerator space triangle straight theta over denominator increment straight t end fraction space equals space straight r space dθ over dt space equals space rω space

Therefore comma space equation space left parenthesis 3 right parenthesis space becomes

straight v space equals space rω
        
    Question 378
    CBSEENPH11016970

    Define average and instantaneous angular acceleration. Give the unit and dimensional formula of angular acceleration.

    Solution
    Average acceleration is the ratio of change in angular velocity of a particle to the time taken to undergo the change in angular velocity.
    Let a particle be revolving with varying angular velocity.
    Let at any instant t, the angular velocity of particle be straight omega and at time t+∆t, it is ω+∆ω.
    Therefore average angular acceleration of particle is,
                         straight alpha subscript av equals Δω over Δt
    In vector form,
                          stack straight alpha subscript av with rightwards arrow on top space equals space fraction numerator straight capital delta straight omega with rightwards arrow on top over denominator Δt end fraction
    Instantaneous acceleration is the acceleration of the particle at a particular instant and is equal to limiting value of the average acceleration of particle as the time interval approaches zero. 
    Mathematically, 
                       space space space space straight alpha with rightwards arrow on top space equals space limit as increment straight t rightwards arrow infinity of open parentheses fraction numerator increment straight omega with rightwards arrow on top over denominator increment straight t end fraction close parentheses equals fraction numerator dω with rightwards arrow on top over denominator dt end fraction
    Also,               straight omega with rightwards arrow on top equals fraction numerator dθ with rightwards arrow on top over denominator dt end fraction   
                          alpha with italic rightwards arrow on top italic space italic equals italic space fraction numerator stack d omega with italic rightwards arrow on top over denominator d t end fraction italic equals fraction numerator d to the power of italic 2 theta with italic rightwards arrow on top over denominator d t to the power of italic 2 end fraction
    Question 379
    CBSEENPH11016974

    Derive the relation between linear acceleration and angular acceleration.

    Solution

    Let a particle be revolving in a circle of radius r.
    Let <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre> be its position vector and <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#6 {main}</pre> be its angular velocity at any instant.
    The linear velocity of particle at that instant is, 
                       straight v with rightwards harpoon with barb upwards on top space equals space omega with rightwards harpoon with barb upwards on top space x space r with rightwards harpoon with barb upwards on top

    The linear acceleration of object is,
                                 
    straight alpha with rightwards arrow on top space equals space fraction numerator straight d straight v with rightwards arrow on top over denominator dt end fraction equals fraction numerator straight d left parenthesis straight omega with rightwards arrow on top cross times straight r with rightwards arrow on top right parenthesis over denominator dt end fraction 
       equals straight omega with rightwards arrow on top cross times fraction numerator straight d straight r with rightwards arrow on top over denominator dt end fraction plus fraction numerator straight d straight omega with rightwards arrow on top over denominator dt end fraction cross times straight r with rightwards arrow on top 
       equals space omega with rightwards arrow on top cross times v with rightwards arrow on top plus alpha with rightwards arrow on top cross times r with rightwards arrow on top 
       equals space stack alpha subscript r with rightwards arrow on top plus stack alpha subscript 1 with rightwards arrow on top 
    The magnitude of net acceleration of particle is,
    straight a space equals space square root of straight a subscript straight t squared plus space straight a subscript straight r squared end root space

Here comma space we space have

straight omega with rightwards harpoon with barb upwards on top space perpendicular space straight v with rightwards harpoon with barb upwards on top space and space straight alpha with rightwards harpoon with barb upwards on top space perpendicular space straight r with rightwards harpoon with barb upwards on top space

therefore space open vertical bar straight omega with rightwards harpoon with barb upwards on top space perpendicular space straight v with rightwards harpoon with barb upwards on top close vertical bar space equals space straight omega space straight v space comma space and space

open vertical bar straight alpha with rightwards harpoon with barb upwards on top space straight x stack space straight r with rightwards harpoon with barb upwards on top close vertical bar space equals space straight alpha space straight r space

Now comma space

straight alpha space equals space square root of left parenthesis ωv right parenthesis squared space plus space left parenthesis αr right parenthesis squared end root space

space space space equals space square root of open parentheses straight omega squared straight r close parentheses squared space plus space left parenthesis αr right parenthesis squared space end root space
                   
    Total acceleration makes an angle θ with radial acceleration and is given by, 
    tan space straight theta space equals space straight alpha subscript straight t over straight alpha subscript straight r space equals space fraction numerator αr over denominator straight omega squared straight r end fraction space equals space straight alpha over straight omega squared space 

    Question 380
    CBSEENPH11016976

    Derive an expression for centripetal acceleration experienced by a particle revolving in circular orbit of radius r with constant angular speed ω.

    Solution
    Let a particle revolve in a circular orbit of radius r with constant angular speed straight omega.
    Let the particle starts from A and at any instant t, the particle be at B and undergo angular displacement θ = ωt. 
    Linear speed of particle is v = rω.
    Therefore the velocity of particle in vector form is, 
    straight v with rightwards harpoon with barb upwards on top space equals space minus straight v space sin space ωt space straight i with hat on top space plus space rω space cos space straight j with hat on top

straight v with rightwards harpoon with barb upwards space on top equals space minus rω space sinωt space straight i with hat on top space plus space rω space cos space ωt space straight j with hat on top space

Now comma space differentiating space both space sides space straight w. straight r. to space straight t comma

fraction numerator straight d straight v with rightwards harpoon with barb upwards on top over denominator dt end fraction space equals space minus rω squared space cos space ωt space straight i with hat on top space minus space rω squared space sin space ωt space straight j with hat on top space

straight a with rightwards harpoon with barb upwards space on top space equals space left parenthesis negative rω squared space cos space ωt right parenthesis space straight i with hat on top space plus space left parenthesis negative rω squared space sin space ωt right parenthesis space straight j with hat on top space

The space magnitude space of space acceleration space is comma space

open vertical bar straight a with rightwards harpoon with barb upwards on top close vertical bar space equals space square root of space left parenthesis negative rω squared space cos space ωt right parenthesis squared plus space left parenthesis negative rω squared space sin space ωt right parenthesis end root space

space space space space space space equals space rω squared space equals space straight v squared over straight r space

The space angle space that space acceleration space makes space with space straight X minus axis space is comma space

tan space straight alpha space equals space fraction numerator negative rω squared space sin space ωt over denominator negative rω squared space cos space ωt end fraction space equals space fraction numerator negative space sin space ωt over denominator negative space cos space ωt end fraction

tan space straight alpha space equals space tan space left parenthesis straight pi space plus space ωt right parenthesis space

straight alpha space equals space straight pi space plus space ωt
    Therefore, acceleration is directed along BO towards the centre. That is why it is known as centripetal acceleration. 
    Question 381
    CBSEENPH11016977

    Explain why the earth is flattened at poles and bulged out at the equator.

    Solution
    During the formation of the earth, it was a liquid drop. As the earth is spins, it experiences the maximum force at equator and gets flattened at poles. 
    Question 382
    CBSEENPH11016978

    If we swing our watch holding it by the chain at high speed, the chain may break. Explain why. 

    Solution
    The tension in the chain of the watch provides the centripetal force required to revolve the watch.
    There is an upper limit for tension that the chain can bear.
    When the speed of rotation is increased, the centripetal force and hence tension exceeds the breaking limit of chain. Thus chain will at once break.
    Question 383
    CBSEENPH11016979

    What is work done by centripetal force? Explain.

    Solution

    The direction of centripetal force is always perpendicular to the direction of instantaneous velocity.
    The instantaneous displacement is in the direction of instantaneous velocity, therefore instantaneous displacement of particle is perpendicular to centripetal force.
    Hence work done by centripetal force is zero.

    Question 384
    CBSEENPH11016980

    Why does cream collect in the middle when milk is churned up?

    Solution
    When milk is churned in a vessel, the milk and the cream particles experience the centripetal force. The cream particles, being lighter, experience lesser centripetal force than heavy milk particles.
    Therefore, two cannot rotate in the same orbit. The cream particles follow circular path of smaller radius and hence are collected in the middle and milk particles are thrown away from axle when milk is churned. 
    Question 386
    CBSEENPH11016982

    One often comes across the following kind of statement concerning circular motion:

    ‘A particle moving uniformly along a circle experiences a force directed towards the centre (centripetal force) and an equal and opposite force directed away from the centre (centrifugal force). The two forces together keep the particle in equilibrium’.

    Explain what is wrong with this statement.

    Solution

    For a person in the laboratory frame, this statement is wrong because w.r.t. observer in laboratory frame, the particle is in uniform circular motion which is  an accelerated motion. The acceleration of the particle is acting along the radius towards center. Therefore, force is required to revolve the particle in circle known as centripetal force and cannot be zero. 
    But with respect to the observer in the frame rotating along with the particle, the particle appears to be at rest. Hence, the net force experienced by particle must be zero.
    Therefore, the observer in this frame finds that some forces must act on the particle equal and opposite to centripetal force which keep the particle in equilibrium.
    Therefore, the statement makes sense only with respect to the observer in the frame rotating along with particle i.e. non-inertial frame. 

    Question 387
    CBSEENPH11016983

    Read the following statements and pick up the true and false statements: 

    (a) The net acceleration of a particle in circular motion is always along the radius of the circle towards the centre. 
    (b) The change in velocity vector of a particle in uniform circular motion in one complete cycle is zero vector. 

    (c) The magnitude of the change in acceleration vector in uniform circular motion over half cycle is zero.

    Solution
    (a) False; the statement is true if the motion is uniform circular motion. 
                             
    It the speed of particle increases or decreases, then there will be tangential acceleration also. Therefore, the net acceleration will subtend some angle with the radius. 
    (b) True; because in uniform circular motion the speed of particle is constant and direction of motion is given by drawing the tangent to the circle at that point.
    Since after completing one cycle the particle again comes at the same point, therefore, the direction of motion and speed is same and hence the velocity is same.
    Therefore, change in velocity is zero. 
                          
    (c) False; the acceleration of a particle at any point is directed towards the centre at that point. 
             
    At A, 
    At space straight A comma space stack straight a subscript straight A with rightwards harpoon with barb upwards on top space equals space straight v squared over straight r comma space along space AO with rightwards harpoon with barb upwards on top space

At space straight B comma space straight a with rightwards harpoon with barb upwards on top subscript straight B space equals space straight v squared over straight r comma space along space BO with rightwards harpoon with barb upwards on top

Therefore space change space in space acceleration space is comma space

straight a with rightwards harpoon with barb upwards on top subscript straight B space minus space straight a with rightwards harpoon with barb upwards on top subscript straight A space equals space fraction numerator 2 straight v squared over denominator straight r end fraction comma space along space BO with rightwards harpoon with barb upwards on top space 
    Hence, not zero.  
    Question 388
    CBSEENPH11016984

    A disc is rotating with angular velocity ω about an axis passing through the centre and perpendicular to the plane of disc. A body of mass m is placed at the distance r from the centre of the disc. If the coefficient of friction between disc and body is μ then at what angular velocity will the body just slip?

    Solution
    Let a body of mass m be placed on the disc at a distance r from the centre.
    Coefficient of friction between disc and body= μ
    Therefore,
    Limiting friction between body and disc = μmg.
                   
    Let straight omega be the angular velocity of the disc.
    The body will be just at the point of slipping when centrifugal force is just equal to the force of friction.

    Hence,                 m r omega to the power of italic 2 italic space italic equals italic space mu m g 
                               straight omega equals square root of μg over straight r end root, is the angular velocity when the body will just slip. 
    Question 389
    CBSEENPH11016986

    When an aeroplane circles the loop, the pilot does not fall down. Why?


    Solution
    The centrifugal force acting radially outward will balance the weight of the pilot. Hence, pilot does not fall down even at the highest point.
    To complete the loop without falling, minimum velocity at the highest point should be
             space space mv squared over straight r equals mg
    rightwards double arrow             straight v equals square root of rg 

    Question 390
    CBSEENPH11016988

    You may have seen in a circus a motorcyclist driving in vertical loops inside a death well (a hollow spherical chamber with holes, so that the spectators can watch from outside). Explain clearly why the motorcyclist does not drop down when he is at the uppermost point, with no support from below. What is the minimum speed required at the uppermost position to perform a vertical loop if the radius of the chamber is 25 m?

    Solution
    The motorcyclist does not drop down when he is at the uppermost point if centrifugal force on motorcyclist is greater or equal to the weight of motorcyclist.
    i.e.
                  fraction numerator m v squared over denominator R end fraction greater or equal than m g 
    rightwards double arrow          v greater or equal than square root of r g end root 
    The minimum speed of motorcyclist not to fall from uppermost point is given by 
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#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    Here, given
                  r = 25m,            g equals 9.8 space m divided by s squared 
    ∴           v subscript m i n end subscript equals square root of 25 cross times 9.8 end root space equals space 15.7 space straight m divided by straight s
    Question 391
    CBSEENPH11016990

    A body is revolving in a circular orbit of radius with constant speed v. If at any instant its velocity is v with rightwards arrow on top comma space then after what time the velocity will change by <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
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#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
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#6 {main}</pre>?

    Solution

    Let <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
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#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> be the velocity of body at any instant and after time capital delta t its velocity becomes stack v space apostrophe with rightwards arrow on top. Therefore change in velocity is, 
                 
    increment straight v with rightwards harpoon with barb upwards on top space equals space stack straight v apostrophe with rightwards harpoon with barb upwards on top space minus space straight v with rightwards harpoon with barb upwards on top space

Here comma space

increment straight v with rightwards harpoon with barb upwards on top space equals space minus 2 straight v with rightwards harpoon with barb upwards on top space

therefore space straight v with rightwards harpoon with barb upwards on top apostrophe space minus straight v with rightwards harpoon with barb upwards on top space equals space minus space 2 straight v with rightwards harpoon with barb upwards on top space

rightwards double arrow straight v with rightwards harpoon with barb upwards on top apostrophe space equals space minus straight v with rightwards harpoon with barb upwards on top space

Therefore comma space angle space between space straight v with rightwards harpoon with barb upwards on top apostrophe space and space straight v with rightwards harpoon with barb upwards on top space is space 180 to the power of straight o space
and space thus space straight B space is space diagonally space opposite space to space straight A.

Hence comma space the space body space completes space straight a space
semi minus circular space motion. space

Distance space travelled space by space body space equals space πr

Time space taken space to space complete space the space semi minus circle comma space

increment straight t space equals space πr over straight v

 
                         
           
              
                      

    Question 392
    CBSEENPH11016991

    A stone tied to the end of a string 80 cm long is whirled in a horizontal circle with constant speed. If the stone makes 14 revolutions in 25 seconds then what is the magnitude and direction of acceleration of the stone?

    Solution

    Given,
    Length of the string, l = 80cm
    No. of revolutions made by the stone in 25 seconds = 14 
    angular speed,  <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    Therefore acceleration of stone is, 
                alpha equals l omega squared equals 80 open parentheses 88 over 25 close parentheses squared
                           equals 991.2 cm divided by straight s squared 
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#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    Here, motion of stone is circular motion. Therefore, acceleration of stone at any instant is directed towards the centre along the radius. 

    Question 393
    CBSEENPH11016993

    An aircraft executes a horizontal loop of radius 1 km with a steady speed of 900 km/hr. Compare its centripetal acceleration with the acceleration due to gravity.

    Solution

    Given that, 
    R a d i u s space o f space t h e space h o r i z o n t a l space l o o p comma space r italic equals italic 1 k m italic equals italic 1000 m
    Speed of the aircraft, nu equals 900 space k m divided by h r space equals space 250 space m divided by s
    Now the centripetal acceleration is, 
    a subscript c italic equals v to the power of italic 2 over r equals fraction numerator left parenthesis 250 right parenthesis squared over denominator 1000 end fraction equals 62.5 straight m divided by straight s squared 
    The centripetal acceleration relative to acceleration due to gravity is, 
                 space space straight a subscript straight c over straight g equals 625 over 98 equals 6.38 

    Question 394
    CBSEENPH11016994

    A stone of mass 0.3 kg tied to the end of a string in a horizontal plane is whirled round in a circle of radius 1m with a speed of 40 rpm. What is the tension in the string? What is the maximum speed with which the stone can be whirled around if the string can withstand a maximum tension of 200N?

    Solution

    Given,
    Mass of the stone, m = 0.3 kg
    Length of the string, l = 1m
    Angular velocity,space straight omega equals 40 space rpm space equals space fraction numerator 4 straight pi over denominator 3 end fraction space rad divided by sec       
    Therefore tension in the string,

    T equals m divided by omega squared equals 0.3 cross times 1 cross times open parentheses fraction numerator 4 straight pi over denominator 2 end fraction close parentheses squared equals 5.26 space straight N 
    Let straight omega subscript 0 be the maximum angular speed of stone with which it can be whirled around.
    The centrifugal force on the stone should not increase above maximum tension 200N. Otherwise the string will break. 
    ∴              m l straight omega subscript 0 superscript 2 space equals space 200 
    rightwards double arrow             0.3 cross times 1 cross times straight omega subscript 0 superscript 2 equals 200
    rightwards double arrow   space space space space space space space space space space straight omega subscript 0 equals square root of fraction numerator 200 over denominator 0.3 end fraction end root space equals space 25.8 space rad divided by straight s   
                    

     

    Question 395
    CBSEENPH11016997

    A long playing record revolves with a speed of 33 1 third space rev divided by min comma and has a radius of 15 cm. Two coins are placed at 4 cm and 14 cm away from the centre of the record. If the coefficient of friction between the coins and the record is 0.15, which of the two coins will revolve with the record?

    Solution
    The coin will revolve along the record, if the centrifugal force on the coin is less or equal to the force of limiting friction.
    If it is not so, the coin will slip.
    i.e. the coin will revolve along with record only if, 
                     space space space m r omega squared less or equal than mu m g
    rightwards double arrow                     <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
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#6 {main}</pre>

    Given here,
            Limiting space friction comma space straight mu equals 0.15
A n g u l a r space s p e e d comma space space straight omega equals 33 1 third r p m space equals space fraction numerator 10 straight pi over denominator 9 end fraction space r a d divided by straight s 

    ∴   <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
              
                = 0.1206m
                = 12.06 cm
    Therefore, the coin at a distance 4 cm will revolve along the record.
    Question 396
    CBSEENPH11016998

    A cyclist is riding with a speed of 27 km/h. As he approaches a circular turn on the road of radius 80 m, he applies brakes and reduces his speed at the constant rate of 0.5 m/s every second. What is the magnitude and direction of the net acceleration of the cyclist on the circular turn?

    Solution

    Given here,
    Speed of the cyclist, = 27 km/h = 7.5m/s

    Radius of the circular tun on the road, r = 80 m
    Therefore the centripetal acceleration of the cyclist at the moment when velocity is 7.5 m/s is,
     space space space space space space space space a subscript c equals v squared over r equals fraction numerator left parenthesis 7.5 right parenthesis squared over denominator 80 end fraction equals 0.7 space straight m divided by straight s squared 
    When brakes are applied, the speed is decreased at the rate of 0.5m/s every second.
    Therefore the tangential acceleration is,
                       straight a subscript straight t space equals space minus 0.5 space straight m divided by straight s squared 
    Therefore the total acceleration of the cyclist is, 
           space space space space space straight a equals square root of straight a subscript straight c superscript 2 plus straight a subscript straight t superscript 2 end root space equals space square root of left parenthesis 0.7 right parenthesis squared plus left parenthesis 0.5 right parenthesis squared end root             

                   equals square root of 0.74 end root space equals space 0.86 space straight m divided by straight s squared              

    Question 397
    CBSEENPH11017001

    The convex over bridge has radius of curvature r. Find the thrust on the bridge at highest point when the car is going over it with speed v.

    Solution

    Let be the mass of the car.
    While crossing the over bridge, the centrifugal force on the car at the highest point is space fraction numerator m v squared over denominator r end fraction. 
    Therefore net force exerted by car on the bridge is given by, 
                              m g minus fraction numerator m v squared over denominator r end fraction

    Question 398
    CBSEENPH11017002

    A car moves in a flat circular track of radius r. What is the maximum speed with which car can move on the track if coefficient of friction is μ? Does the upper limit of speed depend upon the mass of car?

    Solution
    Let a car of mass 'm' move in a flat circular track of radius r with speed v.
    Let μ be the coefficient of friction between tyres and road.
    Normal reaction of the car = mg
    Limiting friction between car and road = μmg
    Centrifugal force experienced by the car = fraction numerator m v to the power of italic 2 over denominator r end fraction italic.
    For car not to slip, the maximum speed of car is given by, 
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#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 
    rightwards double arrow              v subscript m a x end subscript equals square root of mu r g end root 
    The upper limit of speed does not depend upon the mass of car. 
    Question 399
    CBSEENPH11017004

    A cyclist tends to negotiate a curved track. Obtain an expression for the angle which he will have to make with vertical.

    Solution
    Consider a cyclist of negotiating a curve of radius r with velocity v.
    Weight of the cyclist, W = mg
                  
    In order to provide the necessary centripetal force, the cyclist leans through angle straight theta to the power of straight o in inward direction as shown in figure above. 

    The various forces acting on the cyclist are: 
    (i) Weight Mg acting vertically downward at the centre of gravity of cycle and the cyclist. 
    (ii) The reaction R of the ground onto the cyclist, acting along a line, making angle straight theta to the power of straight o with the vertical. 
    The cyclist while taking the turn is in equilibrium, therefore the vertical component Rcosθ of the normal reaction R will balance the weight of the cyclist.
    The horizontal component R sinθ will provide the necessary centripetal force to the cyclist. 
    i.e.,        space space space R space cos theta space equals space M g                  ...(1) 
    and            R space sin theta space equals space fraction numerator M v squared over denominator r end fraction               ...(2) 
    Dividing (2) by (1), we have
                        tan space theta space equals space fraction numerator v squared over denominator r g end fraction 
    rightwards double arrow                     theta space equals space tan to the power of negative 1 end exponent open parentheses fraction numerator v squared over denominator r g end fraction close parentheses 
    Therefore, the cyclist should lean inwards at angle straight theta given by tan to the power of negative 1 end exponent space open parentheses straight v squared over rg close parentheses 
    Question 400
    CBSEENPH11017006

    A cyclist goes round the track of length 440m in 22s. Find the angle that the cyclist makes with the vertical.

    Solution
    Let r be the radius of track and v the speed of cyclist while circling  the track.
    Here,
    Total length of track = 440m.
    Therefore,
                             2 πr equals 440 
    rightwards double arrow                        r equals 70 straight m
    Time taken to complete the track is 22 s.
    Therefore,
                      v equals fraction numerator italic 2 pi r over denominator t end fraction equals 440 over 22 equals 20 space straight m divided by straight s 
    Now,     tan theta space equals space fraction numerator v squared over denominator r g end fraction equals fraction numerator left parenthesis 20 right parenthesis squared over denominator 70 cross times 9.8 end fraction equals 0.583 
    rightwards double arrow                     <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>, is the angle that the cyclist makes with the vertical. 
    Question 401
    CBSEENPH11017008

    Why are roads banked on curves?

    Solution
    To make a body move along a curved path, centripetal force is always required and this force is provided by friction on the flat road. But friction cannot exceed from its limiting value.
    Therefore, when speed of a body becomes greater than a particular value Thus, centripetal force required will be greater than maximum value of friction and hence skidding takes place. To avoid this, tracks are banked. the outer edge of the curved road is raised above the inner edge. A component of normal reaction Rsinθ provides the necessary centripetal force.  
    Question 402
    CBSEENPH11017009

    While negotiating a curve at high speed, stunt driver of a car has the outer wheels of car raised above. Why does he do so?

    Solution
    The tangent of the angle of banking is directly proportional to the square of velocity. When the driver of the car exceeds the optimum speed for a given banked track, he requires greater angle of banking which is obtained by raising the outer wheels. 
    Question 403
    CBSEENPH11017010

    What is angle of banking? Obtain the condition for equilibrium of a body negotiating a smooth banked road.

    Solution

    When the outer part of the road is raised little above the inner part, to take the turn along the circular track, it is called banking of road.
    The angle that the inclined track makes with horizontal is called angle of banking.
    Consider a vehicle of weight mg mving around a banked curved track of radius r and inclination θ with speed v.


    The vehicle is under the action of following forces:
    (i) The weight mg acting vertically downward.

    (ii) The reaction R of the track to the vehicle is acting perpendicular to banked track in upward direction.
    The components of R is resolved into it's components.
    The vertical component Rcosθ of normal reaction R will balance the weight mg of the vehicle.
    The horizontal component R sinθ will provide the necessary centripetal force to the vehicle.
    i.e.           space space R space cos theta space equals space M g                      ...(1)

    and
                      <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>                   ...(2) 
    Dividing (2) and (1),
                     tan space straight theta space equals space fraction numerator v squared over denominator r g end fraction
    rightwards double arrow                  theta space equals space tan to the power of negative 1 end exponent open parentheses fraction numerator v squared over denominator r g end fraction close parentheses 
    The above equation gives the angle of banking.

    Question 404
    CBSEENPH11017011

    A body of mass m is tied with string and is rotated in a vertical circle of radius r. If the body is given an initial velocity of u at the lowest point, then what will be the velocity when string subtends an angle of θ with the vertical?

    Solution
    Let v be the velocity of body when string subtends an angle θ with vertical.

    Height upto which the body rises, h= LM.
     '
    From ΔOMP comma 
    space space space OM space equals italic space r italic space c o s theta
    ∴  h equals LM equals r left parenthesis 1 minus cos theta right parenthesis
    Now according to the law of conservation of energy,
            1 half m u to the power of italic 2 plus 0 equals 1 half m v to the power of italic 2 plus m g r left parenthesis 1 minus cosθ right parenthesis
    ∴                    <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    So, v is the velocity when the string subtends an angle of θ with the vertical.
    Question 405
    CBSEENPH11017012

    A body of mass m is tied with a string and is rotated in a vertical circle of radius r. Find the minimum velocity at lowest point that should be given to the mass so that it completes the loop in vertical circle without slackening of the string.

    Solution
    Let at lowest point, the mass be given the velocity u.
    The velocity V of mass when string subtends an angle of 0 with vertical is,
    <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    The condition for the mass to complete the vertical loop without slackening the string is that the tension in the string at every point should be greater or equal to zero.
    The tension in the string is minimum at highest point of loop.
    Therefore, tension at highest point should be greater or equal to zero.
    i.e.                T subscript h i g h e s t space end subscript space greater or equal than space 0 
    rightwards double arrow                 fraction numerator m v subscript h squared over denominator r end fraction minus m g greater or equal than 0                ...(1) 
    At highest point, straight theta equals 180 degree comma therefore velocity at highest point is,
                    v subscript h equals square root of u squared minus 2 g r left parenthesis 1 minus cos space 180 right parenthesis end root 
    rightwards double arrow            v subscript h to the power of italic 2 italic space italic equals italic space u to the power of italic 2 italic minus italic 4 g r                     ...(2) 
    Substituting<pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> in equation (1), we get 
                         fraction numerator m left parenthesis u squared minus 4 g r right parenthesis over denominator r end fraction minus m g greater or equal than 0
    rightwards double arrow                     u squared minus 4 g r greater or equal than g r
    rightwards double arrow                             u squared greater or equal than 5 g r 
    rightwards double arrow                              u greater or equal than square root of 5 g r end root, is the minimum velcity given to the mass at the lowest point. 
    Question 406
    CBSEENPH11017013

    A body of mass m is tied with string and rotated in vertical circle of radius r. The body is given the velocity u at lowest point space space left parenthesis u greater than square root of 5 g r right parenthesis end root.  What is the tension at the highest point and does the difference in the tensions at the lowest and the highest point depend upon u?

    Solution


    Tension in the string at any point is given by,
                  <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre> 


                    equals m over r left parenthesis u squared minus 2 g r plus 3 r g cosθ right parenthesis
     At highest point,  straight theta equals 180 degree

    Therefore, 

    T subscript H equals m over r left parenthesis u squared minus 2 g r minus 3 r g right parenthesis space equals space m over r left parenthesis u squared minus 5 g r right parenthesis
    At the lowest point, straight theta equals 0 degree
                       <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>


    ∴               T subscript L minus T subscript H equals 6 m g
    The difference in the tension at the highest and the lowest point does not depend upon velocity at the lowest point and is constant, equal to 6 mg. 

    Question 407
    CBSEENPH11017014

    A small mass is suspended from a light string of length r. What should be the least horizontal velocity that must be given to mass in its lowest position so that tension at the highest point is equal to the weight of the mass attached?

    Solution
    Let u be the velocity of mass at the lowest point and v be at the highest point.
                       
    Tension at highest point is equal to weight.
    ∴     space space fraction numerator m v squared over denominator r end fraction minus m g equals T equals m g
    rightwards double arrow                   fraction numerator m v squared over denominator r end fraction equals 2 m g
                                v squared equals 2 r g 
    The mass rises by height 2r in going from the lowest to the highest point,
    ∴                 v squared minus u squared equals negative 2 g left parenthesis 2 r right parenthesis
                       <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    rightwards double arrow                     u squared equals v squared plus 4 g r equals 2 r g plus 4 r g 
    rightwards double arrow                     u squared equals 6 r g 
                               u equals square root of 6 r g end root, is the least horizontal velocity that is given to the mass. 
    Question 408
    CBSEENPH11017017

    The strength of a string is twice the weight of a body attached to one end of string. From what angle the mass must be released so that string just breaks when at the lowest point?

    Solution
    When string subtends an angle θ with vertical, the mass will be at height h = r(l – cosθ) from the lowest point.
                        
    Therefore velocity of mass m at the lowest point when released is,
                         <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    Tension in the string at the lowest point is,
          T equals m g plus fraction numerator m v squared over denominator r end fraction equals m g plus 2 m g left parenthesis 1 minus cos theta right parenthesis
    String will just break when tension at the lowest point is equal to 2mg,
    m g plus 2 m g left parenthesis 1 minus cos theta right parenthesis equals 2 m g
    rightwards double arrow              1 minus cosθ equals 1 half
    rightwards double arrow                   cosθ equals 1 half
    rightwards double arrow                   space space space straight theta equals 60 degree

    Question 409
    CBSEENPH11017018

    A body slides down from inclined plane and at the bottom it enters into circular loop of radius 10m. From what minimum height must it start sliding down?

    Solution
    Let a body of mass m slide down from height h and at bottom enter into circular orbit of radius r.


    Velocity of body at bottom:
    According to the law of conservation of energy,
                 <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
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#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    rightwards double arrow                 v equals square root of 2 g h end root
    To complete the loop of radius r, the minimum velocity at lowest point should be space square root of 5 g r end root
    ∴          square root of 2 g h end root space equals space square root of 5 g r end root
    rightwards double arrow               h equals 5 over 2 r
    Since           <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
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#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
    ∴                 h equals 5 over 2 cross times 10 equals 25 m, is the required minimum height. 
    Question 410
    CBSEENPH11017019

    An aircraft executes a horizontal loop at speed of 720 km/hr with its wings banked at 15°. What is the radius of the loop?

    Solution

    Given,
                      tanθ equals fraction numerator v squared over denominator r g end fraction
    ∴                    r equals fraction numerator v squared over denominator g space tan theta end fraction 
    We have,
    Velocity of the aircraft, v = 720 km/hr = 200 m/s
    Angle space of space banking comma space straight theta equals 15 degree 
    ∴ Radius of the loop, r equals fraction numerator v squared over denominator g space tan theta end fraction equals fraction numerator left parenthesis 200 right parenthesis squared over denominator 9.8 space tan space 15 end fraction
                                     = 15232.86 m
                                     = 15.23 km

    Question 411
    CBSEENPH11017020

    Find the maximum speed at which car turns round a curve of 30 m radius on a level road if the coefficient of friction between the tyres and the road is 0.4.

    Solution

    The maximum speed of car is given by, 
                       v subscript m a x end subscript equals square root of mu r g end root
    Here,
    Limiting friction, mu equals 0.4
    Radius of the track, r = 30 m 
    Acceleration due to gravity,  g italic equals italic 9 italic. italic 8 italic space m italic divided by s to the power of italic 2 

    Maximum speed, <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>
                                     equals space 10.84 space straight m divided by straight s

    Question 412
    CBSEENPH11017021

    At what angle the circular railway track of radius 200 m must be banked for speed 72 km/hr?

    Solution

    Given,
    Radius of the circular railway track, r = 200m
    Speed of the train, v equals 72 space k m divided by h r space equals space 20 space m divided by s
    We know, the angle of banking is,
                   straight theta equals tan to the power of negative 1 end exponent open parentheses fraction numerator v squared over denominator r g end fraction close parentheses
    rightwards double arrow    space space straight theta equals tan to the power of negative 1 end exponent open parentheses fraction numerator left parenthesis 20 right parenthesis squared over denominator 200 cross times 9.8 end fraction close parentheses equals 11 degree 32 apostrophe, is the angle of banking. 

     
    Question 413
    CBSEENPH11017022

    A body of mass m lying on a frictionless table has a light inextensible string attached to it. The string passes through a hole on the surface of table and has a body of mass M suspended from its other end. The system is kept in equilibrium by revolving the body on the table in a circle of radius r. Find the speed of the body to keep the system in equilibrium.

    Solution
    Let v be the speed of mass m.

    As the svstem is in equilibrium, therefore
                   T = Mg                            ...(1)
    and         T equals fraction numerator m v squared over denominator r end fraction                         ...(2)
    From (1) and (2), 
                   fraction numerator m v squared over denominator r end fraction space equals space M g
    So,  space space v equals square root of fraction numerator M r g over denominator m end fraction end root, is the required speed of the body to be in equilibrium. 
    Question 414
    CBSEENPH11017023

    What is conical pendulum? Find its time period of rotation.

    Solution

    A conical pendulum is a small heavy mass attached with massless, flexible and inextensible string suspended from a rigid support and the mass is a constraint to whirl in a horizontal circle with constant speed. 
    Consider a conical pendulum consisting of point heavy mass m attached with massless, flexible and inextensible string of length l.
    Let the pendulum be suspended from a rigid support S and the mass be whirled with constant speed v in a horizontal circle of radius r, such that the string makes an angle θ with the vertical. 
                        

    The different forces acting on the mass are: 
    (i) Weight mg in vertically downward direction. 
    (ii) Tension T in the string along AS.
    Resolve the components of Tension T, as shown in figure.
    The component Tcosθ balances the weight mg of the mass and component T sinθ provides the necessary centripetal force to whirl the mass.  
    Thus, 
    straight T space cos space straight theta space equals space mg space space space space space space space space space space space... space left parenthesis 1 right parenthesis thin space

straight T space sin space straight theta space equals space mv squared over straight r space space space space space space space space... space left parenthesis 2 right parenthesis space

Dividing space equation space left parenthesis 2 right parenthesis space by space left parenthesis 1 right parenthesis comma space we space get

straight v squared over rg space equals space tan space straight theta space

rightwards double arrow space straight v space equals space square root of rg space tan space straight theta end root space space space space space space... space left parenthesis 3 right parenthesis space

From space fig. space straight r space equals space straight l space sin space straight theta space

Substituting space for space straight r space in space equation space left parenthesis 3 right parenthesis comma space we space get space

straight v space equals space square root of straight l space sin space straight theta space straight g space tan space straight theta end root space space space equals space sin space straight theta space square root of fraction numerator gl over denominator cos space straight theta end fraction end root space space
Time space taken space to space complete space one space revolution space
of space conical space pendulum space is comma space

straight T space equals space fraction numerator 2 πr over denominator straight v end fraction space equals space fraction numerator 2 straight pi space straight r space straight l space sin space straight theta over denominator straight v end fraction space

space space equals space 2 straight pi space square root of fraction numerator straight l space cos space straight theta over denominator straight g end fraction end root space

Angular space frequency space of space the space revolution space
of space conical space pendulum space is comma space

straight omega space equals space fraction numerator 2 straight pi over denominator straight T end fraction space equals space square root of fraction numerator straight g over denominator straight l space cos space straight theta end fraction end root space


    Question 415
    CBSEENPH11017024

    A small block of mass m is placed in a hemispherical bowl of radius r and bowl is set rotating about its axis of symmetry with angular velocity straight omega. Find the radius of circle in which block revolves and the angular position of block with vertical at equilibrium position while bowl is rotating.

    Solution
    Let, at equilibrium, the block be at A when the bowl rotates with angular velocity ω.
    Let x = AN be the radius of circle in which block revolves. 
                        
    The forces acting on the block are: 
    (i) Weight mg, in vertically downward direction. 
    (ii) Normal reaction N, along AO.
    Resolve the components of N as shown in the figure above. 
    The component N cosθ balances the weight mg of the mass and component.
    The component T sinθ provides the necessary centripetal force to revolve the block.
    Thus,
    straight N space cos space straight theta space equals space mg space space space space space space space space space space space space space space... space left parenthesis 1 right parenthesis thin space

straight N space sin space straight theta space equals space straight m space straight x space straight omega squared space equals space straight m space straight r space sin space straight theta space straight omega squared space

From space the space fig. space above space

straight x space equals space straight r space sin space straight theta

Therefore comma space

straight N space equals space mr space straight omega squared space space space space space space space space space space space space space space space... space left parenthesis 2 right parenthesis thin space

Dividing space left parenthesis 1 right parenthesis space by space left parenthesis 2 right parenthesis comma space we space get space

cos space straight theta space equals space straight g over rω squared space

rightwards double arrow space straight theta space equals space cos to the power of negative 1 end exponent space open parentheses straight g over rω squared close parentheses space
Now comma space straight x space equals space straight r space sin space straight theta space equals space straight r space square root of 1 space minus space cos squared straight theta end root space

rightwards double arrow space straight x space equals space straight r square root of 1 minus open parentheses straight g over rω squared close parentheses squared end root  
    Hence, the result. 
                   
    Question 416
    CBSEENPH11017025

    A car is taking turn on the rough banked circular track. What should be the velocity of car so that there is no wear and tear of the tyres?

    Solution

    Consider a car of mass m taking the turn on banked rough circular track of radius r.
    Let straight theta be the angle of banking and μ be the coefficient of friction.
    Let u be the velocity of car so that there is no wear and tear.
    Different forces acting on car are:
    (i) Weight of car, acting vertically downwards.
    (ii) Reaction R on car, normal to track.
    (iii) Frictional force f = μR, down the inclined plane.
    (iv) Centrifugal force <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
#1 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php(56): mkdir('/home/config_ad...', 493)
#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
#3 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/RenderImpl.class.php(231): com_wiris_plugin_impl_FolderTreeStorageAndCache->codeDigest('mml=<math xmlns...')
#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
#5 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/service.php(19): com_wiris_plugin_impl_TextServiceImpl->service('mathml2accessib...', Array)
#6 {main}</pre>, radially outwards. 
                   
    Resolving the rectangular component of different forces.
    Since there is no wear and tear on the car, therefore, net force in vertical and horizontal direction is zero. 
    That is, 
    space space space space space mg space plus space straight f space sin space straight theta space equals space straight R space cos space straight theta

rightwards double arrow mg space plus space straight mu space Rsin space straight theta space equals space straight r space cos space straight theta space

rightwards double arrow space space mg space equals space straight R space cos space straight theta space minus space straight mu space straight R space sin space straight theta space space space space... space left parenthesis 1 right parenthesis thin space

and

mv squared over straight r space equals space straight R space sin space straight theta space plus space straight f space cos space straight theta

space space space space space space space space space equals space straight R space sin space straight theta space plus space straight mu space straight R space cos space straight theta space space space space space space... space left parenthesis 2 right parenthesis space

Dividing space left parenthesis 2 right parenthesis space by space left parenthesis 1 right parenthesis comma space we space get

space space space space fraction numerator mv squared divided by straight r over denominator mg end fraction space equals space fraction numerator straight R space sin space straight theta space plus space straight mu space straight R space cos space straight theta space over denominator straight R space cos space straight theta space minus space space straight mu space straight R space sin space straight theta end fraction

rightwards double arrow straight v squared over rg space equals space fraction numerator sin space straight theta space plus space straight mu space cos space straight theta over denominator cos space straight theta space minus space straight mu space sin space straight theta end fraction

rightwards double arrow space straight v squared space equals space rg space fraction numerator straight mu space plus space tan space straight theta over denominator 1 space minus space straight mu space tan space straight theta end fraction

rightwards double arrow space straight v space equals space square root of rg fraction numerator straight mu space plus space tan space straight theta over denominator 1 space minus space straight mu space tan space straight theta end fraction end root
 

    Question 417
    CBSEENPH11017026

    A road is 5m wide. Its radius of curvature is 50m. The outer edge is above the inner edge of road by a distance 0-5m. If the coefficient of friction of road is 0.3, find the upper limit of velocity on the road.

    Solution

    Given that, 
    Radius of the road, r equals 50 m
    Limiting space friction comma space straight mu space equals space 0.3
    Tangent of the angle of banking of road is,

                 <pre>uncaught exception: <b>mkdir(): Permission denied (errno: 2) in /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php at line #56mkdir(): Permission denied</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/util/sys/Store.class.php line 56<br />#0 [internal function]: _hx_error_handler(2, 'mkdir(): Permis...', '/home/config_ad...', 56, Array)
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#2 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/FolderTreeStorageAndCache.class.php(110): com_wiris_util_sys_Store->mkdirs()
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#4 /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/TextServiceImpl.class.php(59): com_wiris_plugin_impl_RenderImpl->computeDigest(NULL, Array)
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#6 {main}</pre>
    Now the upper limit of the velocity is,
               space space space space v italic equals square root of r g fraction numerator mu italic plus t a n theta over denominator italic 1 italic minus mu t a n theta end fraction end root 
    rightwards double arrow           nu equals square root of 50 cross times 10 fraction numerator 0.3 plus 0.1 over denominator 1 minus 0.3 cross times 0.1 end fraction end root 
    So, upper limit of the velocity of road is, v = 14.36 m/s

    Question 418
    CBSEENPH11017031

    A 2000kg car has to take a turn on flat road whose radius is 70m. The coefficient of friction between the car and the road is 0.3. Is the road suitable for a speed 20m/s. If not so, by what angle the outer edge of the road should be banked for safe turn at 20m/s.

    Solution

    Given that, 
    L i m i t i n g space f r i c t i o n comma space straight mu equals 0.3

R a d i u s space o f space t h e space r o a d comma space space r equals 70 space straight m
    The maximum safe speed on the flat road is,
    v subscript 1 equals square root of mu r g end root equals square root of 0.5 cross times 70 cross times 10 end root space equals space 14.49 space straight m divided by straight s
    The flat track is not suitable for speed 20m/s.
    Let for safe turn bank the road at angle straight theta.
    Thus,
                     nu equals square root of r g fraction numerator straight mu plus tan theta over denominator 1 minus μtanθ end fraction end root
    rightwards double arrow        space space 20 equals square root of 70 cross times 10 fraction numerator 0.3 plus tanθ over denominator 1 minus 0.3 space tanθ end fraction end root 
    rightwards double arrow       400 over 700 equals fraction numerator 0.3 plus tanθ over denominator 1 minus 0.3 space tanθ end fraction
    rightwards double arrow   space space tan space straight theta space equals space fraction numerator 1.9 over denominator 8.2 end fraction
    rightwards double arrow           straight theta space equals space 13 degree, is the angle of banking required. 

    Question 419
    CBSEENPH11017033

    In the figure shown, a rod of length 24cm is pivoted near an end in horizontal plane. At the free end a simple pendulum of length 24 cm is suspended. The system is made to rotate with angular velocity ω so that the string makes an angle 37° with vertical. Find ω.


    Solution
    Rod of length = 24 cm 
    Length of simple pendulum = 24 cm 
    Angular velocity, straight omega = 37o, with the vertical.
    From the figure below, the bob will rotate in a horizontal circle of radius of radius r given by, 
                  
    Error converting from MathML to accessible text.
    Question 421
    CBSEENPH11017035

    A force acts for 8 seconds on a body of mass 10 kg. The force then stops acting and the body describes 80 m in next 5 sec. Find the force. 

    Solution

    Here, 
    Mass, m = 10 kg; t = 8 sec 
    After the force stops acting, the body moves with a uniform velocity given by, 
    v = fraction numerator distance space travelled over denominator time space taken end fraction space equals space 80 over 5 space equals space 16 space m divided by s
    Initial velocity of the body, u = 0 
    Using equation of motion, 
    v = u + at 
    16 = 0 + a x 8
    a = 2 m/s2 
    Therefore, 
    Force, F = ma = 10 cross times space 2 = 20 N

    Question 422
    CBSEENPH11017036

    Two billiard balls each of mass 50 g moving in opposite directions with a speed of 36 km/hr collide and rebound with the same velocity. What is the impulse imparted to each ball due to the other. 

    Solution

    Impulse imparted to each ball = change in momentum of each ball. 
                                           = 2 mv
                                           = 2 x (50 x 10-3) x 10
                                           = 1 kg m/s

    Question 423
    CBSEENPH11017037

    A rocket of initial mass 6000 kg ejects mass at a constant rate of 16 kh/s with constant relative speed of 11 km/s. What is the acceleration of the rocket one minute after blast? 

    Solution

    Here, 
    mo = 6000 kg
    dm over dt space equals space 16 space k g divided by s
    u = 11 km/s = 11000 m/s 
    Acceleration,a  = ? 
    Time taken, t = =1 min = 60 sec 
    Mass left after 1 minute  = mo - (dm/dt) t 
                                          = 6000 - ( 16 x 60)
                                          = 5040 kg
    Using the formula, 
    F = m. (dv/dt)= u . (dm/dt), we get
    a = dv/dt = fraction numerator straight u space left parenthesis thin space dm divided by dt right parenthesis over denominator straight m end fraction space equals space fraction numerator 11000 space straight x space 16 over denominator 5040 end fraction space equals space 34.92 space straight m divided by straight s squared

    Question 424
    CBSEENPH11017038

    Two bodies whose masses are m1 = 50 kg and m2 = 150 kg are tied by a light string and are placed on a frictionless horizontal surface. When m1 is pulled by a force F, n acceleration of 5 m/s is produced in both the bodies. Calculate the value of F. What is the tension in the string? 

    Solution

    Here, 
    m1 = 50 kg, m2 = 150 kg 
    Force applied, F = ? 
    Tension in the string, T = ? 
    Acceleration in both the bodies, a = 5 m/s2
    Therefore, 
    F = (m1 + m2) a
    F = (50 +150) 5 = 1000 N 
    Also, T = m2a= 150 x 5 = 750 N

    Question 425
    CBSEENPH11017043

    a hammer weighing 1 kg moving with a speed of 20 m/s strikes the head of a nail driving it 10 cm into a wall. Neglecting the mass of the nail, calculate a) the acceleration during the impact, b) the time interval during impact, c) impulse. 

    Solution

    Here, 
    m = 1 kg, u = 20 m/s 
    s = 10 cm = 0.1 m 
    v = 0, a = ?
    a) As, 
    v2 = u2 + 2as
    therefore space 0 space equals space left parenthesis 20 right parenthesis squared space plus space 2. straight a. space left parenthesis 0.1 right parenthesis space

rightwards double arrow space straight a space equals negative fraction numerator 400 over denominator 2 space cross times 0.1 end fraction space equals space minus 2000 space straight m divided by straight s squared 
    b) v = u + at 
    therefore space 0 space equals space 20 space plus space left parenthesis negative 2000 right parenthesis straight t space

rightwards double arrow space straight t space equals space 20 over 2000
space space space space space space equals 1 over 100 equals 0.01 space sec 
    c) Impulse = F. t
                   = m (v-u)
                   = 1 (0-20)
                   = -20 N.s.

    Question 426
    CBSEENPH11017045

    In an X-ray machine, an electron is subjected to a force of 10-23 N. In how much time will the electron cover a distance of 0.1 m. Take mass of electron as 10-30 kg. 

    Solution

    Given, 
    Force, F = 10-23 N, t = ?
    s = 0.1 m, m = 10-30 kg 
    Acceleration, a = straight F over straight m space equals space 10 to the power of negative 23 end exponent over 10 to the power of negative 30 end exponent space equals space 10 to the power of 7 space m divided by s squared
    Using the equation of motion, 
    straight s space equals space ut space plus space 1 half at squared space

0.1 space equals space 0 space plus space 1 half space cross times space 10 to the power of 7 space straight t squared space
straight t squared space equals space 2 space cross times space 10 to the power of negative 8 end exponent space

rightwards double arrow space straight t space equals space 1.414 space cross times space 10 to the power of negative 4 end exponent space straight s

    Question 427
    CBSEENPH11017050

    A curved road of diameter 1.8 km is banked so that no friction is required at a speed of 30 m/s. What is the banking angle? 

    Solution

    Diameter, 2 r = 1.8 km = 1800 m 
    r = 1800/2 = 900 m 
    Speed, v = 30 m/s
    Banking angle, straight theta = ? 
    We know that, 
    tan space straight theta space equals space fraction numerator straight v squared over denominator straight r space straight g end fraction space equals space fraction numerator 300 space straight x space 30 over denominator 900 space straight x space 9.8 end fraction space equals space 0.102

That space is comma space

straight theta space equals space 6 to the power of straight o
     

    Question 428
    CBSEENPH11017051

    Why is it difficult to move a bike with its brakes on? 

    Solution

    When the brakes of a bike are on, its wheels cannot rotate. They will simply skid. Thus, rolling friction will be converted into sliding friction which is comparatively larger. Hence, it becomes difficult to move the bike. 

    Question 429
    CBSEENPH11017052

    A particle is moving on circular path so that angular speed and linear speed are equal. What is the radius of the path?

    Solution

    When angular speed and linear speed are equal, the radius of the path is 1 m. 
     

    Question 430
    CBSEENPH11017053

    How will you find the direction of velocity of a particle moving on a circular path at any instant?

    Solution

    The direction of velocity at any instant is the direction in which the body tends to move if there is no external force acting on it.
    The body in a circular path moves along the tangent drawn to the circle at a point.

    Hence, the tangent drawn to the circle at any point gives the direction of the velocity of the body at that point.

    Question 431
    CBSEENPH11017054
    Question 432
    CBSEENPH11017055

    What is the direction of force of friction when the vehicle goes round the round about?

    Solution
    When the vehicle goes round about, the force is directed towards the centre of the roundabout.
    Question 433
    CBSEENPH11017056

    A cyclist riding at a speed of 10 m/s, takes a turn round the circular track. For the safe turn he bends at angle 30° with the vertical. Find the radius of track.

    Solution
    Speed of the cyclist, v = 10 m/s
    Angle at which the cyclist bends = 30o
    We know that, 
    tan space theta space equals space fraction numerator v squared over denominator r g end fraction

T h a t space i s comma space

t a n space 30 to the power of o space equals space fraction numerator 10 space cross times space 10 over denominator r space cross times space 9.8 space end fraction

r space equals space fraction numerator 10 space cross times space 10 space x space square root of 3 over denominator 9.8 end fraction space equals space 17.6 space m 
    The radius of the circular track = 17.6 m. 
    Question 434
    CBSEENPH11017057

    If a small can filled with water is rapidly swung in a vertical circle, the water does not fall, why?

    Solution
    If the speed of the can is sufficient, such that the centrifugal force at the highest point is greater than the weight of water in the can, then the water will not fall.
    Question 435
    CBSEENPH11017058

    What is the work done by centripetal force when the body completes half a revolution?

    Solution
    The work done by a body does not depend on the path. Hence, work done by centripetal force remains unchanged
    Question 437
    CBSEENPH11017061

    A car travels on a flat, circular track of radius 200 m at 30 m/s and has a centripetal acceleration = 4.5 m/s2. a) If the mass of the car is 1000 kg, what frictional force is required to provide the acceleration? b) If the coefficient of static friction is 0.8, what is the maximum speed at which the car can circle the track?

    Solution
    Here given,
    Radius, r = 200 m
    Velocity, v = 30 m/s
    Centripetal acceleration, a  = 4.5 m/s2 
    Mass, m = 1000 kg 
    Frictional force required, F = ? 
    Frictional force is the same as accelerating force. 
    Therefore, 
    a) ma = 1000 cross times space 4.5 =4500 N 
    b) Coefficient of static friction, straight mu space equals space 0.8 space space
    Maximum speed, v = ?

    straight v space equals space square root of μrg space equals space square root of 0.8 cross times 200 cross times 9.8 end root space equals space 39.6 space straight m divided by straight s
    Question 438
    CBSEENPH11017062
    Question 439
    CBSEENPH11017228

    What is Aristotle’s law of motion?

    Solution
    Aristotle’s law of motion states that an external force is required to keep the body in motion.
    Question 440
    CBSEENPH11017229
    Question 441
    CBSEENPH11017230

    State Galileo’s law of motion.

    Solution
    Galileo’s law of motion states that if no unbalanced force is acting on the body then, the body continues to move in the same direction with constant speed.
    Question 442
    CBSEENPH11017231

    Define inertia.

    Solution
    Inertia is the property, by virtue of which the body cannot change its state of rest or uniform motion in a straight line, unless acted upon by an unbalanced force. 
    Question 443
    CBSEENPH11017232

    Which law of motion is also called law of inertia?

    Solution
    Newton’s first law of motion is the law of inertia.
    Question 444
    CBSEENPH11017233

    Which law of motion helps us in walking?

    Solution
    Newton’s third law of motion helps us in walking. While walking. a person presses the ground in the backward direction (action) with his feet. The ground pushes the person in the forward direction with an equal force (reaction). The component of the reaction in the horizontal direction makes the person move forward. 
    Question 445
    CBSEENPH11017234

    What do you mean by mechanically isolated system?

    Solution
    When there is no net external force on the system, a system is said to be mechanically isolated. 
    Question 446
    CBSEENPH11017235

    What are absolute units of force?

    Solution
    Newton and dyne. 
    Question 447
    CBSEENPH11017236

    One gf equals to how many dyne? 

    Solution
    980 dyne.
    Question 448
    CBSEENPH11017237

    A body is placed on a frictionless surface in gravity free space. Do you need any force for body to move?

    Solution
    Yes, force is required for the body to move because inertia of body opposes its change of state.
    Question 449
    CBSEENPH11020060

    A pebble of mass 0.05 kg is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble,

    (a) during its upward motion,

    (b) during its downward motion,

    (c) at the highest point where it is momentarily at rest. Do your answers change if the pebble was thrown at an angle of 45° with the horizontal direction?

    Ignore air resistance.

    Solution
    Irrespective of the direction of motion, acceleration due to gravity always acts downward.
    The gravitational force is the only force that acts on the pebble in all three cases.
    Its magnitude is given by Newton’s second law of motion as:
                           F = m x a
    where,
     
    F = net force, 
    m= mass of the pebble, and 
    a is the acceleration.
    Here, a = g = 9.8 m/s2
    Therefore, 
                   F = 0.05 x 10 = 0.5 N.
    The magnitude of net force = 0.5 N, which is acting vertically in the downward direction.
    If the pebble is thrown at an angle of 45° with the horizontal direction, it will have both the horizontal and vertical components of velocity. At the highest point, only the vertical component of velocity becomes zero. However, the pebble will have the horizontal component of velocity throughout its motion. This component of velocity produces no effect on the net force acting on the pebble. 
    Question 450
    CBSEENPH11020061

    Give the magnitude and direction of the net force acting on a stone of mass 0.1 kg,

    (a) just after it is dropped from the window of a stationary train,

    (b) just after it is dropped from the window of a train running at a constant velocity of 36 km/h,

    (c) just after it is dropped from the window of a train accelerating with 1 m s-2,

    (d) lying on the floor of a train which is accelerating with 1 m s-2, the stone being at rest relative to the train.

    Neglect air resistance throughout.

    Solution

    a) We have here, 
    Mass, m = 0.1 kg 
    Acceleration due to gravity, a = +g = 10 m/s2

    Net force, F = ma = 0.1 × 10 = 1.0 N, acting in vertically downward direction.
    b) When the train is running at a constant velocity, its acceleration = 0.
    Due to this motion, no force is acting on the stone.
    Therefore,
    Force on the stone, F = weight of stone = mg
                                     = 0.1 × 10 = 1.0 N

    This force acts in the vertically downward direction. 
    c) Acceleration of the train, a = 1 m s-2
    Additional force, F' = ma = 0.1 × 1 = 0.1 N, acts on the stone in the horizontal direction.
    But once the stone is dropped from the train, F' becomes zero.
    Net force on the stone, F = mg = 0.1 × 10
                                           = 1.0 N, acting vertically downwards.
    d) When the stone is lying on the train, its acceleration is same as that of the train.
    Therefore, the force acting on stone, F = ma 
                                                               = 0.1 × 1 = 0.1 N
    This force is along the horizontal direction of motion of the train.
    In each case, the weight of the stone is being balanced by the normal reaction.

    Question 451
    CBSEENPH11020062

    One end of a string of length is connected to a particle of mass and the other to a small peg on a smooth horizontal table. If the particle moves in a circle with speed the net force on the particle (directed towards the centre) is:

    (i) T, (ii) T - mv2 / l, (iii) T + mv2 / l, (iv) 0,

    is the tension in the string. 

    [Choose the correct alternative]. 

    Solution

    i) T is the correct answer. 
    A particle connected to a string is revolving in a circular orbit around the center.
    For rotation, the centripetal force is provided by the tension produced in the string. 
    Therefore, the net force produced on the particle is the tension, T. 
    That is, 
    Error converting from MathML to accessible text.
    where, 
    F is the net force acting on the particle. 

    Question 452
    CBSEENPH11020063

    A constant retarding force of 50 N is applied to a body of mass 20 kg moving initially with a speed of 15 ms–1. How long does the body take to stop?

    Solution

    Retarding Force, F = - 50 N
    Mass of the body, m = 20 kg 
    Initial velocity of the body, u = 15 m/s
    Final velocity of the body, v = 0 
    Acceleration produced in the body can be calculated as: 
       F = ma
    -50 = 20 x a
      a = fraction numerator negative 50 over denominator space 20 end fraction space equals space minus 2.5 space m divided by s squared
    Using the first equation of motion,

                              v = u + at 
    Time (t) taken by the body to come to rest can be calculated as,

    Therefore,
                     t = negative straight u over straight a space equals space fraction numerator negative 15 over denominator negative 2.5 end fraction space equals space 6 space s

    Question 453
    CBSEENPH11020064

    A constant force acting on a body of mass 3.0 kg changes its speed from 2.0 m s–1 to 3.5 m s–1 in 25 s. The direction of the motion of the body remains unchanged. What is the magnitude and direction of the force?

    Solution
    Given,
    Mass of the body, m = 3 kg
    Initial speed of the body, u = 2 m/s
    Final speed of the body, v = 3.5 m/s
    Time, t = 25 s
    Using the first equation of motion, 
                         v = u + at
    Acceleration(a) produced in the body, a =fraction numerator straight v minus straight u over denominator straight t end fraction
    Therefore, 
    straight a space equals space fraction numerator 3.5 space minus space 2 over denominator 25 end fraction space equals space 0.06 space straight m divided by straight s squared
    Now, according to Newton's second law of motion, we have
    F = ma
       = 3 x 0.06
       = 0.018 N
    Since the application of force does not change the direction of the body, the net force acting on the body is in the direction of its motion. 
    Question 454
    CBSEENPH11020065

    A body of mass 5 kg is acted upon by two perpendicular forces 8 N and 6 N. Give the magnitude and direction of the acceleration of the body.

    Solution

    Mass of the body, m = 5 kg
    The given situation can be represented as follows:


    The resultant of two forces is given by, 
    R = square root of left parenthesis 8 right parenthesis squared plus left parenthesis negative 6 right parenthesis squared end root space equals space square root of 64 space plus space 36 end root space equals space 10 space straight N
    straight theta is the angle made by R with the force of 8 N.
    Therefore, 
    straight theta space equals space tan to the power of negative 1 end exponent space open parentheses negative 6 over 8 close parentheses space equals space minus 36.87 to the power of straight o
    The negative sign indicates that θ is in the clockwise direction with respect to the force of magnitude 8 N.
    According to Newton’s second law of motion
    Acceleration (a) of the body is given as, 
    F = ma 
    Therefore, 
    a =straight F over straight m space equals space 10 over 5 space equals space 2 space m divided by s squared

    Question 455
    CBSEENPH11020066

    The driver of a three-wheeler moving with a speed of 36 km/h sees a child standing in the middle of the road and brings his vehicle to rest in 4.0 s just in time to save the child. What is the average retarding force on the vehicle? The mass of the three-wheeler is 400 kg and the mass of the driver is 65 kg. 

    Solution
    Initial speed of the three-wheeler, u = 36 km/h = 10 m/s
    Final speed of the three-wheeler, v = 0 m/s
    Time, t = 4 s
    Mass of the three-wheeler, m = 400 kg
    Mass of the driver, m' = 65 kg
    Total mass of the system, M = 400 + 65 = 465 kg
    According to the first law of motion,
    Acceleration (a) of the three-wheeler is, 
                           v = u + at 

    Therefore,
     a = fraction numerator left parenthesis straight v space minus space straight u right parenthesis over denominator straight t end fraction space equals space fraction numerator left parenthesis 0 minus 10 right parenthesis over denominator 4 end fraction space equals space minus space 2.5 space straight m divided by straight s squared
    The negative sign indicates that the velocity of the three-wheeler is decreasing with time.
    Now, using Newton’s second law of motion, the net force acting on the three-wheeler is, 
           F = Ma

             = 465 × (–2.5)
     
             = –1162.5 N 
    The negative sign indicates that the force is acting against the direction of motion of the three wheeler. 
    Question 456
    CBSEENPH11020067

    A rocket with a lift-off mass 20,000 kg is blasted upwards with an initial acceleration of 5.0 m s–2. Calculate the initial thrust (force) of the blast.
     

    Solution
    Mass of the rocket, m = 20,000 kg
    Initial acceleration, a = 5 m/s2

    Acceleration due to gravity, g = 10 m/s2

    Using Newton’s second law of motion,
    Net force (thrust) acting on the rocket is given by the relation, 
            F – mg = ma 

                     F = m (g + a)
                        = 20000 × (10 + 5)
                        = 20000 × 15
                        = 3 × 105 N
    Question 457
    CBSEENPH11020068

    A body of mass 0.40 kg moving initially with a constant speed of 10 m s–1 to the north is subject to a constant force of 8.0 N directed towards the south for 30 s. Take the instant the force is applied to be = 0, the position of the body at that time to be = 0, and predict its position at t = –5 s, 25 s, 100 s.
     

    Solution
    Given,
    Mass of the body, m = 0.40 kg
    Initial speed of the body, u = 10 m/s, due north
    Force acting on the body, F = –8.0 N
    Acceleration produced in the body, a = straight F over straight m space equals space fraction numerator negative 8 over denominator 0.40 end fraction space equals space minus 20 space m divided by s squared
    (i)
    At t = –5 s,
    Acceleration, a' = 0
    Initial velocity, u = 10 m/s 
    Using the relation, 
             s = ut + (1/2) a' t

               = 10 × (–5) = –50 m
    (ii)
    At t = 25 s
    Acceleration, a'' = –20 m/s2 
    Initial velocity, u = 10 m/s
    USing the relation, 
                s' = ut' + (1/2) a" t2

                   = 10 × 25 + (1/2) × (-20) × (25)

                   = 250 - 6250
                   = -6000 m
    (iii)
    At t = 100 s, 
    For 0 ≤ t ≤ 30 s 
    Acceleration, a = -20 ms-2
    Initial velocity, u = 10 m/s
    Now, using the equation of motion, we have
                   s1 = ut + (1/2)a"t

                       = 10 × 30 + (1/2) × (-20) × (30)2

                       = 300 - 9000 
                       =  -8700 m
    For 30 < t ≤ 100 s,
    For t= 30 sec, as per the first equation of motion final velocity is given as, 

                       v
     = u + at 

                         = 10 + (–20) × 30
                         = –590 m/s 
    Velocity of the body after 30 s = –590 m/s 
    For motion between 30 s to 100 s, i.e., in 70 s: 
    s2 = vt + open parentheses 1 half close parentheses a" t

        = -590 × 70
        = -41300 m 
    ∴ Total distance, s" = s1 + s2 
                              = -8700 -41300
                              = -50000 m
                              = -50 km.
    Question 458
    CBSEENPH11020069

    A truck starts from rest and accelerates uniformly at 2.0 m s–2. At = 10 s, a stone is dropped by a person standing on the top of the truck (6 m high from the ground). What are the (a) velocity, and (b) acceleration of the stone at = 11 s? (Neglect air resistance.)

    Solution

    Given, 
    Initial velocity of the truck, u = 0
    Acceleration, a = 2 m/s2
    Time, t = 10 s
    a)
    According to the equation of motion, we have
                       v = u + at
                         = 0 + 2x10
                         = 20 m/s
    The final velocity of the truck and hence, of the stone is 20 m/s.
    At t = 11 s, the horizontal component (vx) of velocity, in the absence of air resistance, remains unchanged.
    i.e.,                vx = 20 m/s 
    The vertical component (vy) of velocity of the stone is given by the first equation of motion as,
                         vy = u + aδ

    where,
    δt = 11 – 10 = 1 s, and 
    a
    y = g = 10 m/s

    ∴           vy = 0 + 10 × 1
                      = 10 m/s
    The resultant velocity (v) of the stone is given as:
    straight v space equals space square root of straight v subscript straight x squared space plus space straight v subscript straight y squared end root space

space space space equals space space square root of 20 squared plus space 10 squared end root space

space space space equals space 22.36 space straight m divided by straight s

    Let, straight theta be the angle made by the resultant velocity with the horizontal component of velocity vx,
    Therefore, 
    tan space straight theta space equals space begin inline style straight v subscript straight y over straight v subscript straight x end style

straight theta space equals space tan to the power of negative 1 end exponent space open parentheses 10 over 20 close parentheses

space space space equals space 26.57 to the power of straight o 
    b) 
    When the stone is dropped from the truck, the horizontal force acting on it becomes zero. However, the stone continues to move under the influence of gravity. Hence, the acceleration of the stone is 10 m/s2 and it acts vertically downward. 

    Question 459
    CBSEENPH11020070

    A bob of mass 0.1 kg hung from the ceiling of a room by a string 2 m long is set into oscillation. The speed of the bob at its mean position is 1 m s–1. What is the trajectory of the bob if the string is cut when the bob is

    (a) at one of its extreme positions,

    (b) at its mean position.

    Solution
    (a) When bob is at its extreme position:
    The trajectory of the bob if the string is cut at one of it's extreme positions will be in vertically downward direction.
    At the extreme position, the velocity of the bob becomes zero. If the string is cut at this moment, then the bob will fall vertically on the ground. 
    (b) When the bob is at its mean position:
    When the string is cut at its mean position, the trajectory of the bob will be a parabolic path. 
    At the mean position, the velocity of the bob is 1 m/s.
    The direction of this velocity is tangential to the arc formed by the oscillating bob. If the bob is cut at the mean position, then it will trace a projectile path having the horizontal component of velocity only.
    Hence, the bob will follow a parabolic path. 
    Question 460
    CBSEENPH11020071

    A man of mass 70 kg stands on a weighing scale in a lift which is moving, 

    (a) upwards with a uniform speed of 10 m s–1

    (b) downwards with a uniform acceleration of 5 m s–2

    (c) upwards with a uniform acceleration of 5 m s–2.
     
    What would be the readings on the scale in each case?

    (d) What would be the reading if the lift mechanism failed and it hurtled down freely under gravity?

    Solution
    (a) 
    Mass of the man, m = 70 kg
    Acceleration, a = 0
    Using Newton’s second law of motion, we can write the equation of motion as, 
                          R – mg = ma

    where,
     ma is the net force acting on the man.
    As the lift is moving at a uniform speed, acceleration a = 0
              R = m
                  = 70 × 10
                  = 700 N 
    Therefore, reading on the weighing scale = 700 over straight g
                                                         = 700 over 10
                                                         = 70 space kg
    (b) 
    Mass of the man, m = 70 kg
    Acceleration, a = 5 m/s2 , downward
    Using Newton’s second law of motion, we can write the equation of motion as: 
                          R + mg = ma 

                                   R = m(g – a
                                      = 70 (10 – 5)
                                      = 70 × 5 
                                              = 350 N 
    ∴ Reading on the weighing scale = 350 g = 350 over 10= 35 kg
    (c) 
    Mass of the man, m = 70 kg
    Acceleration, a = 5 m/supward 
    Using Newton’s second law of motion, we can write the equation of motion as:

                            R
     – mg = ma 

                                     R = m(g + a
                                        = 70 (10 + 5)
                                        = 70 × 15
                                        = 1050 N
    Therefore,
    Reading on the weighing scale = 1050 over straight g
                                                   = 1050 over 10
                                                   = 105 kg
    (d) When the lift moves freely under gravity, acceleration a =
    Using Newton’s second law of motion, we can write the equation of motion as, 
                           R + mg = ma 

                                     R = m(g – a
                                        = m(g – g)
                                        = 0 
    ∴ Reading on the weighing scale = 0 over straight g= 0 kg 
    The man will be in a state of weightlessness.
    Question 461
    CBSEENPH11020072

    Figure 5.16 shows the position-time graph of a particle of mass 4 kg. What is the (a) force on the particle for t < 0, t> 4 s,0 < t < 4 s? (b) impulse at t = 0 and t = 4 s? (Consider one-dimensional motion only). 


    Solution
    (a)
    For t < 0
    From the given graph, the position of the particle is coincident with the time axis.
    That is, the displacement of the particle in this time interval is zero. Hence, the force acting on the particle is zero.
    For t > 4 s 
    In the given graph, the position of the particle is parallel to the time axis. It indicates that the particle is at rest at a distance of  3 m from the origin. Hence, no force is acting on the particle.
    For 0 < t < 4 
    The position-time graph has a constant slope in the given graph.
    Hence, the acceleration produced in the particle is zero.
    Therefore, the force acting on the particle is zero.
    (b)
    At t = 0,
    Impulse = Change in momentum 
              = mv – mu 

    Mass of the particle, m = 4 kg 
    Initial velocity of the particle, u = 0 
    Final velocity of the particle, v = 3 over 4 m/s
    ∴ Impulse = 4 x (  3 over 4- 0)
                  = 3 kg m/s
    At t = 4 s,
    Initial velocity of the particle, u = 3 over 4 m/s 
    Final velocity of the particle, v = 0 
    ∴ Impulse = 4 (0 - 3 over 4 ) = -3 kg m/s 
    (a) For t < 0
    It can be observed from the given graph that the position of the particle is coincident with the time axis. It indicates that the displacement of the particle in this time interval is zero. Hence, the force acting on the particle is zero.
    For t > 4 s 
    It can be observed from the given graph that the position of the particle is parallel to the time axis. It indicates that the particle is at rest at a distance of 
    3 m from the origin. Hence, no force is acting on the particle.
    For 0 < t < 4
    It can be observed that the given position-time graph has a constant slope. Hence, the acceleration produced in the particle is zero. Therefore, the force acting on the particle is zero.
    (b) At t = 0
    Impulse = Change in momentum 
               = mv – mu 

    Mass of the particle, m = 4 kg 
    Initial velocity of the particle, u = 0 
    Final velocity of the particle, v = 3 over 4 m/s 
    ∴ Impulse = 4 ( 3 over 4 - 0) = 3 kg m/s
    At t = 4 s
    Initial velocity of the particle, u =  m/s

    Final velocity o9f the particle, v = 0
    ∴ Impulse = 4 (0 - 3 over 4) = -3 kg m/s 
    Question 462
    CBSEENPH11020073

    Two bodies of masses 10 kg and 20 kg respectively kept on a smooth, horizontal surface are tied to the ends of a light string. A horizontal force F = 600 N is applied to

    (i) A, (ii) B along the direction of string.

    What is the tension in the string in each case?

    Solution
    Horizontal force, F = 600 N
    Mass of body A, m1 = 10 kg
    Mass of body B, m2 = 20 kg
    Total mass of the system, m = m1 + m2 = 30 kg
    Using Newton’s second law of motion,
                            F = ma 
    Acceleration (a) produced in the system can be calculated as, 
             straight a space equals space straight F over straight m = 600 over 30 space equals space 20 space m divided by s squared space
    When force is applied on a body A. 
    Equation of motion can be written as, 
    F - T = m1a
    Therefore,
    T = F - m1a
       = 600 - 10 x 20
       = 400 N                                ...  (i)
    When Force is applied on a body B, we have 
    F - T = m2a
    i.e., T = F - m2
    Therefore, 
    T = 600 - 20 x 20 = 200 N         ... (ii)
    From (i) and (ii), we can say that the answer is different in both the cases. 
    Therefore, the answer depends on which end of mass, the force is applied.
    Question 463
    CBSEENPH11020074

    A nucleus is at rest in the laboratory frame of reference. Show that if it disintegrates into two smaller nuclei the products must move in opposite directions.

    Solution
    Let mm1, and m2 be the respective masses of the parent nucleus and the two daughter nuclei.
    The parent nucleus is at rest.
    Initial momentum of the system (parent nucleus) = 0
    Let v1 and v2 be the respective velocities of the daughter nuclei having masses m1 and m2.
    Total linear momentum of the system after disintegration = m1v1 + m2v

    According to the law of conservation of momentum, 
    Total initial momentum = Total final momentum 
                                     0 = m1v1 + m2v

                                    v1 = -m2v2 / m

    Here, the negative sign indicates that the fragments of the parent nucleus move in directions opposite to each other.
    Question 464
    CBSEENPH11020075

    Two billiard balls each of mass 0.05 kg moving in opposite directions with speed 6 m s–1 collide and rebound with the same speed. What is the impulse imparted to each ball due to the other?
     

    Solution

    Mass of each ball, m = 0.05 kg 
    Initial velocity of the ball = 6 m/s 
    Magnitude of the initial momentum of the ball, p1 = 0.05 x 6
        = 0.3 kg m/s
    After collision, the balls change their directions of motion without changing the magnitudes of their velocity.
    Final momentum of each ball, p= - 0.3 kg m/s
    Impulse imparted to each ball = pf - pi = -0.3 -0.3 = -0.6 kg m/s
    The negative sign indicates that the impulses imparted to the balls are opposite in direction.

    Question 465
    CBSEENPH11020076

    A shell of mass 0.020 kg is fired by a gun of mass 100 kg. If the muzzle speed of the shell is 80 m s–1, what is the recoil speed of the gun?

    Solution
    Given, 
    Mass of the gun, M = 100 kg
    Mass of the shell, m = 0.020 kg
    Muzzle speed of the shell, v = 80 m/s
    Recoil speed of the gun = V

    Both the gun and the shell are at rest initially.
    Initial momentum of the system = 0
    Final momentum of the system = mv – MV

    Here, the negative sign appears because the directions of the shell and the gun are opposite to each other.
    Now, as per the law of conservation of momentum. we have
    Final momentum = Initial momentum 
     
              mv – MV = 0 
    ∴                   V = mv / 

                           = 0.020 × 80 / (100 × 1000)
                           = 0.016 m/s
    Question 466
    CBSEENPH11020077

    A batsman deflects a ball by an angle of 45° without changing its initial speed which is equal to 54 km/h. What is the impulse imparted to the ball? (Mass of the ball is 0.15 kg.)

    Solution

    The given question can be illustrated as in the following fig. below.

    Here, 
    AO = incident path of the ball, 
    OB = Path followed by ball after deflection, 
    < AOB = Angle between the incident and deflected paths of the ball = 45o

    Therefore, 
    ∠AOP = ∠BOP = 22.5° = θ
    Initial velocity of the ball = Final velocity of the ball = v
    On resolving the component of velocity along v, we have
    Horizontal component of the initial velocity = vcos θ, along RO 
    Vertical component of the initial velocity = vsin θ, along PO
    Horizontal component of the final velocity = vcos θ, along OS
    Vertical component of the final velocity = sin θ,  along OP
    The horizontal components of velocities suffer no change.
    The vertical components of velocities are in the opposite directions.
    So, change in linear momentum of the ball gives us the impulse which is imparted to the ball. 
    That is, 
    Impulse = mvCosθ - (-mvCosθ) 
                 =  2mvCosθ 
    Mass of the ball, m = 0.15 kg 
    Velocity of the ball, v = 54 km/h
                                    = 15 m/s
    Therefore, 
    Impulse = 2 x 0.15 x 15 cos 22.5o
                 
    = 4.16 kg m/s 



    Question 467
    CBSEENPH11020078

    A stone of mass 0.25 kg tied to the end of a string is whirled round in a circle of radius 1.5 m with a speed of 40 rev./min in a horizontal plane. What is the tension in the string? What is the maximum speed with which the stone can be whirled around if the string can withstand a maximum tension of 200 N?

    Solution
    We have, 
    Mass of the stone, m = 0.25 kg
    Radius of the circle, r = 1.5 m
    Number of revolution per second, n =
    40 over 60 equals space 2 over 3 space r. p. s 40 / 60 = 2 / 3 rps
    Angular velocity, ω = straight v over straight r = 2πn 
    The tension in the string provides the centripetal force. 
    i.e.,
    T = Fcentripetal 

       = mv squared over straight r 
        = mrω
        = mr(2πn)

        = 0.25 × 1.5 × (2 × 3.14 × (open parentheses 2 over 3 close parentheses)2
        = 6.57 N 
    Maximum tension in the string,

             Tmax = 200 N 
               max = mv2max / r

    ∴       vmax = (Tmax × r  / m)1/2 

                     = (200 × 1.5 / 0.25)1/2 

                     = (1200)1/2 
                     = 34.64 m/s 
    Therefore, the maximum speed of the stone is 34.64 m/s. 
    Question 468
    CBSEENPH11020079

    If, in Exercise 5.21, the speed of the stone is increased beyond the maximum permissible value, and the string breaks suddenly, which of the following correctly describes the trajectory of the stone after the string breaks: 

    (a) the stone moves radially outwards, 

    (b) the stone flies off tangentially from the instant the string breaks,

    (c) the stone flies off at an angle with the tangent whose magnitude depends on the speed of the particle ?

    Solution
    (b) The stone flies off tangentially from the instant the string breaks.
    When the string breaks, the stone will move in the direction of the velocity at that instant. According to the first law of motion, the direction of velocity vector is tangential to the path of the stone at that instant. Hence, the stone will fly off tangentially from the instant the string breaks. 
    Question 469
    CBSEENPH11020080

    Figure 5.17 shows the position-time graph of a body of mass 0.04 kg. Suggest a suitable physical context for this motion. What is the time between two consecutive impulses received by the body? What is the magnitude of each impulse?


    Solution
    Between x = 0 and x = 2 cm, ball will be rebounding between two walls.
    After every 2 s,
    Magnitude of the impulse received by the ball = 0.08 × 10–2 kg m/s
    The given graph shows that a body changes its direction of motion after every 2 s.
    Physically, this situation can be visualized as a ball rebounding to and fro between two stationary walls situated between positions x = 0 and x = 2 cm.
    The slope of the position-time graph reverses after every 2 s, the ball collides with a wall after every 2 s.
    Therefore, ball receives an impulse after every 2 s.
    Given, 
    Mass of the ball, m = 0.04 kg
    The slope of the graph gives the velocity of the ball.
    Using the graph,
    Initial velocity, (u) = fraction numerator left parenthesis 2 space minus space 0 right parenthesis space straight x space 10 to the power of negative 2 end exponent over denominator left parenthesis 2 minus 0 right parenthesis end fraction space equals space 10 to the power of negative 2 end exponent space m divided by s
    Velocity of the ball before collision, u = 10–2 m/s
    Velocity of the ball after collision, v = –10–2 m/s 
    Here, the negative sign arises as the ball reverses its direction of motion.
    Magnitude of impulse = Change in momentum 
                                     = | mv - mu |
                                     = | 0.04 (v - u) |
                                     = | 0.04 (-10-2 - 10-2) |
                                     = 0.08 × 10-2 kg m/s 
    Question 470
    CBSEENPH11020081

    A stone of mass tied to the end of a string revolves in a vertical circle of radius R. The net forces at the lowest and highest points of the circle directed vertically downwards are: [Choose the correct alternative]

      Lowest Point Highest Point
    a mg – T1 mg + T2
    b mg + T1 mg – T2
    c mg + T1 – (mv12/ R mg – T2 + (mv12/ R
    d mg – T1 – (mv12/ R mg + T2 + (mv12/ R

    T1 and V1 denote the tension and speed at the lowest point.

    T2 and v2 denote corresponding values at the highest point.

    Solution
    The free body diagram of the stone at the lowest point is shown in the figure below:

                       

    According to Newton’s second law of motion, the net force acting on the stone at this point is equal to the centripetal force.
    i.e.,  Fnet = T - mg = mv subscript 1 squared over straight R                    ...(i)
    where,
    v1 is the velocity at the lowest point. 
    The free body diagram of the stone at the highest point is shown in the following figure. 
                     
    Using Newton’s second law of motion, 

            T + mg = mv subscript 2 squared over straight R                       ...(ii)
    where, v2 is the velocity at the highest point.

    From equations (i) and (ii), 
    Net force acting at the lowest = (T - mg)
    Net force at the highest points = (T + mg) 
    Question 471
    CBSEENPH11020082

    A helicopter of mass 1000 kg rises with a vertical acceleration of 15 m s–2.

    The crew and the passengers weigh 300 kg.

    Give the magnitude and direction of the, 

    (a) force on the floor by the crew and passengers,

    (b) action of the rotor of the helicopter on the surrounding air,

    (c) force on the helicopter due to the surrounding air.

    Solution

    Given, 
    Mass of the helicopter, mh = 1000 kg 
    Mass of the crew and passengers, mp = 300 kg
    Total mass of the system, m = 1300 kg
    Acceleration of the helicopter, a = 15 m/s
    Using Newton’s second law of motion, the reaction force R,
               R – mpg = ma

                              = mp(g + a)
                              = 300 (10 + 15) 
                              = 300 × 25 
                              = 7500 N 
    The reaction force will be directed upwards, the helicopter is accelerating vertically upwards. 
    According to Newton’s third law of motion, the force on the floor by the crew and passengers  = 7500 N, directed downward.
    (b)
    Using Newton’s second law of motion, the reaction force R’ experienced by the helicopter can be calculated as, 
                     R' - mg = ma 

                                 = m(g + a)
                                 = 1300 (10 + 15)
                                 = 1300 × 25 
                                         
                                 = 32500 N 
    The reaction force experienced by the helicopter from the surrounding air is acting upward. Hence, as per Newton’s third law of motion, the action of the rotor on the surrounding air will be 32500 N, directed downward.
    (c) The force on the helicopter due to the surrounding air is 32500 N, directed upwards. 

    Question 472
    CBSEENPH11020083

    A stream of water flowing horizontally with a speed of 15 m s–1 gushes out of a tube of cross-sectional area 10–2m2, and hits a vertical wall nearby. What is the force exerted on the wall by the impact of water, assuming it does not rebound?

    Solution
    Given,
    Speed of the water stream, v = 15 m/s
    Cross-sectional area of the tube, A = 10–2 m2

    Volume of water coming out from the pipe per second,
                     VAv 
                         = 15 × 10–2 m3/s 
    Density of water, ρ = 103 kg/m

    Mass of water flowing out through the pipe per second = ρ ×V 
               = 150 kg/s
    The water strikes the wall and does not rebound.
    Therefore, according to Newton's second law of motion, 
    Force exerted by the water on the wall,
    F = Rate of change of momentum = fraction numerator increment straight p over denominator increment straight t end fraction
                                                        = mv over straight t
                                                        = 150 × 15
                                                        = 2250 N
    Question 473
    CBSEENPH11020084

    Ten one-rupee coins are put on top of each other on a table. Each coin has a mass m. Give the magnitude and direction of,

    (a) the force on the 7th coin (counted from the bottom) due to all the coins on its top,

    (b) the force on the 7th coin by the eighth coin,

    (c) the reaction of the 6th coin on the 7th coin.

    Solution
    (a) Force on the seventh coin is exerted by the weight of the three coins on its top.
    Weight of one coin = mg
    Weight of three coins = 3mg
    Hence, the force exerted on the 7th coin by the three coins on its top is 3mg.
    This force acts vertically downward.
    (b) Force on the seventh coin by the eighth coin is because of the weight of the eighth coin and the other two coins (ninth and tenth) on its top.
    Weight of the eighth coin = mg
    Weight of the ninth coin = mg
    Weight of the tenth coin = mg
    Total weight of these three coins = 3m
    Hence, the force exerted on the 7th coin by the eighth coin is 3mg.
    This force acts vertically downward.

    (c) The 6th coin experiences a downward force because of the weight of the four coins (7th, 8th, 9th, and 10th) on its top. 
    Therefore, the total downward force experienced by the 6thcoin is 4mg. 
    As per Newton’s third law of motion, the 6th coin will produce an equal reaction force on the 7th coin, but in the opposite direction.
    Hence, the reaction force of the 6th coin on the 7thcoin is of magnitude 4mg.
    This force acts in the upward direction. 
    Question 474
    CBSEENPH11020085

    A train runs along an unbanked circular track of radius 30 m at a speed of 54 km/h. The mass of the train is 106 kg. What provides the centripetal force required for this purpose – The engine or the rails? What is the angle of banking required to prevent wearing out of the rail?

    Solution
    Given, 
    Radius of the circular track, r = 30 m
    Speed of the train, v = 54 km/h = 15 m/s
    Mass of the train, m = 106 kg 
    The centripetal force is provided by the lateral thrust of the rail on the wheel.
    As per Newton’s third law of motion, the wheel exerts an equal and opposite force on the rail.
    This reaction force is responsible for the wear and rear of the rail. 
    The angle of banking θ, is related to the radius (r) and speed (v) by the relation:
                 tan θ = v2 / rg 
                          = 152 / (30 × 10) 
                          = 225 / 300 
                       θ = tan-1 (0.75) = 36.87

    Therefore, the angle of banking is about 36.87°.
    Question 475
    CBSEENPH11020086

    A block of mass 25 kg is raised by a 50 kg man in two different ways as shown in Fig. 5.19. What is the action on the floor by the man in the two cases? If the floor yields to a normal force of 700 N, which mode should the man adopt to lift the block without the floor yielding?


    Solution

    Given,

    Mass of the block, m = 25 kg
    Mass of the man, M  = 50 kg 
    Acceleration due to gravity, g = 10 m/s

    Force applied on the block, F = 25 × 10
                                          = 250 N 
    Weight of the man, W = 50 × 10 = 500 N
    Case (a): When the man lifts the block directly
    In this case, the man applies a force in the upward direction.
    This increases his apparent weight. 
    Therefore,
    Action on the floor by the man = 250 + 500 = 750 N
    Case (b): When the man lifts the block using a pulley 
    In this case, the man applies a force in the downward direction. This decreases his apparent weight. 
    Therefore,
    Action on the floor by the man = 500 – 250
                                                   = 250 N
    If the floor can yield to a normal force of 700 N, then the man should adopt the second method to easily lift the block by applying lesser force. 

    Question 476
    CBSEENPH11020087

    A monkey of mass 40 kg climbs on a rope (Fig. 5.20) which can stand a maximum tension of 600 N. In which of the following cases will the rope break: the monkey

    (a) climbs up with an acceleration of 6 m s–2

    (b) climbs down with an acceleration of 4 m s–2

    (c) climbs up with a uniform speed of 5 m s–1

    (d) falls down the rope nearly freely under gravity?

    (Ignore the mass of the rope).


    Solution
    Case (a):
    Mass of the monkey, m = 40 kg
    Acceleration due to gravity, g = 10 m/s 
    Maximum tension that the rope can bear, Tmax = 600 N 
    Acceleration of the monkey, a = 6 m/s2 , upward
    Using Newton’s second law of motion, equation of motion is
                       T – mg = ma 

    ∴                          T = m(g + a
                                  = 40 (10 + 6) 
                                   = 640 N 
    Since T > Tmax, the rope will break in this case.
    Case (b)
    Acceleration of the monkey, a = 4 m/s2 downward 
    Using Newton’s second law of motion, the equation of motion is, 
                                      mg – ma 

    ∴                                        T = (g – a
                                                = 40(10 – 4)  
                                                = 240 N 
    Since T < Tmax, the rope will not break in this case.
    Case (c)
    The monkey is climbing with a uniform speed of 5 m/s.
    Therefore, its acceleration is zero, i.e., a = 0. 
    Using Newton’s second law of motion, equation of motion is, 
                         T – m= ma 

                         T – mg = 0
     
    ∴                           m
                                   = 40 × 10  
                                   = 400 N 
    Since T < Tmax, the rope will not break in this case.
    Case (d) 
    When the monkey falls freely under gravity, acceleration will become equal to the acceleration due to gravity, i.e., a = 
    Using Newton’s second law of motion, we can write the equation of motion as:
                              mg – T = m
    ∴                                 T = m(g – g) = 0 
    Since T < Tmax, the rope will not break in this case. 
    Question 477
    CBSEENPH11020088

    Two bodies and of masses 5 kg and 10 kg in contact with each other rest on a table against a rigid wall (Fig. 5.21). The coefficient of friction between the bodies and the table is 0.15. A force of 200 N is applied horizontally toA. What are

    (a) the reaction of the partition

    (b) the action-reaction forces between 
    and B?

    What happens when the wall is removed? Does the answer to (b) change, when the bodies are in motion? Ignore the difference between μ
    s and μk.


    Solution
    (a)
    Mass of body A, mA = 5 kg
    Mass of body B, mB = 10 kg
    Applied force, = 200 N
    Coefficient of friction, μs = 0.15
    The force of friction is given by the relation, 
                  fs = μ (mA + mB)g 
                     = 0.15 (5 + 10) × 10 
                     = 1.5 × 15
                     = 22.5 N, leftward 
    Net force acting on the partition = 200 – 22.5
                                                     = 177.5 N, rightward 
    According to Newton’s third law of motion, the reaction force of the partition will be in the direction opposite to the net applied force.
    Hence, the reaction of the partition will be 177.5 N,directed leftwards.
    (b) 
    Force of friction on mass A, fA = μmA
                                            = 0.15 × 5 × 10
                                            = 7.5 N leftward 
    Net force exerted by mass A on mass B = 200 – 7.5 = 192.5 N, rightward 
    Now, according to Newton’s third law of motion, an equal amount of reaction force will be exerted by mass B on mass A.
    i.e., Net force acting on mass A by mass B= 192.5 N acting leftward.
    When the wall is removed, the two bodies will move in the direction of the applied force.
    Net force acting on the moving system = 177.5 N
    The equation of motion for the system of acceleration a, can be written as,
    Net force = (mA + mB) a
    ∴ Acceleration, a = fraction numerator Net space Force over denominator straight m subscript straight A space plus space straight m subscript straight B end fraction 
                           = fraction numerator 177.5 over denominator left parenthesis 5 plus 10 right parenthesis end fraction space equals space fraction numerator 177.5 over denominator 15 end fraction space equals space 11.83 space straight m divided by straight s squared
    Net force causing mass A to move,
                     FA = mA

                         = 5 × 11.83
                         = 59.15 N 
    Net force exerted by mass A on mass B = 192.5 – 59.15 = 133.35 N 
    This force will act in the direction of motion.
    According to Newton’s third law of motion, an equal amount of force will be exerted by mass B on mass A, i.e., 133.3 N, acting opposite to the direction of motion. 
    Question 478
    CBSEENPH11020089

    A block of mass 15 kg is placed on a long trolley. The coefficient of static friction between the block and the trolley is 0.18. The trolley accelerates from rest with 0.5 m s–2 for 20 s and then moves with uniform velocity. Discuss the motion of the block as viewed by

    (a) a stationary observer on the ground,

    (b) an observer moving with the trolley.


    Solution
    Given,
    Mass of the block, m = 15 kg
    Coefficient of static friction, μ = 0.18 
    Acceleration of the trolley, a = 0.5 m/s

    According to Newton’s second law of motion,
    Force (F) on the block caused by the motion of the trolley is given by the relation, 
                       F = ma 
                         = 15 × 0.5
                         = 7.5 N
    This force is acted in the direction of motion of the trolley. 
    Force of static friction between the block and the trolley, 
                                 f = μm
                                   = 0.18 × 15 × 10
                                   = 27 N 
    The applied external force is less than the force of static friction between the block and the trolley. Therefore, the block will appear to be at rest, for an observer on the ground. 
    When the trolley moves with uniform velocity, only the force of friction will act in this situation and there will be no applied external force. 
    (b) The person who is moving with the trolley has some acceleration. 
    The frictional force is acting backwards on the trolley and is opposed by a pseudo force of the same magnitude. But, this force is acting in the backward direction. Therefore, for an observer moving with the trolley, the trolley will appear to be at rest.
    Question 479
    CBSEENPH11020090

    The rear side of a truck is open and a box of 40 kg mass is placed 5 m away from the open end as shown in Fig. 5.22. The coefficient of friction between the box and the surface below it is 0.15. On a straight road, the truck starts from rest and accelerates with 2 m s–2. At what distance from the starting point does the box fall off the truck? (Ignore the size of the box).


    Solution
    We have, 
    Mass of the box, m = 40 kg
    Coefficient of friction, μ = 0.15
    Initial velocity, u = 0
    Acceleration, a = 2 m/s2

    Distance of the box from the end of the truck, s' = 5 m
    According to Newton's second law of motion,
    Force on the box caused by the accelerated motion, F = ma 
                   = 40 × 2
                    = 80 N 
    According to Newton’s third law of motion, a reaction force of 80 N is acting on the box in the backward direction.
    The backward motion of the box is opposed by the force of friction f, acting between the box and the floor of the truck.
    This force is given by, 
                     f = μmg  
                       = 0.15 × 40 × 10
                       = 60 N 
    Therefore,
    Net force acting on the block, 
                  Fnet = 80 – 60 = 20 N, acting backward 
    The backward acceleration produced in the box is, 
    Acceleration, aback = straight F subscript net over straight m space equals space 20 over 40 space equals space 0.5 space straight m divided by straight s squared space
    Using the second equation of motion,
    Time t is, 
                       s' = ut + (1/2) abackt

                       5 = 0 + (1/2) × 0.5 × t

    ∴                  t = √20 s 
    Hence, the box will fall from the truck after square root of 20 s from the start. 
    The distance s, travelled bytruck in √20 s is given by, 
                       s = ut + (1/2)at

                         = 0 + (1/2) × 2 × (square root of 20)

                         = 20 m 
    Therefore, at a distance of 20 m, the box will fall off the truck. 
    Question 480
    CBSEENPH11020091

    A disc revolves with a speed of 33 space begin inline style 1 third end style rev/min, and has a radius of 15 cm. Two coins are placed at 4 cm and 14 cm away from the centre of the record. If the coefficient of friction between the coins and the record is 0.15, which of the coins will revolve with the record?

    Solution
    Coin placed at 4 cm from the centre
    Given, 
    Mass of each coin = m

    Radius of the disc, r = 15 cm = 0.15 m
    Frequency of revolution, ν = 100 over 3 rev/min
                                             = fraction numerator 100 over denominator left parenthesis 3 space straight x space 60 right parenthesis end fraction
                                             = open parentheses 5 over 9 close parentheses rev/s
    Coefficient of friction, μ = 0.15
    In the given situation, the coin having a force of friction greater than or equal to the centripetal force provided by the rotation of the disc will revolve with the disc. If this is not the case, then the coin will slip from the disc.
    Case 1: Coin placed at 4 cm
    Radius of revolution, r' = 4 cm = 0.04 m
    Angular frequency, ω = 2πν
                                     = 2 x open parentheses 22 over 7 close parentheses space x space open parentheses 5 over 9 close parentheses 
                                      = 3.49 s-1

    Frictional force, f = μmg
                              = 0.15 × m × 10
                              = 1.5m N 
    Centripetal force on the coin is, 
                  Fcent. = mr'ω

                            = m × 0.04 × (3.49)

                            = 0.49 m N 
    Since f > Fcent, the coin will revolve along with the record.
    Case 2: Coin placed at 14 cm 
    Radius, r" = 14 cm = 0.14 m 
    Angular frequency, ω = 3.49 s–1 

    Frictional force, f' = 1.5 m N 
    Centripetal force is given as, 
                    Fcent. = mr"ω

                             = m × 0.14 × (3.49)2 
                             = 1.7m N 
    Since f < Fcent., the coin will slip from the surface of the record.
    Question 481
    CBSEENPH11020092

    A thin circular loop of radius rotates about its vertical diameter with an angular frequency ωShow that a small bead on the wire loop remains at its lowermost point for straight omega space less-than or slanted equal to space square root of straight g over straight R end rootWhat is the angle made by the radius vector joining the centre to the bead with the vertical downward direction for ω square root of fraction numerator 2 straight g over denominator straight R end fraction end root? Neglect friction.

    Solution
    Let the radius vector joining the bead with the centre make an angle θ, with the vertical downward direction.

                 
    Here,
    OP = R = Radius of the circle
      N = Normal reaction
    The respective vertical and horizontal equations of forces can be written as, 
    mg = N Cosθ                         ... (i) 

    mlω2 = Sinθ                       … (ii)
    In ΔOPQ, we have
    Sin θ = l / R 
    R Sinθ                              … (iii)

    Substituting equation (iii) in equation (ii), we get
     
    m(R Sinθω2 = N Sinθ 

    mR ω2 = N                            ... (iv) 

    Substituting equation (iv) in equation (i), we get
    mg = mR ω2 Cosθ 

    Cosθ = g / Rω2                        ...(v) 

    Since cosθ ≤ 1, the bead will remain at its lowermost point for g / Rω2 ≤ 1.
    i.e., for                      ω ≤ (g / R)1/2  
    For ω = (2g / R)1/2 
    rightwards double arrow      ω2 = 2g / R                  ...(vi) 
    On equating equations (v) and (vi), we get
      fraction numerator 2 straight g over denominator straight R end fraction = g / RCos θ  
     Cos θ = 1 / 2 
    ∴ θ = Cos-1(0.5)  =  600 , is the angle made by the radius vector joining the centre to the bead with the vertical downward direction. 
    Question 482
    CBSEENPH11020262

    Given in the figure are two blocks A and B of weight 20 N and 100 N respectively. These are being pressed against a wall by a force F as shown. If the coefficient of friction between the blocks is 0.1 and between block B and the wall is 0.15, the frictional force applied by the wall on block B is

    • 100N

    • 80 N

    • 120 N

    • 150 N

    Solution

    C.

    120 N

    In the vertical direction, weight are balanced by frictional forces.
    As the blocks are in equilibrium balance forces are in horizontal and vertical direction.
    For the system of blocks (A+B)
    F = N
    For block A, fA = 20 N and for block B.
    fB = fA +100 = 120 N

    Question 484
    CBSEENPH11020270

    A mass ‘m’ is supported by a massless string wound around a uniform hollow cylinder of mass m and radius R. If the string does not slip on the cylinder, with what acceleration will the mass fall on release?

    • 2g/3

    • g/2

    • 5g/6

    • g

    Solution

    B.

    g/2

    For the mass m,
    mg-T = ma

    As we know, a = Rα    ... (i)
    So, mg-T = mRα
    Torque about centre of pully
    T x R = mR2α ...... (ii)
    From Eqs. (i) and (ii), we get 
    a = g/2
    Hence, the acceleration of the mass of a body fall is g/2.

    Question 485
    CBSEENPH11020301

    A thin liquid film formed between a U-shaped wire and a light slider supports a weight of 1.5 x10–2N (see figure). The length of the slider is 30 cm and its weight negligible. The surface tension of the liquid film is

    • 0.0125 Nm-1

    • 0.1 Nm-1

    • 0.05 Nm-1

    • 0.025 Nm-1

    Solution

    D.

    0.025 Nm-1


    The force of surface tension acting on the slider balances the force due to the weight.

    ⇒F = 2Tl = w
    ⇒2T(0.3) = 1.5 x 10–2
    ⇒T = 2.5 x 10–2 N/m

    Question 486
    CBSEENPH11020304

    A liquid in a beaker has temperature θ(t) at time t and θ0 is temperature of surroundings, then according to Newton's law of cooling the correct graph between loge (θ – θ0) and t is

    Solution

    A.

    According to Newton's law of cooling.
    dθ over dt space proportional to left parenthesis straight theta minus straight theta subscript straight o right parenthesis
rightwards double arrow fraction numerator begin display style dθ end style over denominator begin display style dt end style end fraction space equals space minus straight k left parenthesis straight theta minus straight theta subscript straight o right parenthesis
integral fraction numerator begin display style dθ end style over denominator begin display style straight theta minus straight theta subscript straight o end style end fraction space equals space integral negative kdt space
rightwards double arrow space In space left parenthesis straight theta minus straight theta subscript straight o right parenthesis space equals space minus kt plus straight c
    Hence the plot of ln(θ – θ0) vs t should be a straight line with negative slope.

    Question 488
    CBSEENPH11020312

    Work done in increasing the size of a soap bubble from a radius of 3 cm to 5 cm is nearly. (Surface tension of soap solution = 0.03 Nm-1)

    • 4π mJ 

    • 0.2π mJ

    • 2π mJ

    • 0.4π mJ

    Solution

    D.

    0.4π mJ

    Work done = Change in surface energy
    ⇒ W = 2T x 4π (R22-R12)
     = 2 x 0.03 x 4π [ (5)2-(3)2] x 10-4
     = 0.4 π mJ

    Question 491
    CBSEENPH11020334

    Two fixed frictionless inclined planes making an angle 30° and 60° with the vertical are shown in the figure. Two blocks A and B are placed on the two planes. What is the relative vertical acceleration of A with respect to B?

    • 4.9 ms-2 in horizontal direction

    • 9.8 ms-2 in vertical direction

    • zero

    • 4.9 ms-2 in vertical direction

    Solution

    D.

    4.9 ms-2 in vertical direction

    For the motion of block along inclined plane
    mg sin θ =ma
    a = g sin θ
    where a is along the inclined plane.
    The vertical component of acceleration is g sin2θ
    Therefore, the relative vertical acceleration of  A with respect to B is 
    g (sin260 - sin230) = g/2 = 4.9 ms-2 (in vertical direction)

    Question 514
    CBSEENPH11020467

    An automobile travelling with speed of 60 km/h, can brake to stop within a distance of 20 cm. If the car is going twice as fast, i.e 120 km/h, the stopping distance will be

    • 20 m

    • 40 m

    • 60 m

    • 80 m

    Solution

    D.

    80 m

    Third equation of motion gives
    v2 = u2 + 2as ⇒ 2
    s ∝ u (since v = 0)
    where a = retardation of body in both the cases
    therefore, straight s subscript 1 over straight s subscript 2 space equals space fraction numerator straight u subscript 1 superscript 2 over denominator straight u subscript 2 superscript 2 end fraction .... (i)
    Here, s1 = 20 m, u1 = 60 km/h, u2 = 120 km/h. Putting the given values in eq. (i), we get
    20 over straight s subscript 2 space equals space open parentheses 60 over 120 close parentheses squared
space straight s subscript 2 space equals space 20 space straight x space open parentheses 120 over 60 close parentheses squared
space equals space 20 space straight x space 4 space
space equals space 80 space straight m

    Question 515
    CBSEENPH11020468

    A machine gun fires a bullet of mass 40 g with a velocity 1200 ms−1. The man holding it can exert a maximum force of 144 N on the gun. How many bullets can he fire per second at the most?

    • one 

    • Four

    • Two

    • Three

    Solution

    D.

    Three

    The force exerted by machine gun on man's hand in firing a bullet = change in momentum per second on a bullet or rate of change of momentum
    space equals open parentheses 40 over 1000 close parentheses space straight x space 1200 space equals space 48 space straight N
    The force exerted by man on machine gun = 144 N Hence, number of bullets fired =144/48 = 3

    Question 517
    CBSEENPH11020501

    It is found that if a neutron suffers an elastic collinear collision with deuterium at rest, fractional loss of its energy is pd; while for its similar collision with carbon nucleus at rest, fractional loss of energy is pc. The values of pd and pc are respectively :

    • (0,1)

    • (.89,.28)

    • (.28,.89)

    • (0,0)

    Solution

    B.

    (.89,.28)

    For collision of a neutron with deuterium:

    Applying conservation of momentum:

    mv + 0 = mv1 + 2mv2 .....(i)
    v2 -v1 = v ...... (ii)

    Therefore, Collision is elastic, e = 1

    From equ (i) and equ (ii) v1 = -v/3

    Pd = 12mv2 -12mv1212mv2 = 89 = 0.89

    Now, for the collision of neutron with carbon nucleus

    Applying conservation of momentum

    mv + 0 = mv1 + 12mv2 ....; (iii)

    v = v2-v1  ....(iv)

    v1 = -1113 vPc = 12mv2 - 12m1113v212mv2 = 48169 0.28

    Question 518
    CBSEENPH11020513

    A car is negotiating a curved road of radius R. The road is banked at angle straight theta. The coefficient of friction between the tyres of the car and the road is straight mu subscript straight s. The maximum safe velocity on this road is,

    • square root of gR open parentheses fraction numerator straight mu subscript straight s plus tan space straight theta over denominator 1 minus straight mu subscript straight s space tan space straight theta end fraction close parentheses end root
    • square root of straight g over straight R open parentheses fraction numerator straight mu subscript straight s plus tan space straight theta over denominator 1 minus straight mu subscript straight s space tanθ end fraction close parentheses end root
    • square root of straight g over straight R squared open parentheses fraction numerator mu subscript s plus tan theta over denominator 1 minus mu subscript s tan theta end fraction close parentheses end root
    • square root of g R squared open parentheses fraction numerator straight mu subscript straight s plus tanθ over denominator 1 minus straight mu subscript straight s tanθ end fraction close parentheses end root

    Solution

    A.

    square root of gR open parentheses fraction numerator straight mu subscript straight s plus tan space straight theta over denominator 1 minus straight mu subscript straight s space tan space straight theta end fraction close parentheses end root

    A car is negotiating a curved road of radius R. The road is banked at angle straight theta and the coefficient of friction between the tyres of car and the road is straight mu subscript straight s.
    The given situation is illustrated as:

    In the case of vertical equilibrium,
    N cos straight theta = mg + f1 sin straight theta
    rightwards double arrowmg = N cos straight theta space minus space straight f subscript 1 space sin space straight theta    ... (i)
    In the case of horizontal equilibrium,
    straight N space sin space straight theta space plus space straight f subscript 1 space cosθ space equals space mv squared over straight R  ... (ii)
    Dividing Eqns. (i) and (ii), we get
    straight v squared over Rg equals fraction numerator sin space theta space plus space mu subscript s space cos theta over denominator cos space theta space minus space mu subscript s space sin space theta end fraction space open square brackets f subscript 1 proportional to mu subscript s close square brackets

rightwards double arrow v space equals square root of R g open parentheses fraction numerator tan space theta space plus mu subscript s over denominator cos space theta space minus space mu subscript s space sin space theta space end fraction close parentheses end root
rightwards double arrow space v space equals space square root of R g open parentheses fraction numerator tan space theta space plus space mu subscript s over denominator 1 minus mu subscript s space tan space theta end fraction close parentheses end root

    Question 519
    CBSEENPH11020529
    Question 520
    CBSEENPH11020538

    Three blocks A, B, and C of masses 4 kg, 2 kg and 1 kg respectively, are in contact on a 14 N is applied to the 4 kg block, then the contact force between A and B is

    • 2 N

    • 6 N

    • 8 N 

    • 18 N 

    Solution

    B.

    6 N

    Given, mA = 4 kg
    mB = 2 kg 
    => mC =1 kg

    So total mass (M)  = 4+2+1 = 7 kg
    Now, F = Ma 
    14 = 7a
    a=2 m/s2

    F-F' = 4a
    F' = 14-4x2
    F' = 6N

    Question 523
    CBSEENPH11020550
    Question 525
    CBSEENPH11020565

    A system consists of three masses m1, m2 and m3 connected by a string passing over a pulley P. The mass m1 hangs freely and m2 and m3 are on a rough horizontal table (the coefficient of friction= straight mu). the pulley is frictionless and of negligible mass. The downward acceleration of mass m1 is,

    • fraction numerator straight g space left parenthesis 1 minus gμ right parenthesis over denominator straight g end fraction
    • fraction numerator 2 gμ over denominator 3 end fraction
    • fraction numerator straight g space left parenthesis 1 minus 2 straight mu right parenthesis over denominator 3 end fraction
    • fraction numerator straight g left parenthesis 1 minus 2 straight mu right parenthesis over denominator 2 end fraction

    Solution

    C.

    fraction numerator straight g space left parenthesis 1 minus 2 straight mu right parenthesis over denominator 3 end fraction

    First of all consider the forces on the blocks,

    For the 1st block,
    mg - T1 = m x a        ... (i)
    Let us consider second and third block as a system,
    Then,
    T12 μmg = 2m x a
    mg (1 space minus 2 straight mu) = 3m x a
    straight a space equals space 2 over 3 left parenthesis 1 minus 2 straight mu right parenthesis 

    Question 526
    CBSEENPH11020573

    The ratio of the accelerations for a solid sphere (mass m and radius R) rolling down an incline of angle straight theta without slipping and slipping down the incline without rolling is,

    • 5:7

    • 2:3

    • 2:5

    • 7:5

    Solution

    A.

    5:7

    A solid sphere rolling without slipping down an inclined plane is,

    We have in this case,
    a subscript 1 space equals space fraction numerator straight g space sin space straight theta over denominator 1 plus space begin display style straight k squared over straight R squared end style end fraction space equals space fraction numerator g space sin space theta over denominator 1 space plus open parentheses space begin display style bevelled 2 over 5 end style close parentheses end fraction
F o r space s o l i d space s p h e r e comma space K squared space equals space 2 over 5 R squared
space space space space space space equals space fraction numerator g space sin space theta over denominator begin display style bevelled 7 over 5 end style end fraction
rightwards double arrow space a subscript 1 space equals space 5 over 7 space g space sin space theta
    For a sphere slipping down an inclined plane,
    straight a subscript 2 space equals space straight g space sin space straight theta

That space is comma

space space space space space space straight a subscript 1 over straight a subscript 2 space equals space fraction numerator bevelled 5 over 7 space straight g space sin space straight theta over denominator straight g space sin space straight theta end fraction
rightwards double arrow space straight a subscript 1 over straight a subscript 2 space equals space 5 over 7
    Question 531
    CBSEENPH11020625
    Question 534
    CBSEENPH11020640

    Uniform magnetic field acting along AB. If the magnetic force on the arm BC Is F, the force on the arm AC is

    • -F
    • F
    • square root of 2 bold F
    • negative square root of 2 space bold F

    Solution

    A.

    -F
    FAB = 0
    FAB + FBC + FCA = 0
    FBC + FCA = 0
    FBC + FCA = 0
    FCA = - FBC = - F
    Question 535
    CBSEENPH11020663

    Two stone of masses m and 2m are whirled in horizontal circles, the heavier one in a radius r/2 and the lighter one in radius r. The tangential speed of lighter one in radius r. The tangential speed of lighter stone is n times that of the value of heavier stone when they experience same centripetal forces. The value of n is

    • 2

    • 3

    • 4

    • 1

    Solution

    A.

    2

    Given, that two stones of masses m and 2m are whirled in horizontal circles, the heavier one in a radius r/2 and lighter one in radius r as shown in a figure.


    As lighter stone is n times that of the value of heavier stone when they experience same centripetal forces, we get
    (Fc)heavier = (Fc)lighter
    fraction numerator 2 straight m left parenthesis straight v right parenthesis squared over denominator straight r divided by 2 end fraction space equals space fraction numerator straight m left parenthesis nv right parenthesis squared over denominator 2 end fraction

straight n squared space equals space 4

straight n space equals space 2

    Question 537
    CBSEENPH11020674
    Question 538
    CBSEENPH11020679

    A block of mass m is in contact with the cart C as shown in the figure


    The coefficient of static friction between the block and the cart is μ. The acceleration α of the cart that will prevent the block from falling satisfies

    • straight alpha space greater than mg over straight mu
    • straight alpha space greater than space straight g over μm
    • straight alpha space greater or equal than space straight g over straight mu
    • straight alpha space less than space straight g over straight mu

    Solution

    C.

    straight alpha space greater or equal than space straight g over straight mu

    When a cart moves with some acceleration toward right then a pseudo force (mα) acts on block towards left. The force (mα) is action force by a block on the cart. Now, block will remain static w.r.t cart if frictional force 
    μR space greater or equal than space mg

rightwards double arrow space straight mu space mα space greater or equal than space mg space space space left square bracket space as space straight R space equals space mα right square bracket

rightwards double arrow space straight alpha space greater or equal than space straight g over straight mu

    Question 542
    CBSEENPH11020716

    An explosion blows a rock into three parts. Two parts go off at right angles to each other. These two are 1 kg first part moving with a velocity of 12 ms-1 and 2 kg second part moving with a velocity of 8 ms-1.If the third part flies off with a velocity of 4 ms-,its mass would be

    • 5 kg 

    • 7 kg

    • 17 kg

    • 3 kg

    Solution

    A.

    5 kg 

    apply the law of conservation of linear momentum. 
    momentum of first part = 1 x 12 = 12 kg ms-1
    Momentum of the second part  = 2 x 8 = 16 kg ms-1 '
    Resultant monmentum
    = [(12)2 +(16)2]1/2 = 20 kg ms-1
    The third part should also have the same momentum.


    Let the mass of third part be M, then 
    4 x M = 20
    M = 5 kg
    Question 543
    CBSEENPH11020722

    The mass of a lift is 2000 kg. When the tension in the supporting cable is 28000 N, then its acceleration is 

    • 30 ms-2 downward

    • 4 ms-2  upwards

    •  4 ms-2  downwards

    • 14 ms-2  upwards

    Solution

    B.

    4 ms-2  upwards

    Apparent weight > actual weight, then the lift is accelerating upward.
    Lift is accelerating upward at the rate of a 
    Hence, equation of motion is written as
    R - mg = ma
    28000-20000 = 2000a
    a = 8000/2000 = 4 ms-2 upwards.

    Question 544
    CBSEENPH11020736

    A body under the action of a force,straight F with rightwards arrow on top space equals space 6 space bold i with bold hat on top space minus space 8 space bold j with bold hat on top space plus 10 space bold k with hat on top acquires an acceleration of 1 ms-2. The mass of this body must be

    • 2 square root of 10 space kg
    • 20 kg

    • 10 kg

    • 10 square root of 2 space kg

    Solution

    D.

    10 square root of 2 space kg

    According to Newton's second law of motion, force = mass x acceleration.
    Here, 
    straight F with rightwards arrow on top equals space 6 space bold i with bold hat on top space minus 8 space bold j with bold hat on top space plus 10 space bold k with hat on top
vertical line straight F vertical line space equals space square root of 36 plus 64 plus 100 end root

space equals space 10 square root of 2 space straight N
straight a space equals space 1 space ms to the power of negative 2 end exponent
straight m space equals space fraction numerator 10 square root of 2 over denominator 1 end fraction space equals 10 space square root of 2 straight g

    Question 547
    CBSEENPH11020753

    A shell of mass 200 g is ejected from a gun of mass 4 g by an explosion that generates 1.05 kJ of energy. The initial velocity of the shell is

    • 100 ms-1

    • 80 ms-1

    • 40 ms-1

    • 120 ms-1

    Solution

    A.

    100 ms-1

    In the given problem conservation of linear momentum and energy hold good.
    Conservation of momentum yields.
    m1v1 + m2v2 = 0
    4v1 + 0.2 v2  = 0
    Conservation of energy yields

    1 half straight m subscript 1 straight v subscript 1 squared space plus 1 half space straight m subscript 2 straight v subscript 2 squared space equals space 1050 space space space... left parenthesis straight i right parenthesis
or
1 half space straight x 4 straight v subscript 1 squared space plus space 1 half 0.1 straight v subscript 2 squared space equals space 1050 space space space.... space left parenthesis ii right parenthesis

solving space eqs. space left parenthesis straight i right parenthesis space and space left parenthesis ii right parenthesis space we space have
straight v subscript 1 space equals space 100 space straight m divided by straight s

    Question 549
    CBSEENPH11020772

    A block B is pushed momentarily along a horizontal surface with an initial velocity v. if μ is the coefficient of sliding friction between B  and the surface, block B will come to rest after a time:

    • v/gμ

    • gμ/v

    • g/v

    • v/g

    Solution

    A.

    v/gμ

    Block B will come to rest, if force applied to it will vanish due to frictional force acting between block B and surface, ie,
    force applied = frictional force
    i.e., μmg = ma
    μmg = m (v/t)
    t =v/μg

    Question 550
    CBSEENPH11020795

    300 J of work is done in sliding a 2 kg block up an inclined plane of height 10 m. Taking =10 m/s2, work done against friction is

    • 200 J

    • 100 J

    • Zero

    • 1000 J

    Solution

    B.

    100 J

    Net work done in sliding a body up to a height h on inclined plane
        = Work done against gravitational force + Work done against frictional force
    rightwards double arrow space space space space space straight W space equals space straight W subscript straight g plus straight W subscript straight f space space space space space space space space space space space... left parenthesis straight i right parenthesis
But space space space space space straight W space equals space 300 space straight J
space space space space space space straight W subscript straight g space equals space mgh space equals space 2 space cross times 10 cross times 10 space equals space 200 space straight J
putting space in space Eq. space left parenthesis straight i right parenthesis comma space we space get
space space space space space space space space space 300 space equals space 200 plus straight W subscript straight f
rightwards double arrow space space space space space space space space space space space space space straight W subscript straight f space equals space 300 space minus space 200 space equals space 100 space straight J
     

    Question 551
    CBSEENPH11020796

    A 0.5 kg ball moving with a speed of 12 m/s strikes a hard wall at an angle of 30° with the wall. It is reflected with the same speed and at the same angle. If the ball is in contact with the wall for 0.25 s, the average force acting on the wall is

    • 48 N

    • 24 N

    • 12 N

    • 96 N

    Solution

    B.

    24 N

    The vector OA with rightwards arrow on top represents the momentum of the object before the collision, and the vector OB with rightwards arrow on top that after the collision. The vector AB with rightwards arrow on top represents the change in momentum of the object increment straight P with rightwards arrow on top.


    As the magnitudes of  OA with rightwards arrow on top space and space OB with rightwards arrow on top are equal, the components of OA with rightwards arrow on top space and space OB with rightwards arrow on top along the wall are equal and in the same direction, while those perpendicular to the wall are equal and opposite. Thus, the change in momentum is due only to the change in direction of the perpendicular components.
    Hence,  increment straight p space equals space OB space sin space 30 degree space minus space left parenthesis negative OA space sin space 30 degree right parenthesis
                        equals mv space sin space 30 degree space minus space left parenthesis negative mv space sin space 30 degree right parenthesis
equals 2 space mv space sin space 30 degree
    Its time rate will appear in the form of average force acting on the wall.
     therefore space space space space space space space space space straight F space cross times space straight t space equals space 2 mv space sin space 30 degree
or space space space space space space space space space space space space space space straight F space equals space fraction numerator 2 mv space sin space 30 degree over denominator straight t end fraction
       Given comma space straight m space equals space 0.5 space kg comma space space straight v space equals space 12 space straight m divided by straight s comma space space space straight t equals space 0.25 space straight s
space space space space space space space space space space straight theta space equals space 30 degree
Hence comma space space space straight F space equals space fraction numerator 2 cross times 0.5 cross times 12 space sin space 30 degree over denominator 0.25 end fraction space equals space 24 space straight N

    Question 552
    CBSEENPH11020797

    The moment of inertia of a uniform circular disc of radius R and mass M about an axis touching the disc at its diameter and normal to the disc is

    • MR squared
    • 2 over 5 MR squared
    • 3 over 2 MR squared
    • 1 half MR squared

    Solution

    C.

    3 over 2 MR squared

    The moment of inertia about an axis passing through centre of mass of disc and perpendicular to its plane is
                         straight I subscript CM space equals space 1 half MR squared
    where M is the mass of disc and R its radius. According to theorem of parallel axis, MI of circular disc about an axis touching the disc at it diameter and normal to the disc is
                       straight I equals space straight I subscript CM space plus space MR squared
                         equals 1 half MR squared plus space MR squared
equals 3 over 2 MR squared

    Question 553
    CBSEENPH11020806

    A uniform rod of length l and mass m is free to rotate in a vertical plane about A. The rod initially in horizontal position is released. The initial angular acceleration of the rod is: (Moment of inertia of rod about A is fraction numerator straight m l squared over denominator 3 end fraction)

    • fraction numerator 3 straight g over denominator 2 straight l end fraction
    • fraction numerator 2 straight l over denominator 3 straight g end fraction
    • fraction numerator 3 straight g over denominator 2 straight l squared end fraction
    • mg straight l over 2

    Solution

    A.

    fraction numerator 3 straight g over denominator 2 straight l end fraction

    The moment of inertia of the uniform rod about an axis through the end and perpendicular to length is
               straight I space equals space fraction numerator straight m l squared over denominator 3 end fraction
    where m is mass of rod and l its length.
    Torque left parenthesis straight t space equals space Iα right parenthesis acting on centre of gravity of rod is given by
                      straight t equals mg l over 2
    or               Iα space equals space mg l over 2
    or space space fraction numerator straight m l squared over denominator 3 end fraction straight alpha space equals space mg l over 2
therefore space space space space straight alpha space equals fraction numerator 3 straight g over denominator 2 l end fraction

     

    Question 556
    CBSEENPH11020849

    A body initially at rest and sliding along a frictionless track from a height h (as shown in the figure) just completes a vertical circle of diameter AB = D. The height h is equal to

    • 3D/2

    • D

    • 5D/4

    • 7D/5

    Solution

    C.

    5D/4

    As track is frictionless, so total mechanical energy will remain constant,

    i.e., 0 + mgh = 12mvL2 + 0using v2 - u2 = 2gh,h = vL22g ( u = 0)For completing the vertical circle VL 5gRor, h = 5gR2g = 52R = 54D

    Question 557
    CBSEENPH11020851
    Question 558
    CBSEENPH11020852

    A moving block having mass m, collides with another stationary block having mass 4m. The lighter block comes to rest after collision. When the initial velocity of the lighter block is v, then the value of coefficient of restitution (e) will be

    • 0.5

    • 0.25

    • 0.4

    • 0.8

    Solution

    B.

    0.25

    According to law of conservation of linear momentum,

    mv + 4m x 0 = 4mv' + 0

    v' = v4e = relative velocity of separation Relative velocity of approach =v4ve = 14 = 0.25

    Question 560
    CBSEENPH11020859

    A cylindrical tube of uniform cross-sectional area A is fitted with two air tight frictionless pistons. The pistons are connected to each other by a metallic wire. Initially, the pressure of the gas is p0 and temperature is T0, atmospheric pressure is also p0. Now, the temperature of the gas is increased to 2T0, the tension of the wire will be

    • 2p0A

    • p0A

    • p0A/2

    • 4p0A

    Solution

    B.

    p0A

    The volume of the gas is constant i.e., V = constant

    ∴ p ∝ T i.e pressure will be doubled if the temperature is doubled.

    Let F be the tension in the wire. Then, the equilibrium of anyone pipes gives.

    F = (p-p0) A = 2(p0 - p0) = p0A

    Question 561
    CBSEENPH11020864

    A skier starts from rest at point A and slides down the hill without turning or breaking. The friction coefficient is μ. When he stops at point B, his horizontal displacement is S. what is the height difference between points A and B?

    (The velocity of the skier is small so that the additional pressure on the snow due to the curvature can be neglected. Neglect also the friction of air and the dependence of μ on the velocity of the skier.)

    • h = μS

    • h = μ/S

    • h = 2μS

    • h = μS2

    Solution

    A.

    h = μS

    According to the question, the condition is shown in the figure

    For a sufficiently safe-horizontal displacement △S can be considered straight. If the corresponding length of path element is △L, the friction force is given by μmg(△S/△L ).△L =  μmg△S

    Adding up, we find that along the whole path the total work done by the friction force is μmgs. By energy conservation, this must equal the decrease mgh in potential energy of skier.

    Hence, h = μS

     

    Question 563
    CBSEENPH11020869

    The upper half of an inclined plane of inclination θ is perfectly smooth while the lower half rough. A block starting from rest at the top of the plane will again come to  rest at the bottom if the coefficient of friction between the block and the lower half of the plane is given by

    • μ = 2 tanθ

    • μ = tanθ

    • μ = 2/(tan θ)

    • μ = 1/ tan θ

    Solution

    A.

    μ = 2 tanθ

    suppose the length of plane is L. When block descends the plane, rise in kinetic energy = fall in potential energy = mg L sinθ i

    Work done by friction = μ x (reaction) x distance 

    0 + μ (mg cos θ) x (L/2)

    =  μ (mg cos θ) x (L/2)

    Now, work done = change in KE

    mgL sin θ = μ (mg cos θ) x (L/2)

    ⇒ tan θ  = μ/2 

     μ = 2 tan θ

    Question 564
    CBSEENPH11020870

    Two masses 10 kg and 20 kg respectively are connected by a massless springs as shown in the figure. A force of 200 N acts on the 20 kg mass. At the instant shown is a figure the 10 kg mass has an acceleration of 12 m/s2. The value of the acceleration of 20 kg mass is

    • 4 m/s2

    • 10 m/s2

    • 20 m/s2

    • 30 m/s2

    Solution

    A.

    4 m/s2

    The equation of motion of m1 = 10 kg mass is

    F1 = m1a1 = 10 x 12  = 120 N

    Force on 10 kg mass is 120 N to the right. As action and reaction are equal and opposite, the reaction force F- on 20 kg mass F = 120 N to the left.

    therefore, equation of motion of mass m2 = 20 kg is

    200 - F = 20 a2

    200-120 = 20a2

    80 = 20a2

    a2 = 80 /20 = 4 m/s2

    Question 566
    CBSEENPH11020879
    Question 567
    CBSEENPH11024908

    State Hooke's Law.

    Solution

    Hooked law states that for a small force, Stress is directly proportional to strain. the constant of proportionality is the modulus of elasticity

    Question 568
    CBSEENPH11024909

    State Boyle's Law

    Solution

    Boyle’s law is a gas law which states that the pressure exerted by a gas (of a given mass, kept at a constant temperature) is inversely proportional to the volume occupied by it. In other words, the pressure and volume of a gas are inversely proportional to each other as long as the temperature and the quantity of gas are kept constant. Boyle’s law was put forward by the Anglo-Irish chemist Robert Boyle in the year 1662.

    For a gas, the relationship between volume and pressure (at constant mass and temperature) can be expressed mathematically as follows.

    P ∝ (1/V)

    Where P is the pressure exerted by the gas and V is the volume occupied by it. This proportionality can be converted into an equation by adding a constant, k.

    P = k*(1/V) ⇒ PV = k

    The pressure v/s volume curve for a fixed amount of gas kept at constant temperature is illustrated below.

    It can be observed that a straight line is obtained when the pressure exerted by the gas (P) is taken on the Y-axis and the inverse of the volume occupied by the gas (1/V) is taken on the X-axis.

    Question 569
    CBSEENPH11024910

    Action and reaction forces do not balance each other why?

    Solution

    They do not balance each other as the action and reaction forces acts on two different bodies.

    Question 570
    CBSEENPH11026112

    A cone filled with water is revolved in a vertical circle of radius 4 m and the water does not fall down. What must be the maximum period of revolution?

    • 2 s

    • 4 s

    • 1 s

    • 6 s

    Solution

    B.

    4 s

    When a cone filled with water is revolved in a vertical circle, then the velocity at the highest point is given by

    ν g ralso ν = rω =r2πTHence, 2πrTr gT2πrr g=2πrg=2π49.8=2×3.14×0.6389=4.0 secso Tmax =4 sec

    Question 571
    CBSEENPH11026130

    Which is true for rolling frictrion   μr , static friction μs  and  kinetic friction  μk :

    • μs > μk >  μr

    • μs <  μk <  μr

    • μs <  μk >  μr

    • μs > μr  > μk

    Solution

    A.

    μs > μk >  μr

    Static friction is a force that keeps an object at rest. It must be overcome to start moving the object. When external force exceeds the maximum limit of static friction body begins to move.
     
    Once the body is in motion it is subjected to sliding or kinetic friction which opposes relative motion between the two surfaces in contact. At every instant, there is just one point of contact
    between the body and the plane and this point has no motion relative to the plane. In this ideal situation, kinetic or static friction is zero and the body should continue to roll with constant velocity. 
    In practice, this will not happen and some resistance to motion (rolling friction) does occur, i.e. to keep the body rolling. Static friction is always greater than kinetic friction and rolling friction is less than kinetic friction.
                            μs >  μk > μr
    Question 572
    CBSEENPH11026131

    Two weights w1 and w2 are suspended to the two strings on a frictionless pulley. When the pulley is pulled up with an acceleration g, then the tension in the string is :

    • 4 w1w2w1 + w2

    • w1w2w1 + w2

    • 2 w1 w2w1 w2

    • w1 + w22

    Solution

    A.

    4 w1w2w1 + w2

    Equation of motion for first weight

    This is a frictionless and inextensible pulley.

    for first weight

    T - m1g = m1 ( a - g)

    ⇒ T - 2m1 g = m1a

    For second weight 

    m2g -T = m2 ( a - g)

    2 mg - T = m2 a

    on solving equations (1) & (2)

    T =4 m1 m2m1 + m2T = 4 m1 m2w1 + w2

    Question 573
    CBSEENPH11026153

    A bomb of mass 30 kg at rest explodes into two pieces of masses 18 kg and 12 kg. The velocity of 18 kg mass is 6 ms'.The kinetic energy of the other mass is

    • 256 J

    • 486 J

    • 524 J

    • 324 J

    Solution

    B.

    486 J

    The linear momentum of the exploding part will remain conserved.

    Applying coservation of linear momentum,

    m1 u1 =m2 u2

    Here, m1 = 18kg, m2 = 12kg

    u1 = 6ms-1, u2 = ?

    u2 = 18×612 9 ms-1

    Thus kinetic energy of 12 kg mass

    K2 =12m1u22       = 12×12×92       = 6× 81K2   = 486 J

    Question 574
    CBSEENPH11026154

     A drum of radius R and mass M, rolls down without slipping along an inclined plane of angle θ. The frictional force

    • converts translational energy to rotational energy

    • dissipates energy as heat

    • decreases the rotational motion

    • decreases the rotational and translational motion

    Solution

    A.

    converts translational energy to rotational energy

    When a body rolls down without slipping along an inclined plane of inclination θ, it rotates about a horizontal axis through its centre of mass and also its centre of mass moves. Therefore, rolling motion may be regarded as a rotational motion about an axis through its centre of mass plus a translational motion of the centre of mass. As it rolls down, it suffers loss in gravitational potential energy provided translational energy due to frictional force is converted into rotational energy.

    Question 575
    CBSEENPH11026176

    A block B is pushed momentarily along a horizontal surface with an initial velocity v. If µ is the coefficient of sliding friction between B and the surface, block B will come to rest after a time

    • ν

    • ν

    • gν

    • νg

    Solution

    A.

    ν

    Block B will come to rest, if force applied to it will vanish due to frictional force acting between block B and surface, ie,
      force applied  =  frictional force

       μmg = mα

    where μ - constant of proportionality 

     μmg =  μmg = m νt      t   = νμg

    Question 576
    CBSEENPH11026187

    Diwali rocket is ejecting 50 g of gases/s at a velocity of 400 m/s. The accelerating force on the rocket will be

    • 22 dyne

    • 20 N

    • 20 dyne

    • 100 N

    Solution

    B.

    20 N

    The accelerating force on the rocket

         = upward thrust = mt.u

    Given:-

    mt = 50 ×10-3 kg/s , 

    ν  = 400 m/s

    Accelerating force = 50 × 10 ×  400

                               = 20 N

    Question 577
    CBSEENPH11026213

    A weightless thread can bear tension upto 37 N. A stone of mass 500 g is tied to it and revolved in a circular path of radius 4 min a vertical plane. If g = 10 ms-2,  then the maximum angular velocity the stone will be

    • 2 rad s-1

    • 4 rad s-1

    • 8 rad s-1

    • 16 rad s-1

    Solution

    B.

    4 rad s-1

    According to second law,  the force f provding the acceleration is 

                F = mv2R

    where m is the mass of the body. This force directed towards the centre is called the centripetal force. For a stone rotated in a circle by a string, the centripetal force is provided by the string.

    Maximum tension in the thread is given by 

               Tmax = mg + mv2r

    ⇒         Tmax = mg + mrω2               (  v = r ω )

                  ω2Tmax - mgmr

    Given

             Tmax = 37 N,   m = 500 g = 0.5 kg,

              g = 10 ms-2,    r = 4 m

    ∴              ω2 = 37 - 0.5 × 100.5 × 4

                    ω2 = 37 - 52

    ⇒              ω2 = 16

    ⇒              ω = 4 rad s-1

    Question 578
    CBSEENPH11026238

    A bullet of mass 20 g and moving with 600 m/s collides with a block of mass 4 kg hanging with the string. What is velocity of bullet when it comes out of block, if block rises to height 0.2 m after collision?

    • 200 m/s

    • 150 m/s

    • 400 m/s

    • 300 m/s

    Solution

    A.

    200 m/s

    Linear momentum is a product of the mass (m) of an object and the velocity (v) of the object.

    According to conservation of linear momentum

              m1 v1 = m1 v + m2 v2

    where v1 is the velocity of the bullet before the collision, v  is the velocity of bullet after the collision and v2 is the velocity of the block.

    ∴     0.02 × 600 = 0.02 v + 4 v2

    We have

           v2 = 2gh

               = 2 × 10 × 0.2

           v2 = 2 m/s

    ∴      0.02 × 600 = 0.02 v + 4 × 2

    ⇒             0.02 v = 12 - 8

    ⇒                    v = 40.02

    ⇒                      v = 200 m/s

    Question 579
    CBSEENPH11026239

    Voltage in the secondary coil of a transformer does not depend upon

    • frequency of the source

    • voltage in the primary coil

    • ratio of number of turns in the two coils

    • Both (b) and (c)

    Solution

    A.

    frequency of the source

    NPNS = VPVS = n = Turns ratio

    Ratio of number of  turns in the two coils results induced emf. 

    Voltage in the primary coil also affects the induced emf.

    Question 580
    CBSEENPH11026251

    A shell of mass 20 kg at rest explodes into two fragments whose masses are in the ratio 2 : 3. The smaller fragment moves with a velocity of 6 m/s. The kinetic energy of the larger fragment is

    • 96 J

    • 216 J

    • 144 J

    • 360 J

    Solution

    A.

    96 J

    Total mass of the shell = 20 kg

    Ratio of the masses of the fragments are 8 kg and 12 kg 

    Now according to the conservation of momentum 

                 m1 v1 = m2 v2

    ∴            8 × 6 = 12  × v

    v (velocity of the larger fragment) = 4 m/s

      Kinetic energy = 12mv2

                            = 12 × 12 × 42

       Kinetic energy = 96 J

    Question 581
    CBSEENPH11026252

    An electric bulb has a rated power of 50 W at 100 V. If it is used on an AC source 200 V, 50 Hz, a choke has to be used in series with it. This choke should have an inductance of

    • 0.1 mH

    • 1 mH

    • 0.1 mH

    • 1.1 H

    Solution

    D.

    1.1 H

    Power P = I × V

                 = V R × V

               P = V2 R

    Resistance of bulb

              R = V2P

               R = 100250

                   = 200 Ω 

    Current through bulb

                (I) = VR

                     = 100200

                  I  =  0.5 A

    In a circuit containing inductive reactance (XL ) and resistance (R ), impedance (Z) of the circuit is

                 Z = R2 + ω2 L2              ...... (i)

                 Z = 2000.5

                 Z = 400 Ω

    Now,      XL2 = Z2 - R2

                 XL2  = ( 400 )2  - ( 200 )2

               2 πf L2 = 12 × 104

                 L = 23 × 1002π × 50

                      = 23 π

                  L = 1.1 H

    Question 582
    CBSEENPH11026254

    Three concurrent co-planar forces 1 N, 2 N and 3 N acting along different directions on a body

    • can keep the body in equilibrium if 2 N and 3 N act at right angle

    • can keep the body in equilibrium if 1 N and 2 N act at right angle

    • cannot keep the body in equilibrium

    • can keep the body in equilibrium in 1 N and 3 N act at an acute angle

    Solution

    C.

    cannot keep the body in equilibrium

    If we keep 1 N and 2 N forces act in the same direction then these are balanced by 3 N force, but this is against statement of question.

    Hence, options (c) is correct.

    Question 583
    CBSEENPH11026268

    Two rectangular blocks A and B of masses 2 kg and 3 kg respectively are connected by a spring of constant 10.8 Nm-1 and are placed on a frictionless horizontal surface. The block A was given an initial velocity of 0.15 ms-1 in the direction shown in the figure. The maximum compression of the spring during the motion is

            

    • 0.01 m

    • 0.02 m

    • 0.05 m

    • 0.03 m

    Solution

    C.

    0.05 m

    As the block A moves with velocity 0.15 ms-1  it compresses the spring which pushes B towards right. A goes on compressing the spring till the velocity acquired by B becomes equal to the velocity of A, i.e., 0.15 ms-1.Let this velocity be v.

    Now, spring is in a state of maximum compression. Let x be the maximum compression at this stage.

               

    According to the law of conservation of linear momentum, we get

                 mA u = mA + mB v

    ⇒               v = mA umA + mB        

    ⇒                   = 2 × 0.152 + 3

                      v = 0.06 ms-1

    According to the law of conservation of energy

             12 m A u2 = 12 mA +  mB v2 + 12 k x2

              12 mA u2 - 12 mA  +  mB v2 = 12 k x2   

       12× 2 × 0.152 - 12 2 + 3 0.062  = 12  k x2

                             0.0225 - 0.009 = 12 k x2

                              0.0135 = 12 k x2

    ⇒                              x = 0.027k

    ⇒                                 = 0.02710.8

                               x = 0.05 m

    Question 584
    CBSEENPH11026274

    A boy on a cycle pedals around a circle of 20 metres radius at a speed of 20 m/s. The combined mass of the body and the cycle makes with the vertical so that it may not fall is (g = 9.8 m/s2 )

    • 60.25o

    • 63.90o

    • 26.12o

    • 30.00o

    Solution

    B.

    63.90o

    A body that travels an equal distance in equal amounts of time along a circular path has a constant speed but not constant velocity. This is because velocity is a vector and thus it has magnitude as well as direction.

                   v = ( Rg tanθ )1/2

                   tan θ = v2r g

                   = 40020 × 900

             tan θ = 63.70o 

             tanθ ≈ 63.90o

    Question 585
    CBSEENPH11026281

    A car of mass 1000 kg moves on a circular track of radius 40 m. If the coefficient of friction is 1.28. The maximum velocity with which the car can be moved, is

    • 22.4 m/s

    • 112 m/s

    • 0.64 × 401000 × 100m/s

    • 1000 m/s

    Solution

    A.

    22.4 m/s

    A car moves in a circular track so it perform circular motion. 

    According to second law the force providing this acceleration is

               fcmv2R

    But according to static friction

             fs ≤ μs N

         f = m v2R  μs N

         vmax = μ R g

    Given:-  μ = 1.28 

    where μ is  the coefficient of friction 

      R = 40 m 

    The maximum velocity

              vmaxμ R g 

                       = 1.28 × 40 × 9.8

                 vmax = 22.4 m/s 

    Question 586
    CBSEENPH11026288

    A moving body of mass m and velocity 3 km/h collides with a rest body ofmass 2 m and stick to it. Now the combined mass starts to move. What will be the combined velocity?

    • 4 km/h

    • 1 km/h

    • 2 km/h

    • 3 km/h

    Solution

    B.

    1 km/h

    The law of conservation of momentum states that for two objects colliding in an isolated system, the total momentum before and after collision is equal.

    Applying law of conservation of momentum

                 m1 v1 = MV

    [ m2 v= 0 because v2 = 0 and M = m1 + m2 , V = final velocity 

                 m × 3 = ( m + 2m ) v1

     So,          v1 = 1 km/h

    Question 587
    CBSEENPH11026316

    A block B is pushed momentarily along a horizontal surface with an initial velocity v. If is the coefficient of vlicling friction between B and the surface, block B will come to rest after a time

       

    • vg μ

    • g μv

    • gv

    • vg

    Solution

    A.

    vg μ

    Block will come to rest, if force applied on it. It will vanish due to frictional force acting between block Band surface, i.e.,

        Force applied = Frictional force

    ⇒     μ mg  = ma

    ⇒      μ mg = m vt

    ⇒            t = vμ g

    Question 588
    CBSEENPH11026324

    A block slides down on an incline of angle 30° with an acceleration g4, Find the kinetic 4 friction coefficient.

    • 122

    • 0.6

    • 12 3

    • 12

    Solution

    C.

    12 3

    Consider the situation

       

    Let the mass of block be m. 

    The equation of forces

                mg sin 30o - f = mg4

                                 f  =   mg 12 - mg4              .....( since sin30o12 )

                                  f = m g4 

    Also, N = mg cos 30° = mg 32

    As the block is slipping on the incline, friction is

                   μn = FN

                        = mg4 mg 32

                   μn = 123

    Question 589
    CBSEENPH11026339

    A pendulum having a bob of mass m is hanging in a ship sailing along the equator form east to west. When the strip is stationary with respect to water, the tension in the string is T0, The difference between T0 and
    earth attraction on the bob, is

    • mg + m ω2 R2

    • 2 R3

    • 2 R2

    • 2 R

    Solution

    D.

    2 R

    The attraction of earth on the bob = Weight of bob = mg

    Speed of ship due to rotation of the earth

    Velocity = angular velocity × radius

          v = ω R

    Also due to the rotation of the earth, a centripetal force acts on the bob = mω2 R on earth 

    The net force

        T0 = mg  + mω2 R

    ⇒  T0 - mg = mω2 R

    Question 590
    CBSEENPH11026342

    A ball of mass m hits the floor with a speed v making an angle of incidence θ with the normal. The coefficient of restitution is e. The speed of reflected ball and the angle of reflection of the ball will be

         

    • v' = v, θ = θ'

    • v' = v2, θ = 2θ'

    • v' = 2v, θ = 2θ'

    • v' = 3v2, θ = 2θ'3

    Solution

    A.

    v' = v, θ = θ'

    The coefficient of restitution is a number which indicates how much kinetic energy ( energy of motion ) remains after collision of two object.

    The parallel component of velocity of the ball remains unchanged. This gives

               v' sinθ = v sinθ                   .... (i)

    For the components normal to the floor, the velocity of separation = v' cosθ

    Hence  v' cosθ' = e v cosθ               .....(ii)

    From equations (i) and (ii)

                v' = sin2θ + e2 cos2θ

    and       tanθ' = tanθe

    For elastic collision, e = 1, so that 

              θ' = θ  and  v' = v

    Question 591
    CBSEENPH11026343

    A particle slides on surface of a fixed smooth sphere starting from top most point. The angle rotated by the radius through the particle, when it leaves contact with the sphere, is

    • θ =cos-1 13

    • θ = cos-1 23

    • θ = tan-1 13

    • θ = sin-1 43

    Solution

    B.

    θ = cos-1 23

    See the diagram

       

    Let the velocity be v when the body leaves the surface. From free body diagram,

                         mv2 R = mg cosθ

    ⇒                    v2 = R g cosθ

    Again from work-energy  principle

              change in KE = work done

    ⇒             12 mv2 - 0 =  mg ( R -R cosθ )  

                    v2 = 2gR ( 1 - cosθ)

    From Eqs. (i) and (ii)

              Rg cosθ = 2gR ( 1 - cosθ )

    ⇒           3 gR cosθ = 2gR

    ⇒              θ = cos-123

    Question 592
    CBSEENPH11026347

    A monkey of mass 15 kg is climbing on a rope with one end fixed to the ceiling. If it wishes to go up with an acceleration 1 m/s, how much force should it apply to the rope if rope is 5 m long and the monkey starts from rest?

    • 150 N

    • > 160 N

    • 165 N

    • 150 < T ≤ 160 N

    Solution

    C.

    165 N

    The mass of monkey = 15 kg

       Acceleration     a = 1 m/s2

    For the motion of the monkey T - [ 15 g + 15 (1) ]

    Hence, T is tension in the string T = 15 g + 15

                  T = 15 ( 10 + 1)

                      = 15 × 11

                 T = 165 N

    The monkey should apply 165 N force to the rope.

    Question 593
    CBSEENPH11026350

    A body weighing 8 g when placed in one pan and 18 g when placed on the other pan of a false balance. If the beam is horizontal when both the pans are empty, then the true weight of the body is

    • 13 g

    • 9 g

    • 22 g

    • 12 g

    Solution

    D.

    12 g

    Consider the diagram in balance

           

             8x = wy

    ⇒        xy = w8                       ...(i)

    Also,  wx = 18 y

    ⇒         xy = 18w                     ....(ii)

    Dividing Eq. (i) and Eq. (ii) we get

                xyxy = w818w

    ⇒         1 = w218 × 8

    ⇒          w = 18 × 8

                 w = 12 g

    Question 594
    CBSEENPH11026352

    Universal time is based on

    • rotational effect of the earth about its axis

    • vibrations of cesium atom

    • orbital motion of the earth around the sun

    • oscillation of quartz crystal

    Solution

    A.

    rotational effect of the earth about its axis

    Time defined in terms of the rotation of the earth is called Universal Time (UT).

    Question 595
    CBSEENPH11026380

    A liquid of density 800  kg/m3 is filled in a cylindrical vessel upto a height of 3 m. This cylindrical vessel stands on a horizontal plane. There is a circular hole on the side of the vessel. What should be the minimum diameter of the hole to move the vessel on the
    floor, if plug is removed. Take the coefficient of friction between the bottom of the vessel and the plane as 0.5 and total mass of vessel plus vessel as 95 kg.

    • 0.107 m

    • 0.053 m

    • 0.206 m

    • 0.535 m

    Solution

    A.

    0.107 m

    Given:-

    ρ = 800 kg/m3

    h = 3 m

    μ = 0.5 

    m = 95 kg

    dmin = ?

        

    Let area of hole be a

    ∴ Reaction force F = ρ av2

                               = ρa. 2gh                            [v =2g h ]

     and fmax = μN

                   = μ mg

    μ= coefficient of friction

    ⇒          F ≥ fmax

    ⇒          2 ρ agh ≥ μ mg

    ⇒             a ≥ μ m2 ρ h

                    = 0.5 × 952 × 800 × 3

                a = 0.009

                 π r2 ≥ 0.009

                r ≥ 0.009π

            rmin ≥ 0.0535 m

         dmin = 2 rmin 

                 = 2 × 0.0535

         dmin = 0.107 m

    Question 597
    CBSEENPH11026403

    Assertion:  Pseudo force is an imaginary force which is recognised only by a non-inertial observer to explain the physical situation according to Newton's laws.

    Reason: Pseudo force has no physical origin, i.e., it is not caused by one of the basic interactions in nature. It does not exist in the action-reaction pair.

    • If both assertion and reason are true and reason is the correct explanation of assertion.

    • If both assertion and reason are true but reason is not the correct explanation of assertion.

    • If assertion is true but reason is false.

    • If both assertion and reason are false.

    Solution

    B.

    If both assertion and reason are true but reason is not the correct explanation of assertion.

    Pseudo force is an imaginary force which is recognised only by a non-inertial observer to explain the physical situation according to Newton's laws. The magnitude of this force F is equal to the mass m of the object and acceleration α of the frame of reference. The direction of force is opposite to the direction of acceleration.

          F = - m α

    Question 598
    CBSEENPH11026433

    Assertion:  When a charged particle moves in a circular path. It produces electromagnetic wave.

    Reason:  Charged particle has acceleration

    • If both assertion and reason are true and reason is the correct explanation of assertion.

    • If both assertion and reason are true but reason is not the correct explanation of assertion.

    • If assertion is true but reason is false.

    • If both assertion and reason are false

    Solution

    A.

    If both assertion and reason are true and reason is the correct explanation of assertion.

    Magnetic forces can cause charged particles to move in circular or spiral paths. Particle accelerators keep protons following circular paths and magnetic force. The magnetic force is perpendicular to the velocity and so velocity changes in direction but not in magnitude.

    Accelerated charges radiate electromagnetic waves.

    Question 599
    CBSEENPH11026446

    The radii of circular paths of two particles of same mass are in ratio 6 : 8 then what will be velocities ratio if they have a constant centripetal force?

    • 3 : 4

    • 4  : 3

    • 2  : 3

    • 3  :  2

    Solution

    D.

    3  :  2

    The radii of circular paths of two particles of same mass m1 = m2 and ratio is given by

                  r1r2 = 68

                  m1m2 = 1

    Centripetal force is a force that acts on a moving body in a circular path and is directed towards the centre around which the body is moving.

           m1 v1 2r1 = m2 v2 2r2

    ⇒        v1 2v2  2 = r1r2 × m2m1

                     = 68 × 1

    ∴         v1v2 = 32

    Question 600
    CBSEENPH11026457

    Assertion:  Use of ball bearing between two moving parts of a machine is a common practice. 

    Reason:  Ball bearing reduces rolling friction and provides good stability.

    • If both assertion and reason are true and reason is the correct explanation of assertion.

    • If both assertion and reason are true but reason is not the correct explanation of assertion. 

    • If assertion is true but reason is false.

    • If both assertion and reason are false.

    Solution

    A.

    If both assertion and reason are true and reason is the correct explanation of assertion.

    A bearing is a machine element that constrains relative motion to only the desired motion, and reduces friction between moving parts. The simplest form of bearing, the plain bearing, consist of a shaft rotating in a hole.

    Question 601
    CBSEENPH11026462

    Assertion:  Centripetal force is always required for motion in curved path. 

    Reason:  On a banked curved track, vertical component of normal reaction provides the necessary centripetal force.

    • If both assertion and reason are true and reason is the correct explanation of assertion.

    • If both assertion and reason are true but reason is not the correct explanation of assertion.

    • If assertion is true but reason is false.

    • If both assertion and reason are false.

    Solution

    C.

    If assertion is true but reason is false.

    While taking a turn on banked road, the necessary centripetal force is provided by the horizontal component of normal reaction and vertical component balances the weight of the vehicle. Also roads are usually banked for average speed of vehicles passing over them. However, ifthe speed of a vehicle is somewhat less or more than this, the self adjusting static friction will operate between the tyres and the road.

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