Science Chapter 12 Friction

relative motion

nature

heat

reduces

less

C.

Static, sliding, rolling

A.

Wet marble floor, dry marble floor, newspaper and towel.

Frictional force will act opposite to the movement of book i.e. upwards and along the plane of the desk. 

When soap water is spilled on the floor, the friction exerted by the floor is reduced. Hence, the feet will not get a proper grip on the floor and one tends to slip, making it difficult to walk.

Sportsmen use shoes with spike to increase the friction between their feet and the ground so that their shoes do not slip while they run or play. 

Seema will apply a larger force, because heavier objects generates more frictional force due to the two surfaces being pressed together harder.

For an object which is sliding, the contact point on the two surfaces, do not get enough time to lock into the contact points on the floor. Hence, there is less resistance to motion.
So, the sliding friction is smaller than the static friction. 

Friction is both a friend and a foe. This can be shown in the following below cases.

Friction is a friend because:

1. It allows us to walk wihtout slipping on ground.

2. It allows us to have a grip and catch different objects.

3. We can use transportation because of friction. 

4. Speeds of moving objects can be controlled using friction. 

Friction is a foe because :

1. It causes wear and tear in objects.

2. It causes wind and water erosion. 

3. Moving heavy objects consumes more energy because additional energy is required to move the object against the direction of friction.

4. All moving parts of machines need lubrication for proper functioning. 

Objects moving in fluids must have a special shape called streamlined shape.

Streamlining is done to reduce the friction in the fluid so that the boats, ships etc. can travel faster with lesser energy consumption.

A body in streamlined shapes has the least possible resistance to a current of air, water or other fluids.

Friction is the force acting between two surfaces in contact with each other. Force of friction will increase if the two surfaces are pressed harder. 

It is caused by the irregularities in the surfaces. 

Frictional forces oppose the direction of motion.  

When surface with irregularities comes in contact with each other, they move against each other. These irregularities interlock and pose resistance to the motion. This is how friction is caused.

Sliding friction is smaller than the static friction.

When the objects are sliding against each other, the interlock between the surfaces stays for a short while and is overcome due to inertia of motion. 

In case of a static object, a lot of energy has to be spent in overcoming the resistance posed by the interlocked irregularities. 

Frictional force helps us to write using chalk on black board. 

The chalk deposits would not stick to the board and no mark would be left behind, had it been there was no friction.

The water on a wet marble floor fills the microscopic irregularities on the surface and hence present lesser frictional force. It is therefore difficult to move on a wet marble floor. 

Friction causes wear and tear in soles of our shoes. As we walk, friction tends to oppose the direction of motion. 

If friction was absent, the chalk deposits would not stick to the board and no mark would be left behind.
This would have made it impossible to write with the chalk on the blackboard. 

If there was no friction, then once in motion, the object would never stop. We would have been continuously moving.

Friction reduces the smoothness of a surface. For example, moving parts of a machine, shoes that we wear.

Hence, lubrication is required for such materials to reduce the friction. Thus, cost of maintenance increases.

The jar of a mixer becomes hot due to friction arising between the moving machine parts. 

Tyres of cars and trucks are treaded to prevent the loss of traction due to the buildup of a layer of water between the road and the tyre in wet conditions. The grooves in tyres allow water to be expelled from beneath the tyre. 

Treading maintains friction between the road and the tyre for proper control of the vehicle.

Kabaddi players rub their hands with soil so that they get a better grip on their opponents. The game involves grabbing the players of the opponent team. The sweat on the bodies do not let the players to get a proper grip on on the opponents. 

1. Friction causes wear and tear of machine parts that rub against each other continuously, leading to costly maintenance. 

2. Additional energy is consumed to overcome friction, in addition to the physical work required for actual work.

Lubricants are substances used to reduce friction between moving parts of a machine. 

Shoes are manufactured with grooves on their soles. These grooves ensure we get a grip on the ground. This does not cause the wearer to slip due to lack of friction and helps from falling.  

To reduce the friction in machines, air cushion can be used in places of lubricants.

Hovercraft uses air cushion instead of lubricants to reduce friction in moving parts. 

When luggages are fitted with rollers, the resisatnce to motion is decreased by offering a smooth flow of motion. Hence, friction is reduced and rolling of luggage is anyday easier than lifting them. 

The Ceiling fan uses ball bearings to reduce friction. 

Friction is called a necessary evil because it wears out the materials be it screws, ball bearing or soles of shoes. This wear and tear increases the maintenance cost of the things. 

But, at the same time life would have been a trouble without friction, as the energy could not have been transferred. In the absence of friction, if the object started moving it would not have stopped and kept moving on. 

Lubrication of the moving parts of a machine reduces friction. 

Friction is required in case of moving vehicles for them to have proper traction of the roads. Friction can be increased by treading the tyres of the vehicles for a smooth run in wet roads. 

When objects move in fluid, the must have a streamlined shape which will resist the motion of objects in the liquid by reducing the current. 

The objects that have streamlined shape are:

1. Boats or Ships

2. Aeroplanes 

When one body rolls over the surface of another body, the resistance to it's motion is called as rolling friction.
e.g. wheels on a cart. 

The frictional force will act in an upward direction so as to oppose the sliding of the book on the table.

When the horse cart is stationary, the muscular force provided by the horse is used to overcome the static friction as well as to provide acceleration to the cart.

As the cart begins to move, the friction becomes lesser since it is rolling friction and the muscular force of the horse is utilised to only overcome this friction.

Hence, initiailly to set the cart in motion, the horse needs to do more work than, when the cart is in motion. 

Friction is caused by the inter locking of the irregularities in the two surfaces. When two surfaces are pressed harder together, the force of friction increases.

Therefore, it is easy to drag a mat when no body is sitting on it rather than when a person is sitting on it. When person is sitting on the mat, the force of friction exerted is more.

When a book kept on a table is gently pushed, it stops after some distance because of the force of friction that acts between the surface of the book and the table. The friction opposes the motion of the book from moving forward. Hence, the relative movement between the two surfaces in contact is opposed. 

Friction is caused by the irregularities on surfaces. There is a relative movement, when two surface comes in contact with each other, and the force resists the motion. This resistance to the motion is friction. 

When the surfaces in contact are at rest, the imperfections interlock and hence more force is requried to overcome the static friction. 

However, when the objects are moving, the interlocking is transient and hence the force required to overcome this sliding friction is lesser. 

When a person is walking on the road, the surface of the road will offer friction opposing our motion. Therefore, shoes oppose this friction and continues with the motion. Therefore, our shoes gets worn out in the process of overcoming the friction caused by the surface we walk on.

Rubbing hands against each other causes friction between them, generating heat. As heat is generated from our body, body becomes cold and hands become warm.

A better grip is required to control the movement of objects. For example, if the friction is absent, vehicles will slip on the roads and they will never stop. So, roads are built with concretes to increase friction. Similarly we increase friction in vehicles by applying brakes to stop the vehicle. Therefore, friction is required to be increased. 

Friction is a necessary evil which causes a lot of wear and tear in machine parts that move against each other. It erodes the surfaces and destroys their symmetries. Hence, in such cases it is desirable to reduce friction. 
Friction is reduced by lubricating surfaces to decrease the friction. 

We can increase friction between the surfaces in contact  by increasing the irregularities of the surfaces in contact. 

For example: 
1. Gymnasts and weight lifters rub chalk powder on their hands before their events to increase the grip in their hands. 
2. spikes are used in shoes of the athletes so that they can interlock better with the irregularities of the ground to have a  better grip. 

Friction can be decreased by using rollers, sliding surfaces and lubricating the surfaces in contact. Using this method increases the smoothness of the surface thereby, decreasing the friction.

Materials like ball beamings, wheels and lubricants like oils, grease, cream etc., are used to decrease the friction.  

Aeroplanes have been designed as birds. Birds have a light body, shaped such as to have a minimum wind resistance when they fly to overcome the friction. Thus, aeroplanes have been designed such as to have a streamlined body to overcome the frictional force of air. 

Fishes can easily swim in water because they have streamlined bodies, which produces minimum friction in water and energy required to overcome this friction is very less.

When the sheet is not crumpled, paper has a larger surface area. And, when a paper is crumpled the surface area of the paper reduces. Therefore, the paper which is not crumpled experiences more  force of friction and falls slower in spite of the same gravity that is acting on them as compared to the one which is crumpled.

Types of friction are:

1. Sliding friction

2. Static friction

3. Rolling friction.

The magnitude of frictional force exerted by a liquid on an object depends on the following factors:

1. Speed of the object,

2. Shape of the object,

3. Size of the object, and

4. Nature of the fluid or viscosity of the fluid. 

The disadvantages of fluid friction are: 

1. Fluid friction reduces the speed of objects moving through the fluids by opposing their motion. 

2. When objects move through fluids (air or water), they lose some of their energy in over coming the fluid friction. This decreases their efficiency. 

Sportsmen wear shoes with spikes in order to increase the friction. Since, they need to run fast a greater friction is required between the soles of their shoes and ground to prevent them from slipping. 

The pressure exerted by gases and fluids is known as fluid pressure. Fluids exert a force on an object in it which is called fluid friction. This fluid friction is also called drag.

Fluid friction depends are as following:

(i) Speed with respect to fluid. 

(ii) Shape of the object.

(iii) Nature of the fluid. 

Friction is equal and opposite force exerted by relative motion between two surfaces.
The direction of friction is such that it opposes the direction of motion.
Friction acts between two surfaces with irregularities. When these surfaces come in contact their irregularities get locked with each other, and their movement causes friction. 

Force of friction depends upon the nature of the surfaces in contact and the force with which these two surfaces are pressed against each other.

The nature of the surface, on which the object moves affects the force of friction. Rough surfaces, having more irregularities increases frictional force, while smooth surfaces produce less friction. 
Smooth surfaces having irregularities allow easy movement. Interlocking of irregularities with other surface is less for smooth surface as compared to rough surface. 

The below activity will illustrate that smoothness of surface reduces friction.

1. Take a pencil or pen and put it on a table.  
2. Slightly push the pencil and see how much distance it covers. Mark the point where the pencil stops after the distance covered.
3. Now take some cream or oil. Make a small coating of oil on the table or spread polythene on the table if possible.
4. Again take the pencil and put it at the same initial point. 
5. Now push the pencil slightly and let it stop on its own as done earlier.
6. Mark the point where the pencil stops.
7. Observe the difference in the distance.
8. We will see that the distance covered in the second case is more as compared to the first case.

That is less friction is exerted in the second case which eases the movement of the pencil. 
This implies, the smoothness of the surface reduces friction. 

opposes

streamlined

lesser

friction

friction

irregularities 

friction

rolling friction

streamlined

streamlined

A.

contact force

C.

weight

C.

force of friction

B.

the opposite direction

D.

water, ice, marble, cemented floor, towel

C.

fluid friction, dynamic friction, rolling friction, static friction

B.

decrease friction

A.

greasing

C.

friction can be increased

B.

tend to bring a body from rest to motion

B.

opposes the motion

(i) This diagram will illustrate the rolling friction.

 

(ii) Types of friction are:

(a) Static friction

(b) Sliding friction

(c) Rolling friction

(i) The figure shows an aeroplane and bird.

(ii) The similarities are:

(a) Both aeroplane and bird fly in the air.

(b) Both have a specific shape to reduce friction.

(iii) The name of the specific shape is streamlined.