Science Chapter 11 Force And Pressure

Objects are pushed or pulled inorder to change it's state of motion.

Examples of push are:
1) We push a door to open it.
2) A football player kicks the ball and pushes it. 

Examples of pull are:
1) We pull the desk to change its position.
2) The rope is pulled, to draw a bucket of water from a well.

The shape of an object can be changed due to applied force. For example:

1. When force is applied on a lump of dough on a plate. 
2. The spring fitted under the seat of a bicycle gets compressed when you sit on the seat. 

pull

attracts

push

repels

shape

muscular

contact

gravity

friction

Agent exerting the force is Fingers

Object on which the force acts is Lemon

Effect of force is Lemon juice is extracted by squeezing.

Agent - Hand of the person squeezing the tube.
Object - Toothpaste tube
Effect -  The paste comes out of the tube on squeezing.

Agent - Suspended Load
Object - The spring
Effect - Elongation of the spring upon suspension of load.

Hammering the iron requires a force and this force causes the hot iron to change the shape of iron. Thus, hot iron can be moulded in the shape as required. 

Electrostatic force is responsible for the attraction between the balloon and the wall. 

The bucket is help up by the action of muscles in our body in the upward direction and force of gravity acting downwards. The force resulting due to the action of muscles in our body is known as muscular force. 
Thus, the bucket is held in position by the action of upward and downward force. 

The forces do not bring the change in its state of motion because they are acting in opposite direction with equal magnitude. Hence, bucket stays in the same position. 

The two forces acting on the rocket are :-

1. Gravitational force which is acting vertically downwards
2. Frictional force due to the air. 

D.

Atmospheric pressure

A push or pull on an object is called Force. That is an external force acting on the body is called force. 

Force can act to either change the state of an object or its shape. 

Interaction of one object with another object results in force between the objects. That is, interaction is the factor which is essential for force to act. 

When two forces act in the same direction, the resultant force increases. The magnitude of one adds up with the magnitude of second one. 

When two forces act in opposite directions on an object then the magnitude of the net force will be the difference between two forces.
The resultant force will decrease. 

If the two teams pull equally hard on the rope in a game of Tug of War, then the rope does not move in either direction. The magnitude of pull shoul not be same in both direction for the ropes to move. 

When the magnitude of two forces acting on a body in opposite direction is equal, then net force is equal to zero. 

The force resulting due to the action of muscles is known as muscular force. 

Two examples of muscular force are:
1. Lifting a bucket of water
2. Kicking a football.

The force applied on an object, when our body is in contact with the object is known as contact forces.

Two examples of Contact Forces are:-

(i) Muscular force, and

(ii) Force of friction.

The forces acting on two or more objects from a distance without making contact on any of the objects, are called non-contact forces. 

Examples: 
Gravitational force, magnetic force and electrostatic force. 

The force exerted by a magnet is an example of a non-contact force. 

The force exerted by a magnet on any object having magnetic properties is called magnetic force.

When two magnets are brought near each other, 

(i) The like poles repel each other, and

(ii) Unlike poles attract each other.

The force exerted by a charged body on another charged or uncharged body is known as Electrostatic force.

Every object whether large or small, exerts a force on every other object in the universe. This force is known as gravitational force. 

The force due to which every object falls on earth is called the force of Gravity. 
Force of gravity is an attractive force. 

Pressure is the force acting on a unit area of a surface.

Mathematically, it is given by

                 $\mathrm{Pressure} = \frac{\mathrm{Force}}{\mathrm{Area} }$ = $\frac{\mathrm{F}}{\mathrm{A}}$

Mathematically, pressure is given by,

               Pressure = $\frac{\mathrm{Force}}{\mathrm{Area}}$ 

Pressure can be increased by decreasing the area of the surface on which force is to be applied. 

Camels have broad feet i.e., feet having a large are so that the pressure exerted on the sand is less due to their heavy weight. 

We know that, 

Pressure = $\frac{\mathrm{Force}}{\mathrm{Area}}$

Therefore, camels do not dig deep in the sand dunes and can move freely.

Edge of nails is made sharp(less area) so that the object experiences more pressure. This makes easier for a person to penetrate the wood with the force of a hammer. 

Liquid exerts pressure on the walls of the container in which it is filled and on the base due to the column of liquid over it. 

Pressure exerted by the weight of air is known as atmospheric pressure. 

When air is pumped into a tube, the pressure inside the tube becomes more than the atmospheric pressure and exerts outward pressure, thus inflating the tube. 

We don't feel or are not crushed by the atmospheric pressure because, the pressure inside our bodies is equal to the atmospheric pressure and cancels the pressure from outside. 

Force is an external influence (push or pull) on a body that tends to either change the state of a body or its shape.

Actions like pushing, pulling, picking, hitting, lifting, running and bending are some of the examples of force.

An object exists in a state of rest or in a state of motion. 

When a force is applied on a body at rest, it can start moving. e.g. a stationary football is kicked.

When a force is applied on a body that is in motion, it can also bring it to rest. e.g brakes in a vehicle. 

Similarly, a force can change the direction of a moving body as well as the speed of moving body. e.g. a cricket ball hit with a bat. 

On exerting force, an object that is not rigid may change its shape.

e.g. flour can be kneaded in any shape.

1. Contact forces

The forces which come in action, when two objects come in contact with each other are called contact forces.

e.g. Muscular force and Friction are contact forces.

2. Non-Contact forces

The forces which the body exerts on another, without any contact with one another are called non-contact forces.

e.g. Electrostatic and Magnetic forces are non-contact forces. 

The force exerted by a charged body on another charged or uncharged body is called electrostatic force.

This force is known as non-contact force because these forces comes into play even when the bodies are not in contact with each other. 

Pressure is the amount of force acting on a unit area of a surface.

Mathematically, pressure is given by

Pressure = Force / Unit Area

That is, pressure and area on which the force is applied has an inverse dependance.

Keeping force constant, if the area on which the force applied is increased, the pressure decreases. 

We know that

Pressure = $\frac{\mathrm{Force}}{\mathrm{Area}}$

A school bag has a weight of the books and exerts a vertically downward force.

If the string is thin, it has less area of cross-section, and therefore, exerts greater pressure on the hand for the given weight of the school bag.

Therefore, it becomes difficult to hold the school bag with a thin string. 

A truck carries load whose weight is largely dependent on the rear axle.

Since,
Pressure = $\frac{\mathrm{Force}}{\mathrm{Area}}$

The double tyres increase area of cross-section without increasing friction, and hence decrease pressure on the axle. 

Pressure is given by force per unit area. 

The pressure exerted by the sharp knife edge is more than that exerted by the blunt one since the area in contact for the sharp knife is smaller than the blunt one.

Thus, it is easier to cut with a sharp knife than with a blunt one. 

The foundation of a building is made wide so that it do not dug deep under the ground during extremely high pressure generated by the weight of the building.

As, Pressure = $\frac{\mathrm{Force}}{\mathrm{Area}}$

The broad foundation distributes the weight of the building over a larger area and keeps the building safe from falling. 

Cutting instruments like  blades and axes are sharpened to decrease the area of cross-section that is in contact with the object being cut.

Using the formula,

Pressure = Force /Area, we can say that with decrease in area the pressure exerted by them on the surface increases. Thus, they can easily penetrate the surface that they are meant to cut without applying much force. 

When a liquid is enclosed in a container, 

(a) As depth increases, pressure exerted by the liquid also increases.

(b) Pressure of a liquid is independent of the shape and size of the container. 

When pressure is exerted on an object from opposite sides in opposite directions, the two forces either get cancelled if they are equal in magnitude or the force that is more in magnitude comes in play.
The object gets pulled to that side where the magnitude of force is greater. 

The Force exerted on a surface per unit area is called pressure.

Pressure = Force /Area 

Pressure is inversely proportional to the area on which it is applied.

That is, when the area is increased the pressure exerted by applying a force is less. 

An external force sets the object in motion. Here, the fan is placed inside the boat. Therefore, there is no motion due to the fan blowing wind into the sail. 
Here, when the air from fan pushes the sail, air also pushes the fan in opposite direction.

The motion of walking is such that at a given point of time, only one of his feet is on the grond.

Since,

Pressure = Force/ Unit Area,

the weight of the person falls on a smaller area of the ground when he's walking than when he's standing on his two feet.

Hence, when the person walks, he exerts more pressure on the ground than when he's standing on his two feet. 

The types of forces in the given cases are:-

(a) Force of Gravity 

(b) Frictional force

(c) Muscular force

(d) Electrostatic force

(e) Magnetic force. 

Since, pressure is equal to force per unit area, in order to produce a sufficiently large pressure on the cloth being stitched so that the needle can easily pass through the cloth, the tip of the sewing needle is made sharp.

Examples to show that air exerts pressure:

1. When air is pumped inside a balloon, it expands in size. This shows that the air inside exerts pressure on the walls of the balloon. 

2. A sealed packet of chips swells up in the mountains. This is because the atmospheric pressure at higher altitudes is less whereas the packet is filled with gas at normal atmospheric pressure. It therefore swells up in the mountains, as the pressure inside is more than the atmospheric pressure. 

When air is pumped into the balloon from the sealed jar, the balloon will expand rapidly and burst. Balloon contains gas whose pressure which is more than the atmospheric pressure. This atmospheric pressure opposes the pressure of the gas inside the balloon. 

If the air in the jar is pumped out, the pressure exerted by the gas in the balloon will not be opposed. Therefore, the balloon expands rapidly and bursts.

The force acting on objects, when two objects come in contact with each other are known as contact forces.

Examples of contact forces are given below: 

1. Muscular Force: Force exerted due to the action of muscles. This force is categorised as contact force because, it comes in play only when an object is being handled.
Example: bending, running, moving etc. 

2. Friction: Force exerted on an object when its surface comes in contact with another surface. This force comes into action only when the two surfaces come in contact with each other and hence are called contact forces. 

Relationship between pressure and surface area of the object can be illustrated using the following activity.

Materials Required:

A soft bulletin board, chart paper and drawing pins.

Procedure:

1. Pin the chart paper on a soft bulletin board with flat rounded end of the drawing pin.

2. Now try pinning the chart papers with the sharp pointed ends of the drawing pin.

Observations:

In the first case, when the rounded end of the pin was used, only an impression of the pin formed on the chart paper.

In the second case, the sharp end makes a hole in the chart paper and hence can be fixed on the bulletin board.

Conclusion: 

The surface area of the rounded end of the drawing pin is larger than the surface area of the sharp end. Therefore, a greater pressure is produced with the sharp end of the drawing pin. 

This proves that, Pressure = Force per unit Area. 

The presence of atmospheric pressure can be demonstrated using the following activity. 

Procedure:

1. Take a metallic can and add a little water in it.

2. Remove it's lid or cap and heat the can, as shown in Fig. A 



3. The water boils and steam starts coming out from the mouth of the can.

4. The steam forces out most of the air from the can.

5. Now the can is closed with an air tight lid.

6. Now on pouring cold water on the can, we observe that the can crushes.

Explanation: 

When the can is heated and then capped, the hot air is filled inside the can.

Due to the cold water, the steam inside the can, condenses to liquid state. Thus, a partial vacuum is created. In this case, the external atmospheric pressure becomes greater than the inside pressure. Hence high external pressure crushes the container. 

force

contact

muscular force

pressure

State

shape

newton

Contact force

force

electrostatic

non-contact

C.

Pull as well as Push

D.

Electrostatic force

B.

Difference between two forces

C.

State of rest or of motion

C.

Magnetic force

B.

Increases

C.

Feet are broader

C.

Force per unit area

D.

Reduce the pressure

C.

Increases

(i) Liquids exert pressure on the walls of the container. This can be demonstrated by fitting a rubber sheet on the glass tube and filling the glass tube with water. 



(ii) On increasing the level of water in the bottle, there is a greater bulge in the balloon. Pressure increases because of increase in water column.