Force & Pressure, Chapter Notes, Class 8, Science PDF Download

Force and Pressure

Force

A pull or push over an object is called force.

Example – to open a drawer, one has to pull it; and to close the drawer one has to push it. Thus, in both the conditions a person applies force by pull or push.

To kick a ball, one has to push it, i.e. a force is applied.

To ride a bicycle one has to push the pedal. In this condition, force is applied while pushing the pedal.

Forces are due an interaction:

To apply a force over an object interaction between object and source of force is necessary.

Example –

To open a drawer one has to pull by holding it, drawer will not open otherwise .

To kick a ball one has to touch it, otherwise force will not be applied over the ball.

Direction and Magnitude of Force:

The measure of amount or strength of force is called the magnitude of force. Thus, strength or amount of force is expressed in terms of magnitude.

Condition when more than two forces are applied over an object:

Forces applied in same direction:

When more than one force is applied in the same direction, the total force is the magnitude of both the forces.

Example – It is difficult to push a car by single person, but two or more persons can collectively push the same car easily. This is because when more than one person tries to push the car, magnitudes of forces applied by each of the person are summed up and applied over than car, and car is pushed easily because of greater magnitude of force.

That means if three units of force are applied over an object by each of the two persons,

The total unit of force = sum of both forces = 3 unit + 3 unit = 6 unit

Forces applied in opposite direction:

When two forces are applied over an object from opposite directions, the total effective force is the difference of two forces.

Example –

(a) When two persons are pushing a box in opposite directions and the magnitude of both the forces is equal, then box will not move, because the difference of two forces will be equal to zero.

Let a person applying 4 unit of force in one direction and other person is also applying 4 unit of force in opposite direction on the same wooden box.

Therefore, the magnitude of total force = 4 unit – 4 unit = 0

(b) If one person is applying a force of 4 unit in one direction and another person is applying a force of 6 unit in opposite direction,

Then total magnitude of force = 6 unit – 4 unit = 2 unit

Thus, force will act in the direction of larger magnitude of force.

Force can change the state of motion:

State of rest:

An object is called in the state of rest when it is not moving. This means a stationary object is called in the state of rest. For example – a building, an electric pole, a ball kept over the ground and not moving, etc.

State of motion:

A moving object is called in the state of motion. For example – a moving car, a moving ball, etc.

A force can speed up a moving object. Force can decrease the speed of a moving object. Force can stop a moving object. Force can change the direction of a moving object. Force can move a stationary object.

Example –

In the game of cricket, when a batsman pushes a fast moving ball in the same direction of the movement of ball, the speed of the ball is increased.

When a batsman pushes a fast moving ball in the opposite direction the speed of ball decreases; because of force being applied from his bat.

When a player in the game of football kicks the ball to his mate, he changes the direction of the moving football by applying force through his kick.

A goalkeeper stops the football going towards the goal post by applying a force by his hand.

Force can change the shape of an object:

When you apply force on an inflated balloon; using your hand from both sides, the force of pressure changes the shape of balloon.

You mom change the shape of dough into a bread by applying force with a rolling pin.

A blacksmith changes the shape of an iron rod by applying force using a hammer.

When a rubber band is stretched in opposite directions, its shape is changed.