Force - Class 9, Force and law of motion

Question 1 Define the term force?

Question 2 Define the term direction of force?

Question 3 What are the various effects of force?

Question 4 Give an example where a force stops a moving body?

Question 5 Give an example where a force changes the shape of a body?

Question 6 What do you understand by balanced forces. Give example?

Question 7 What do you understand by unbalanced forces. Give example?

Contents

1 Force and Law of motion
2 Effects of Force
3 Types of forces
- 3.1 1) Balanced Force
- 3.2 2) Unbalanced Force

Force and Law of motion

A push or pull on a body is called force.

The direction in which a body is pulled or pushed is called direction of force.

Forces are used in our everyday actions like pushing, pulling, stretching, lifting, twisting, pressing etc.

For Example: Kick a football(push), pull drawer from table, lift a box from floor, stretch a rubber, twist a wet cloth etc.

Effects of Force

1)It can move a stationary body.

2)It can stop a moving body.

3)It can change the speed of moving body.

4)It can change the direction of moving body.

5)It can change the shape of a body.

For Example :1) If we kick a football kept on ground with our foot, it will start moving. Force of our foot moves stationary ball.

2)A football moving on ground stops after sometimes. This is due to friction of ground.

3)If we are moving a bicycle at a certain speed. If someone pushes it, speed increases but if someone pulls it speed decreases. Thus force changes speed of moving body. Ball thrown downwards, moves with a fast speed due to force of gravity(pulling force of earth)but if thrown upwards, speed decreases(because earth applies a pulling force of gravity on a ball in downward direction)

4)In a tennis match, when a moving tennis ball is hit by a racket, then the direction of tennis ball changes and it goes in different direction. The force exerted by tennis player racket changes the direction of moving ball.

5)If we take a spring and pull it on both the ends with our hands, shape and size of spring changes, shape of dough changes on pressing with rolling pin.

Types of forces

1) Balanced Force

If the resultant of all forces acting on body is zero, the forces are called balanced forces.

For Example: Suppose a heavy box is lying on the ground. The box does not move. This is due to four forced acting on it.

a)Force of friction

b)Force of gravity

c)Force of push or pull

d)Force of reaction

Since the box does not move at all, we conclude that the resultant of all forces acting on it is zero. If the number of balanced forces act on stationary body, the body continues to remain in its stationary position. If the number of balanced forces act on a body in uniform motion, the body continues to be in its state of uniform motion. Balanced forces can change the shape of a body. For Example: When we squeeze a rubber ball.

2) Unbalanced Force

If the resultant of all forces acting on body is not zero, the forces are called unbalanced forces.Unbalanced forces can move a stationary body or they can stop a moving body.
Unbalanced forces acting on a body can change its speed or direction of motion.
For Example : A toy car lying on a ground.

The forces acting on it are:

1)Force of friction

2)Force of gravity(weight of toy car)

3)Force of reaction(exerted by ground)

4)Force of our pushForce of reaction=Force of gravity(balanced)

Force of push > Force of friction(unbalanced)

Resultant of all forces acting on toy car is not zero.

For Example: In a tug of war, if one of the team suddenly releases the rope, an unbalanced force acts on other team due to which it falls backwards.

An unbalanced force can stop a moving body.

For Example: When a ball is rolling on ground, an unbalanced force of friction acts on it which brings the ball to stop after sometime. If the speed or direction of motion of body changes the some balanced force is acting on it. When we stop pedalling a moving bicycle, it slow downs due to unbalanced forces of friction acting on it. The force of friction opposes the motion of bicycle.

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