# Force And Laws of Motion Chapter 9 Class 9 Science NCERT Textbook PDF

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### Chapter 9: Force and Laws of Motion

In the previous chapter, we described the motion of an object along a straight line in terms of its position, velocity, and acceleration.

We saw that such a motion can be uniform or non-uniform. We have not yet discovered what causes the motion.

Why does the speed of an object change with time? Do all motions require a cause? If so, what is the nature of this cause? In this chapter, we shall make an attempt to quench all such curiosities.

For many centuries, the problem of motion and its causes had puzzled scientists and philosophers. A ball on the ground, when given a small hit, does not move forever.

Such observations suggest that rest is the “natural state” of an object. This remained the belief until Galileo Galilei and Isaac Newton developed an entirely different approach to understanding motion.

In our everyday life, we observe that some effort is required to put a stationary object into motion or to stop a moving object.

We ordinarily experience this as a muscular effort and say that we must push or hit or pull on an object to change its state of motion.

The concept of force is based on this push, hit, or pull. Let us now ponder about a ‘force’. What
is it? In fact, no one has seen, tasted, or felt a force.

However, we always see or feel the effect of a force. It can only be explained by describing what happens when a force is applied to an object.

Pushing, hitting, and pulling objects are all ways of bringing objects in motion (Fig. 9.1). They move because we make a force act on them.

From your studies in earlier classes, you are also familiar with the fact that a force can be used to change the magnitude of the velocity of an object (that is, to make the object move faster or slower) or to change its direction of motion.

We also know that a force can change the shape and size of objects.

#### 9.1 Balanced and Unbalanced Forces

Fig. 9.3 shows a wooden block on a horizontal table. Two strings X and Y are tied to the two
opposite faces of the block as shown.

If we apply a force by pulling the string X, the block begins to move to the right. Similarly, if we pull the string Y, the block moves to the left. But, if the block is pulled from both the sides with equal forces, the block will not move.

Such forces are called balanced forces and do not change the state of rest or of motion of
an object.

Now, let us consider a situation in which two opposite forces of different magnitudes pull the block. In this case, the block would begin to move in the direction of the greater force.

Thus, the two forces are not balanced and the unbalanced force acts in the direction the block moves. This suggests that an unbalanced force acting on an object brings it in motion.

What happens when some children try to push a box on a rough floor? If they push the box with a small force, the box does not move because of friction acting in a direction opposite to the push [Fig. 9.4(a)].

This friction force arises between two surfaces in contact; in this case, between the bottom of the box and the floor’s rough surface.

It balances the pushing force and therefore the box does not move. In Fig. 9.4(b), the children push the box harder but the box still does not move.

This is because the friction force still balances the pushing force.

If the children push the box harder still, the pushing force becomes bigger than the friction force [Fig. 9.4(c)]. There is an unbalanced force. So the box starts moving.

### NCERT Solutions Class 9 Science Chapter 9 Force and Laws of Motion

1. Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five-rupee coin and a one-rupee coin?

Solution

Since inertia is dependent on the mass of the object, the object with the greater mass will hold greater inertia. The following objects hold greater inertia because of their mass.

1. Stone
2. Train
3. Five-Rupee coin

2. In the following example, try to identify the number of times the velocity of the ball changes: “A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”. Also identify the agent supplying the force in each case.

Solution

The velocity of football changes four times.

First, when a football player kicks a football to another player, second when that player kicks the football to the goalkeeper. Third when the goalkeeper stops the football. Fourth, when the goalkeeper kicks the football towards his team player.

Agent supplying the force:

a) The First case is the First player

b) The Second case is the Second player

c) The Third case is Goalkeeper

d) The Fourth case is Goalkeeper

3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Solution

When the branch of the tree is shaken, the branch moves in a to-and-fro motion. However, the inertia of the leaves in attached to the branch resists the motion of the branch.

Therefore, the leaves that are weakly attached to the branch fall off due to inertia whereas the leaves that are firmly attached to the branch remain attached.

4. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Solution

Initially, when the bus accelerates in a forward direction from a state of rest, the passengers experience a force exerted on them in the backward direction due to their inertia opposing the forward motion.

Once the bus starts moving, the passengers are in a state of motion in the forward direction. When the brakes are applied, the bus moves towards a position of rest.

Now, a force in the forward direction is applied on the passengers because their inertia resists the change in the motion of the bus. This causes the passengers to fall forwards when the brakes are applied.

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