NCERT Book Class 9 Science Chapter 8 Force and laws of motion

C hapter 8
FORCE AND LAWS OF MOTION
In the previous chapter, we described the In our everyday life we observe that some
motion of an object along a straight line in effort is required to put a stationary object
terms of its position, velocity and acceleration. into motion or to stop a moving object. We
We saw that such a motion can be uniform ordinarily experience this as a muscular effort
or non-uniform. We have not yet discovered and say that we must push or hit or pull on
what causes the motion. Why does the speed an object to change its state of motion. The
of an object change with time? Do all motions concept of force is based on this push, hit or
require a cause? If so, what is the nature of pull. Let us now ponder about a ‘force’. What
this cause? In this chapter we shall make an is it? In fact, no one has seen, tasted or felt a
attempt to quench all such curiosities. force. However, we always see or feel the effect
For many centuries, the problem of of a force. It can only be explained by
motion and its causes had puzzled scientists describing what happens when a force is
and philosophers. A ball on the ground, when applied to an object. Pushing, hitting and
given a small hit, does not move forever. Such pulling of objects are all ways of bringing
observations suggest that rest is the “natural objects in motion (Fig. 8.1). They move because
state” of an object. This remained the belief we make a force act on them.
until Galileo Galilei and Isaac Newton From your studies in earlier classes, you
developed an entirely different approach to are also familiar with the fact that a force can
understand motion. be used to change the magnitude of 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 (Fig. 8.2).

(a) The trolley moves along the (b) The drawer is pulled.
direction we push it.
(a)

(b)

(c) The hockey stick hits the ball forward Fig. 8.2: (a) A spring expands on application of force;
Fig. 8.1: Pushing, pulling, or hitting objects change (b) A spherical rubber ball becomes oblong
their state of motion. as we apply force on it.