Page 99 - Understanding NCERT Science 09

8.1 Balanced and Unbalanced box with a small force, the box does not move
because of friction acting in a direction
Forces opposite to the push [Fig. 8.4(a)]. This friction

force arises between two surfaces in contact;
Fig. 8.3 shows a wooden block on a horizontal in this case, between the bottom of the box
table. Two strings X and Y are tied to the two
and floor’s rough surface. It balances the
opposite faces of the block as shown. If we pushing force and therefore the box does not
apply a force by pulling the string X, the block
begins to move to the right. Similarly, if we move. In Fig. 8.4(b), the children push the box
harder but the box still does not move. This is
pull the string Y, the block moves to the left. because the friction force still balances the
But, if the block is pulled from both the sides
with equal forces, the block will not move. pushing force. If the children push the box
Such forces are called balanced forces and harder still, the pushing force becomes bigger
than the friction force [Fig. 8.4(c)].
do not change the state of rest or of motion of
an object. Now, let us consider a situation in There is an unbalanced force. So the box
which two opposite forces of different starts moving.
magnitudes pull the block. In this case, the What happens when we ride a bicycle?
block would begin to move in the direction of When we stop pedalling, the bicycle begins
the greater force. Thus, the two forces are to slow down. This is again because of the
not balanced and the unbalanced force acts friction forces acting opposite to the direction
in the direction the block moves. This of motion. In order to keep the bicycle moving,
suggests that an unbalanced force acting on we have to start pedalling again. It thus
an object brings it in motion. appears that an object maintains its motion
under the continuous application of an
unbalanced force. However, it is quite
incorrect. An object moves with a uniform
velocity when the forces (pushing force and
frictional force) acting on the object are
balanced and there is no net external force
on it. If an unbalanced force is applied on
the object, there will be a change either in its
speed or in the direction of its motion. Thus,
to accelerate the motion of an object, an
Fig. 8.3: Two forces acting on a wooden block unbalanced force is required. And the change
in its speed (or in the direction of motion)
What happens when some children try to would continue as long as this unbalanced
push a box on a rough floor? If they push the force is applied. However, if this force is

(a) (b) (c)

Fig. 8.4

88 SCIENCE

Rationalised 2023-24