Page 126 - Understanding NCERT Science 09

done on an object when a force of 1 N
displaces it by 1 m along the line of action of
the force.
Look at Eq. (10.1) carefully. What is the
work done when the force on the object is
zero? What would be the work done when
the displacement of the object is zero? Refer
to the conditions that are to be satisfied to
say that work is done.
Example 10.1 A force of 5 N is acting on
an object. The object is displaced
through 2 m in the direction of the force
(Fig. 10.2). If the force acts on the object Let the object stop after a displacement s. In
all through the displacement, then work
done is 5 N × 2 m =10 N m or 10 J.
Fig. 10.2
uestion
Q 1. A force of 7 N acts on an object.
The displacement is, say 8 m, in
the direction of the force
(Fig. 10.3). Let us take it that the
force acts on the object through
the displacement. What is the
work done in this case?
Fig. 10.3
Consider another situation in which the
force and the displacement are in the same
direction: a baby pulling a toy car parallel to
the ground, as shown in Fig. 10.4. The baby
has exerted a force in the direction of
displacement of the car. In this situation, the
work done will be equal to the product of the
force and displacement. In such situations,
the work done by the force is taken as positive.
WORK AND ENERGY
Rationalised 2023-24
Fig. 10.4
Consider a situation in which an object is
moving with a uniform velocity along a
particular direction. Now a retarding force, F,
is applied in the opposite direction. That is,
the angle between the two directions is 180º.
such a situation, the work done by the force,
F is taken as negative and denoted by the
minus sign. The work done by the force is
F × (–s) or (–F × s).
It is clear from the above discussion that
the work done by a force can be either positive
or negative. To understand this, let us do the
following activity:
Activity _____________10.4
force exerted by you on the object. The
Lift an object up. Work is done by the
•
object moves upwards. The force you
exerted is in the direction of
displacement. However, there is the
Which one of these forces is doing
• force of gravity acting on the object.
positive work?
• Which one is doing negative work?
• Give reasons.
Work done is negative when the force acts
opposite to the direction of displacement.
Work done is positive when the force is in the
direction of displacement.
Example 10.2 A porter lifts a luggage of
15 kg from the ground and puts it on
his head 1.5 m above the ground.
Calculate the work done by him on the
luggage.
Solution:
Mass of luggage, m = 15 kg and
displacement, s = 1.5 m.
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