Page 125 - Understanding NCERT Science 09

in day-to-day life. For each of these Activity _____________10.3
activities, ask the following questions
and answer them: • Think of situations when the object
(i) What is the work being done on? is not displaced in spite of a force
(ii) What is happening to the object? acting on it.
(iii) Who (what) is doing the work? • Also think of situations when an object
gets displaced in the absence of a force
acting on it.
10.1.2SCIENTIFIC CONCEPTION OF WORK
• List all the situations that you can
think of for each.
To understand the way we view work and
• Discuss with your friends whether
define work from the point of view of science,
work is done in these situations.
let us consider some situations:
Push a pebble lying on a surface. The
pebble moves through a distance. You exerted 10.1.3 WORK DONE BY A CONSTANT
a force on the pebble and the pebble got FORCE
displaced. In this situation work is done. How is work defined in science? To
A girl pulls a trolley and the trolley moves understand this, we shall first consider the
through a distance. The girl has exerted a
case when the force is acting in the direction
force on the trolley and it is displaced.
of displacement.
Therefore, work is done. Let a constant force, F act on an object.
Lift a book through a height. To do this Let the object be displaced through a
you must apply a force. The book rises up. distance, s in the direction of the force (Fig.
There is a force applied on the book and the
10.1). Let W be the work done. We define work
book has moved. Hence, work is done.
to be equal to the product of the force and
A closer look at the above situations displacement.
reveals that two conditions need to be Work done = force × displacement
satisfied for work to be done: (i) a force should W = F s (10.1)
act on an object, and (ii) the object must be
displaced.
If any one of the above conditions does
not exist, work is not done. This is the way
we view work in science.
A bullock is pulling a cart. The cart
moves. There is a force on the cart and the
cart has moved. Do you think that work is
done in this situation?
Activity _____________10.2

• Think of some situations from your Fig. 10.1
daily life involving work.
• List them. Thus, work done by a force acting on an
• Discuss with your friends whether object is equal to the magnitude of the force
work is being done in each situation. multiplied by the distance moved in the
• Try to reason out your response. direction of the force. Work has only
• If work is done, which is the force acting magnitude and no direction.
on the object?
• What is the object on which the work In Eq. (10.1), if F = 1 N and s = 1 m then
is done? the work done by the force will be 1 N m.
• What happens to the object on which Here the unit of work is newton metre (N m)
work is done? or joule (J). Thus 1 J is the amount of work

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