Chapter 11
WORK AND ENERGY
In the previous few chapters we have talked draws diagrams, organises her thoughts,
about ways of describing the motion of collects question papers, attends classes,
objects, the cause of motion and gravitation. discusses problems with her friends, and
Another concept that helps us understand and performs experiments. She expends a lot of
interpret many natural phenomena is ‘work’. energy on these activities. In common
Closely related to work are energy and power. parlance, she is ‘working hard’. All this ‘hard
In this chapter we shall study these concepts. work’ may involve very little ‘work’ if we go by
All living beings need food. Living beings the scientific definition of work.
have to perform several basic activities to You are working hard to push a huge rock.
survive. We call such activities ‘life processes’. Let us say the rock does not move despite all
The energy for these processes comes from the effort. You get completely exhausted.
food. We need energy for other activities like However, you have not done any work on the
playing, singing, reading, writing, thinking, rock as there is no displacement of the rock.
jumping, cycling and running. Activities that You stand still for a few minutes with a
are strenuous require more energy. heavy load on your head. You get tired. You
Animals too get engaged in activities. For have exerted yourself and have spent quite a
example, they may jump and run. They have bit of your energy. Are you doing work on the
to fight, move away from enemies, find food load? The way we understand the term ‘work’
or find a safe place to live. Also, we engage in science, work is not done.
some animals to lift weights, carry loads, pull You climb up the steps of a staircase and
carts or plough fields. All such activities reach the second floor of a building just to
require energy. see the landscape from there. You may even
Think of machines. List the machines that climb up a tall tree. If we apply the scientific
you have come across. What do they need for definition, these activities involve a lot of work.
their working? Why do some engines require In day-to-day life, we consider any useful
fuel like petrol and diesel? Why do living physical or mental labour as work. Activities
beings and machines need energy? like playing in a field, talking with friends,
humming a tune, watching a movie, attending
11.1 Work a function are sometimes not considered to
be work. What constitutes ‘work’ depends
What is work? There is a difference in the on the way we define it. We use and define
way we use the term ‘work’ in day-to-day life the term work differently in science. To
and the way we use it in science. To make understand this let us do the following
this point clear let us consider a few examples. activities:
11.1.1 NOT MUCH‘WORK’ IN SPITE OF Activity _____________ 11.1
WORKING HARD!
• We have discussed in the above
Kamali is preparing for examinations. She paragraphs a number of activities
spends lot of time in studies. She reads books, which we normally consider to be work
2020-21
, in day-to-day life. For each of these
activities, ask the following questions
Activity _____________ 11.3
and answer them: • Think of situations when the object is
(i) What is the work being done on? not displaced in spite of a force acting
(ii) What is happening to the object? 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.
11.1.2 SCIENTIFIC CONCEPTION OF WORK • List all the situations that you can
To understand the way we view work and think of for each.
• 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
11.1.3 WORK DONE BY A CONSTANT FORCE
a force on the pebble and the pebble got How is work defined in science? To
displaced. In this situation work is done. understand this, we shall first consider the
A girl pulls a trolley and the trolley moves case when the force is acting in the direction
through a distance. The girl has exerted a of displacement.
force on the trolley and it is displaced. Let a constant force, F act on an object.
Therefore, work is done. Let the object be displaced through a
Lift a book through a height. To do this distance, s in the direction of the force (Fig.
you must apply a force. The book rises up. 11.1). Let W be the work done. We define work
There is a force applied on the book and the to be equal to the product of the force and
book has moved. Hence, work is done. displacement.
A closer look at the above situations Work done = force × displacement
reveals that two conditions need to be W = Fs (11.1)
satisfied for work to be done: (i) a force should
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 _____________ 11.2
• Think of some situations from your Fig. 11.1
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
on the object?
magnitude and no direction.
• What is the object on which the work In Eq. (11.1), 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
WORK AND ENERGY 147
2020-21
WORK AND ENERGY
In the previous few chapters we have talked draws diagrams, organises her thoughts,
about ways of describing the motion of collects question papers, attends classes,
objects, the cause of motion and gravitation. discusses problems with her friends, and
Another concept that helps us understand and performs experiments. She expends a lot of
interpret many natural phenomena is ‘work’. energy on these activities. In common
Closely related to work are energy and power. parlance, she is ‘working hard’. All this ‘hard
In this chapter we shall study these concepts. work’ may involve very little ‘work’ if we go by
All living beings need food. Living beings the scientific definition of work.
have to perform several basic activities to You are working hard to push a huge rock.
survive. We call such activities ‘life processes’. Let us say the rock does not move despite all
The energy for these processes comes from the effort. You get completely exhausted.
food. We need energy for other activities like However, you have not done any work on the
playing, singing, reading, writing, thinking, rock as there is no displacement of the rock.
jumping, cycling and running. Activities that You stand still for a few minutes with a
are strenuous require more energy. heavy load on your head. You get tired. You
Animals too get engaged in activities. For have exerted yourself and have spent quite a
example, they may jump and run. They have bit of your energy. Are you doing work on the
to fight, move away from enemies, find food load? The way we understand the term ‘work’
or find a safe place to live. Also, we engage in science, work is not done.
some animals to lift weights, carry loads, pull You climb up the steps of a staircase and
carts or plough fields. All such activities reach the second floor of a building just to
require energy. see the landscape from there. You may even
Think of machines. List the machines that climb up a tall tree. If we apply the scientific
you have come across. What do they need for definition, these activities involve a lot of work.
their working? Why do some engines require In day-to-day life, we consider any useful
fuel like petrol and diesel? Why do living physical or mental labour as work. Activities
beings and machines need energy? like playing in a field, talking with friends,
humming a tune, watching a movie, attending
11.1 Work a function are sometimes not considered to
be work. What constitutes ‘work’ depends
What is work? There is a difference in the on the way we define it. We use and define
way we use the term ‘work’ in day-to-day life the term work differently in science. To
and the way we use it in science. To make understand this let us do the following
this point clear let us consider a few examples. activities:
11.1.1 NOT MUCH‘WORK’ IN SPITE OF Activity _____________ 11.1
WORKING HARD!
• We have discussed in the above
Kamali is preparing for examinations. She paragraphs a number of activities
spends lot of time in studies. She reads books, which we normally consider to be work
2020-21
, in day-to-day life. For each of these
activities, ask the following questions
Activity _____________ 11.3
and answer them: • Think of situations when the object is
(i) What is the work being done on? not displaced in spite of a force acting
(ii) What is happening to the object? 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.
11.1.2 SCIENTIFIC CONCEPTION OF WORK • List all the situations that you can
To understand the way we view work and think of for each.
• 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
11.1.3 WORK DONE BY A CONSTANT FORCE
a force on the pebble and the pebble got How is work defined in science? To
displaced. In this situation work is done. understand this, we shall first consider the
A girl pulls a trolley and the trolley moves case when the force is acting in the direction
through a distance. The girl has exerted a of displacement.
force on the trolley and it is displaced. Let a constant force, F act on an object.
Therefore, work is done. Let the object be displaced through a
Lift a book through a height. To do this distance, s in the direction of the force (Fig.
you must apply a force. The book rises up. 11.1). Let W be the work done. We define work
There is a force applied on the book and the to be equal to the product of the force and
book has moved. Hence, work is done. displacement.
A closer look at the above situations Work done = force × displacement
reveals that two conditions need to be W = Fs (11.1)
satisfied for work to be done: (i) a force should
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 _____________ 11.2
• Think of some situations from your Fig. 11.1
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
on the object?
magnitude and no direction.
• What is the object on which the work In Eq. (11.1), 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
WORK AND ENERGY 147
2020-21
