NCERT Textbook Chapter 11 - Work & Energy , Science, Class 9 Class 9 Notes | EduRev

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Class 9 : NCERT Textbook Chapter 11 - Work & Energy , Science, Class 9 Class 9 Notes | EduRev

 Page 1


In the previous few chapters we have talked
about ways of describing the motion of
objects, the cause of motion and gravitation.
Another concept that helps us understand and
interpret many natural phenomena is ‘work’.
Closely related to work are energy and power.
In this chapter we shall study these concepts.
All living beings need food.  Living beings
have to perform several basic activities to
survive.  We call such activities ‘life processes’.
The energy for these processes comes from
food. We need energy for other activities like
playing, singing, reading, writing, thinking,
jumping, cycling and running.  Activities that
are strenuous require more energy.
Animals too get engaged in activities.  For
example, they may jump and run.  They have
to fight, move away from enemies, find food
or find a safe place to live. Also, we engage
some animals to lift weights, carry loads, pull
carts or plough fields.  All such activities
require energy.
Think of machines. List the machines that
you have come across. What do they need for
their working? Why do some engines require
fuel like petrol and diesel?  Why do living
beings and machines need energy?
11.1 Work
What is work? There is a difference in the
way we use the term ‘work’ in day-to-day life
and the way we use it in science.  To make
this point clear let us consider a few examples.
11.1.1 NOT MUCH ‘WORK’ IN SPITE OF
WORKING HARD!
Kamali is preparing for examinations. She
spends lot of time in studies. She reads books,
draws diagrams, organises her thoughts,
collects question papers, attends classes,
discusses problems with her friends, and
performs experiments.  She expends a lot of
energy on these activities. In common
parlance, she is ‘working hard’. All this ‘hard
work’ may involve very little ‘work’ if we go by
the scientific definition of work.
You are working hard to push a huge rock.
Let us say the rock does not move despite all
the effort. You get completely exhausted.
However, you have not done any work on the
rock as there is no displacement of the rock.
You stand still for a few minutes with a
heavy load on your head.   You get tired. You
have exerted yourself and have spent quite a
bit of your energy. Are you doing work on the
load?  The way we understand the term ‘work’
in science, work is not done.
You climb up the steps of a staircase and
reach the second floor of a building just to
see the landscape from there.  You may even
climb up a tall tree.  If we apply the scientific
definition, these activities involve a lot of work.
In day-to-day life, we consider any useful
physical or mental labour as work. Activities
like playing in a field, talking with friends,
humming a tune, watching a movie, attending
a function are sometimes not considered to
be work.  What constitutes ‘work’ depends
on the way we define it. We use and define
the term work differently in science.  To
understand this let us do the following
activities:
Activity _____________11.1
• We have discussed in the above
paragraphs a number of activities
which we normally consider to be work
11 11
11 11 11
W W W W WORK ORK ORK ORK ORK     AND AND AND AND AND E E E E ENERGY NERGY NERGY NERGY NERGY
Chapter
Page 2


In the previous few chapters we have talked
about ways of describing the motion of
objects, the cause of motion and gravitation.
Another concept that helps us understand and
interpret many natural phenomena is ‘work’.
Closely related to work are energy and power.
In this chapter we shall study these concepts.
All living beings need food.  Living beings
have to perform several basic activities to
survive.  We call such activities ‘life processes’.
The energy for these processes comes from
food. We need energy for other activities like
playing, singing, reading, writing, thinking,
jumping, cycling and running.  Activities that
are strenuous require more energy.
Animals too get engaged in activities.  For
example, they may jump and run.  They have
to fight, move away from enemies, find food
or find a safe place to live. Also, we engage
some animals to lift weights, carry loads, pull
carts or plough fields.  All such activities
require energy.
Think of machines. List the machines that
you have come across. What do they need for
their working? Why do some engines require
fuel like petrol and diesel?  Why do living
beings and machines need energy?
11.1 Work
What is work? There is a difference in the
way we use the term ‘work’ in day-to-day life
and the way we use it in science.  To make
this point clear let us consider a few examples.
11.1.1 NOT MUCH ‘WORK’ IN SPITE OF
WORKING HARD!
Kamali is preparing for examinations. She
spends lot of time in studies. She reads books,
draws diagrams, organises her thoughts,
collects question papers, attends classes,
discusses problems with her friends, and
performs experiments.  She expends a lot of
energy on these activities. In common
parlance, she is ‘working hard’. All this ‘hard
work’ may involve very little ‘work’ if we go by
the scientific definition of work.
You are working hard to push a huge rock.
Let us say the rock does not move despite all
the effort. You get completely exhausted.
However, you have not done any work on the
rock as there is no displacement of the rock.
You stand still for a few minutes with a
heavy load on your head.   You get tired. You
have exerted yourself and have spent quite a
bit of your energy. Are you doing work on the
load?  The way we understand the term ‘work’
in science, work is not done.
You climb up the steps of a staircase and
reach the second floor of a building just to
see the landscape from there.  You may even
climb up a tall tree.  If we apply the scientific
definition, these activities involve a lot of work.
In day-to-day life, we consider any useful
physical or mental labour as work. Activities
like playing in a field, talking with friends,
humming a tune, watching a movie, attending
a function are sometimes not considered to
be work.  What constitutes ‘work’ depends
on the way we define it. We use and define
the term work differently in science.  To
understand this let us do the following
activities:
Activity _____________11.1
• We have discussed in the above
paragraphs a number of activities
which we normally consider to be work
11 11
11 11 11
W W W W WORK ORK ORK ORK ORK     AND AND AND AND AND E E E E ENERGY NERGY NERGY NERGY NERGY
Chapter
Activity _____________11.3
• Think of situations when the object is
not displaced in spite of a force acting
on it.
• Also think of situations when an object
gets displaced in the absence of a force
acting on it.
• List all the situations that you can
think of for each.
• Discuss with your friends whether
work is done in these situations.
11.1.3 WORK DONE BY A CONSTANT FORCE
How is work defined in science? To
understand this, we shall first consider the
case when the force is acting in the direction
of displacement.
Let a constant force, F act on an object.
Let the object be displaced through a
distance, s in the direction of the force (Fig.
11.1). Let W be the work done. We define work
to be equal to the product of the force and
displacement.
Work done = force × displacement
W= F s (11.1)
in day-to-day life. For each of these
activities, ask the following questions
and answer them:
(i) What is the work being done on?
(ii) What is happening to the object?
(iii) Who (what) is doing the work?
11.1.2 SCIENTIFIC CONCEPTION OF WORK
To understand the way we view work and
define work from the point of view of science,
let us consider some situations:
Push a pebble lying on a surface. The
pebble moves through a distance. You exerted
a force on the pebble and the pebble got
displaced. In this situation work is done.
A girl pulls a trolley and the trolley moves
through a distance.  The girl has exerted a
force on the trolley and it is displaced.
Therefore, work is done.
Lift a book through a height. To do this
you must apply a force.  The book rises up.
There is a force applied on the book and the
book has moved.  Hence, work is done.
A closer look at the above situations
reveals that two conditions need to be
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
daily life involving work.
• List them.
• Discuss with your friends whether
work is being done in each situation.
• Try to reason out your response.
• If work is done, which is the force acting
on the object?
• What is the object on which the work
is done?
• What happens to the object on which
work is done?
Fig. 11.1
Thus, work done by a force acting on an
object is equal to the magnitude of the force
multiplied by the distance moved in the
direction of the force. Work has only
magnitude and no direction.
In Eq. (11.1), if F = 1 N and s = 1 m then
the work done by the force will be 1 N m.
Here the unit of work is newton metre (N m)
or joule  (J).  Thus 1 J is the amount of work
WORK AND ENERGY 147
Page 3


In the previous few chapters we have talked
about ways of describing the motion of
objects, the cause of motion and gravitation.
Another concept that helps us understand and
interpret many natural phenomena is ‘work’.
Closely related to work are energy and power.
In this chapter we shall study these concepts.
All living beings need food.  Living beings
have to perform several basic activities to
survive.  We call such activities ‘life processes’.
The energy for these processes comes from
food. We need energy for other activities like
playing, singing, reading, writing, thinking,
jumping, cycling and running.  Activities that
are strenuous require more energy.
Animals too get engaged in activities.  For
example, they may jump and run.  They have
to fight, move away from enemies, find food
or find a safe place to live. Also, we engage
some animals to lift weights, carry loads, pull
carts or plough fields.  All such activities
require energy.
Think of machines. List the machines that
you have come across. What do they need for
their working? Why do some engines require
fuel like petrol and diesel?  Why do living
beings and machines need energy?
11.1 Work
What is work? There is a difference in the
way we use the term ‘work’ in day-to-day life
and the way we use it in science.  To make
this point clear let us consider a few examples.
11.1.1 NOT MUCH ‘WORK’ IN SPITE OF
WORKING HARD!
Kamali is preparing for examinations. She
spends lot of time in studies. She reads books,
draws diagrams, organises her thoughts,
collects question papers, attends classes,
discusses problems with her friends, and
performs experiments.  She expends a lot of
energy on these activities. In common
parlance, she is ‘working hard’. All this ‘hard
work’ may involve very little ‘work’ if we go by
the scientific definition of work.
You are working hard to push a huge rock.
Let us say the rock does not move despite all
the effort. You get completely exhausted.
However, you have not done any work on the
rock as there is no displacement of the rock.
You stand still for a few minutes with a
heavy load on your head.   You get tired. You
have exerted yourself and have spent quite a
bit of your energy. Are you doing work on the
load?  The way we understand the term ‘work’
in science, work is not done.
You climb up the steps of a staircase and
reach the second floor of a building just to
see the landscape from there.  You may even
climb up a tall tree.  If we apply the scientific
definition, these activities involve a lot of work.
In day-to-day life, we consider any useful
physical or mental labour as work. Activities
like playing in a field, talking with friends,
humming a tune, watching a movie, attending
a function are sometimes not considered to
be work.  What constitutes ‘work’ depends
on the way we define it. We use and define
the term work differently in science.  To
understand this let us do the following
activities:
Activity _____________11.1
• We have discussed in the above
paragraphs a number of activities
which we normally consider to be work
11 11
11 11 11
W W W W WORK ORK ORK ORK ORK     AND AND AND AND AND E E E E ENERGY NERGY NERGY NERGY NERGY
Chapter
Activity _____________11.3
• Think of situations when the object is
not displaced in spite of a force acting
on it.
• Also think of situations when an object
gets displaced in the absence of a force
acting on it.
• List all the situations that you can
think of for each.
• Discuss with your friends whether
work is done in these situations.
11.1.3 WORK DONE BY A CONSTANT FORCE
How is work defined in science? To
understand this, we shall first consider the
case when the force is acting in the direction
of displacement.
Let a constant force, F act on an object.
Let the object be displaced through a
distance, s in the direction of the force (Fig.
11.1). Let W be the work done. We define work
to be equal to the product of the force and
displacement.
Work done = force × displacement
W= F s (11.1)
in day-to-day life. For each of these
activities, ask the following questions
and answer them:
(i) What is the work being done on?
(ii) What is happening to the object?
(iii) Who (what) is doing the work?
11.1.2 SCIENTIFIC CONCEPTION OF WORK
To understand the way we view work and
define work from the point of view of science,
let us consider some situations:
Push a pebble lying on a surface. The
pebble moves through a distance. You exerted
a force on the pebble and the pebble got
displaced. In this situation work is done.
A girl pulls a trolley and the trolley moves
through a distance.  The girl has exerted a
force on the trolley and it is displaced.
Therefore, work is done.
Lift a book through a height. To do this
you must apply a force.  The book rises up.
There is a force applied on the book and the
book has moved.  Hence, work is done.
A closer look at the above situations
reveals that two conditions need to be
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
daily life involving work.
• List them.
• Discuss with your friends whether
work is being done in each situation.
• Try to reason out your response.
• If work is done, which is the force acting
on the object?
• What is the object on which the work
is done?
• What happens to the object on which
work is done?
Fig. 11.1
Thus, work done by a force acting on an
object is equal to the magnitude of the force
multiplied by the distance moved in the
direction of the force. Work has only
magnitude and no direction.
In Eq. (11.1), if F = 1 N and s = 1 m then
the work done by the force will be 1 N m.
Here the unit of work is newton metre (N m)
or joule  (J).  Thus 1 J is the amount of work
WORK AND ENERGY 147 SCIENCE 148
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. (11.1) 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 11.1 A force of 5 N is acting on
an object. The object is displaced
through 2 m in the direction of the force
(Fig. 11.2). If the force acts on the object
all through the displacement, then
work done is 5 N × 2 m  =10 N m or
10 J.
Fig. 11.4
Consider a situation in which an object
is being displaced by the action of forces and
we identify one of the forces, F acting opposite
to the direction of the displacement s, that
is, the angle between the two directions is
180º.  In 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 _____________11.4
• Lift an object up. Work is done by the
force exerted by you on the object.  The
object moves upwards. The force you
exerted is in the direction of
displacement. However, there is the
force of gravity acting on the object.
• Which one of these forces is doing
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 11.2 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.
Fig. 11.2
uestion
1. A force of 7 N acts on an object.
The displacement is, say 8 m, in
the direction of the force
(Fig. 11.3).  Let us take it that the
force acts on the object through
the displacement. What is the
work done in this case?
Fig. 11.3
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. 11.4.  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.
Q
Page 4


In the previous few chapters we have talked
about ways of describing the motion of
objects, the cause of motion and gravitation.
Another concept that helps us understand and
interpret many natural phenomena is ‘work’.
Closely related to work are energy and power.
In this chapter we shall study these concepts.
All living beings need food.  Living beings
have to perform several basic activities to
survive.  We call such activities ‘life processes’.
The energy for these processes comes from
food. We need energy for other activities like
playing, singing, reading, writing, thinking,
jumping, cycling and running.  Activities that
are strenuous require more energy.
Animals too get engaged in activities.  For
example, they may jump and run.  They have
to fight, move away from enemies, find food
or find a safe place to live. Also, we engage
some animals to lift weights, carry loads, pull
carts or plough fields.  All such activities
require energy.
Think of machines. List the machines that
you have come across. What do they need for
their working? Why do some engines require
fuel like petrol and diesel?  Why do living
beings and machines need energy?
11.1 Work
What is work? There is a difference in the
way we use the term ‘work’ in day-to-day life
and the way we use it in science.  To make
this point clear let us consider a few examples.
11.1.1 NOT MUCH ‘WORK’ IN SPITE OF
WORKING HARD!
Kamali is preparing for examinations. She
spends lot of time in studies. She reads books,
draws diagrams, organises her thoughts,
collects question papers, attends classes,
discusses problems with her friends, and
performs experiments.  She expends a lot of
energy on these activities. In common
parlance, she is ‘working hard’. All this ‘hard
work’ may involve very little ‘work’ if we go by
the scientific definition of work.
You are working hard to push a huge rock.
Let us say the rock does not move despite all
the effort. You get completely exhausted.
However, you have not done any work on the
rock as there is no displacement of the rock.
You stand still for a few minutes with a
heavy load on your head.   You get tired. You
have exerted yourself and have spent quite a
bit of your energy. Are you doing work on the
load?  The way we understand the term ‘work’
in science, work is not done.
You climb up the steps of a staircase and
reach the second floor of a building just to
see the landscape from there.  You may even
climb up a tall tree.  If we apply the scientific
definition, these activities involve a lot of work.
In day-to-day life, we consider any useful
physical or mental labour as work. Activities
like playing in a field, talking with friends,
humming a tune, watching a movie, attending
a function are sometimes not considered to
be work.  What constitutes ‘work’ depends
on the way we define it. We use and define
the term work differently in science.  To
understand this let us do the following
activities:
Activity _____________11.1
• We have discussed in the above
paragraphs a number of activities
which we normally consider to be work
11 11
11 11 11
W W W W WORK ORK ORK ORK ORK     AND AND AND AND AND E E E E ENERGY NERGY NERGY NERGY NERGY
Chapter
Activity _____________11.3
• Think of situations when the object is
not displaced in spite of a force acting
on it.
• Also think of situations when an object
gets displaced in the absence of a force
acting on it.
• List all the situations that you can
think of for each.
• Discuss with your friends whether
work is done in these situations.
11.1.3 WORK DONE BY A CONSTANT FORCE
How is work defined in science? To
understand this, we shall first consider the
case when the force is acting in the direction
of displacement.
Let a constant force, F act on an object.
Let the object be displaced through a
distance, s in the direction of the force (Fig.
11.1). Let W be the work done. We define work
to be equal to the product of the force and
displacement.
Work done = force × displacement
W= F s (11.1)
in day-to-day life. For each of these
activities, ask the following questions
and answer them:
(i) What is the work being done on?
(ii) What is happening to the object?
(iii) Who (what) is doing the work?
11.1.2 SCIENTIFIC CONCEPTION OF WORK
To understand the way we view work and
define work from the point of view of science,
let us consider some situations:
Push a pebble lying on a surface. The
pebble moves through a distance. You exerted
a force on the pebble and the pebble got
displaced. In this situation work is done.
A girl pulls a trolley and the trolley moves
through a distance.  The girl has exerted a
force on the trolley and it is displaced.
Therefore, work is done.
Lift a book through a height. To do this
you must apply a force.  The book rises up.
There is a force applied on the book and the
book has moved.  Hence, work is done.
A closer look at the above situations
reveals that two conditions need to be
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
daily life involving work.
• List them.
• Discuss with your friends whether
work is being done in each situation.
• Try to reason out your response.
• If work is done, which is the force acting
on the object?
• What is the object on which the work
is done?
• What happens to the object on which
work is done?
Fig. 11.1
Thus, work done by a force acting on an
object is equal to the magnitude of the force
multiplied by the distance moved in the
direction of the force. Work has only
magnitude and no direction.
In Eq. (11.1), if F = 1 N and s = 1 m then
the work done by the force will be 1 N m.
Here the unit of work is newton metre (N m)
or joule  (J).  Thus 1 J is the amount of work
WORK AND ENERGY 147 SCIENCE 148
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. (11.1) 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 11.1 A force of 5 N is acting on
an object. The object is displaced
through 2 m in the direction of the force
(Fig. 11.2). If the force acts on the object
all through the displacement, then
work done is 5 N × 2 m  =10 N m or
10 J.
Fig. 11.4
Consider a situation in which an object
is being displaced by the action of forces and
we identify one of the forces, F acting opposite
to the direction of the displacement s, that
is, the angle between the two directions is
180º.  In 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 _____________11.4
• Lift an object up. Work is done by the
force exerted by you on the object.  The
object moves upwards. The force you
exerted is in the direction of
displacement. However, there is the
force of gravity acting on the object.
• Which one of these forces is doing
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 11.2 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.
Fig. 11.2
uestion
1. A force of 7 N acts on an object.
The displacement is, say 8 m, in
the direction of the force
(Fig. 11.3).  Let us take it that the
force acts on the object through
the displacement. What is the
work done in this case?
Fig. 11.3
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. 11.4.  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.
Q
WORK AND ENERGY 149
raised hammer falls on a nail placed on a
piece of wood, it drives the nail into the wood.
We have also observed children winding a toy
(such as a toy car) and when the toy is placed
on the floor, it starts moving. When a balloon
is filled with air and we press it we notice a
change in its shape. As long as we press it
gently, it can come back to its original shape
when the force is withdrawn. However, if we
press the balloon hard, it can even explode
producing a blasting sound. In all these
examples, the objects acquire, through
different means, the capability of doing work.
An object having a capability to do work is
said to possess energy. The object which does
the work loses energy and the object on which
the work is done gains energy.
How does an object with energy do work?
An object that possesses energy can exert a
force on another object. When this happens,
energy is transferred from the former to the
latter. The second object may move as it
receives energy and therefore do some work.
Thus, the first object had a capacity  to do
work. This implies that any object that
possesses energy can do work.
The energy possessed by an object is thus
measured in  terms of  its  capacity of doing
work. The unit of energy is, therefore, the same
as that of work, that is, joule (J). 1 J is the
energy required to do 1 joule of work.
Sometimes a larger unit of energy called kilo
joule (kJ) is used. 1 kJ equals 1000 J.
11.2.1 FORMS OF ENERGY
Luckily the world we live in provides energy
in many different forms.  The various forms
include potential energy, kinetic energy, heat
energy, chemical energy, electrical energy and
light energy.
Think it over !
How do you know that some entity is a
form of energy? Discuss with your friends
and teachers.
Work done, W= F × s = mg × s
= 15 kg × 10 m s
-2 
× 1.5 m
= 225 kg m s
-2
 m
= 225 N m =  225 J
Work done is 225 J.
uestions
1. When do we say that work is
done?
2. Write an expression for the work
done when a force is acting on
an object in the direction of its
displacement.
3. Define 1 J of work.
4. A pair of bullocks exerts a force
of 140 N on a plough.  The field
being ploughed is 15 m long.
How much work is done in
ploughing the length of the field?
11.2 Energy
Life is impossible without energy. The demand
for energy is ever increasing.  Where do we
get energy from? The Sun is the biggest
natural source of energy to us. Many of our
energy sources are derived from the Sun.  We
can also get energy from the nuclei of atoms,
the interior of the earth, and the tides.  Can
you think of other sources of energy?
Activity _____________11.5
• A few sources of energy are listed above.
There are many other sources of
energy. List them.
• Discuss in small groups how certain
sources of energy are due to the Sun.
• Are there sources of energy which are
not due to the Sun?
The word energy is very often used in our
daily life, but in science we give it a definite
and precise meaning. Let us consider the
following examples: when a fast moving
cricket ball hits a stationary wicket, the wicket
is thrown away. Similarly, an object when
raised to a certain height gets the capability
to do work. You must have seen that when a
Q
Page 5


In the previous few chapters we have talked
about ways of describing the motion of
objects, the cause of motion and gravitation.
Another concept that helps us understand and
interpret many natural phenomena is ‘work’.
Closely related to work are energy and power.
In this chapter we shall study these concepts.
All living beings need food.  Living beings
have to perform several basic activities to
survive.  We call such activities ‘life processes’.
The energy for these processes comes from
food. We need energy for other activities like
playing, singing, reading, writing, thinking,
jumping, cycling and running.  Activities that
are strenuous require more energy.
Animals too get engaged in activities.  For
example, they may jump and run.  They have
to fight, move away from enemies, find food
or find a safe place to live. Also, we engage
some animals to lift weights, carry loads, pull
carts or plough fields.  All such activities
require energy.
Think of machines. List the machines that
you have come across. What do they need for
their working? Why do some engines require
fuel like petrol and diesel?  Why do living
beings and machines need energy?
11.1 Work
What is work? There is a difference in the
way we use the term ‘work’ in day-to-day life
and the way we use it in science.  To make
this point clear let us consider a few examples.
11.1.1 NOT MUCH ‘WORK’ IN SPITE OF
WORKING HARD!
Kamali is preparing for examinations. She
spends lot of time in studies. She reads books,
draws diagrams, organises her thoughts,
collects question papers, attends classes,
discusses problems with her friends, and
performs experiments.  She expends a lot of
energy on these activities. In common
parlance, she is ‘working hard’. All this ‘hard
work’ may involve very little ‘work’ if we go by
the scientific definition of work.
You are working hard to push a huge rock.
Let us say the rock does not move despite all
the effort. You get completely exhausted.
However, you have not done any work on the
rock as there is no displacement of the rock.
You stand still for a few minutes with a
heavy load on your head.   You get tired. You
have exerted yourself and have spent quite a
bit of your energy. Are you doing work on the
load?  The way we understand the term ‘work’
in science, work is not done.
You climb up the steps of a staircase and
reach the second floor of a building just to
see the landscape from there.  You may even
climb up a tall tree.  If we apply the scientific
definition, these activities involve a lot of work.
In day-to-day life, we consider any useful
physical or mental labour as work. Activities
like playing in a field, talking with friends,
humming a tune, watching a movie, attending
a function are sometimes not considered to
be work.  What constitutes ‘work’ depends
on the way we define it. We use and define
the term work differently in science.  To
understand this let us do the following
activities:
Activity _____________11.1
• We have discussed in the above
paragraphs a number of activities
which we normally consider to be work
11 11
11 11 11
W W W W WORK ORK ORK ORK ORK     AND AND AND AND AND E E E E ENERGY NERGY NERGY NERGY NERGY
Chapter
Activity _____________11.3
• Think of situations when the object is
not displaced in spite of a force acting
on it.
• Also think of situations when an object
gets displaced in the absence of a force
acting on it.
• List all the situations that you can
think of for each.
• Discuss with your friends whether
work is done in these situations.
11.1.3 WORK DONE BY A CONSTANT FORCE
How is work defined in science? To
understand this, we shall first consider the
case when the force is acting in the direction
of displacement.
Let a constant force, F act on an object.
Let the object be displaced through a
distance, s in the direction of the force (Fig.
11.1). Let W be the work done. We define work
to be equal to the product of the force and
displacement.
Work done = force × displacement
W= F s (11.1)
in day-to-day life. For each of these
activities, ask the following questions
and answer them:
(i) What is the work being done on?
(ii) What is happening to the object?
(iii) Who (what) is doing the work?
11.1.2 SCIENTIFIC CONCEPTION OF WORK
To understand the way we view work and
define work from the point of view of science,
let us consider some situations:
Push a pebble lying on a surface. The
pebble moves through a distance. You exerted
a force on the pebble and the pebble got
displaced. In this situation work is done.
A girl pulls a trolley and the trolley moves
through a distance.  The girl has exerted a
force on the trolley and it is displaced.
Therefore, work is done.
Lift a book through a height. To do this
you must apply a force.  The book rises up.
There is a force applied on the book and the
book has moved.  Hence, work is done.
A closer look at the above situations
reveals that two conditions need to be
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
daily life involving work.
• List them.
• Discuss with your friends whether
work is being done in each situation.
• Try to reason out your response.
• If work is done, which is the force acting
on the object?
• What is the object on which the work
is done?
• What happens to the object on which
work is done?
Fig. 11.1
Thus, work done by a force acting on an
object is equal to the magnitude of the force
multiplied by the distance moved in the
direction of the force. Work has only
magnitude and no direction.
In Eq. (11.1), if F = 1 N and s = 1 m then
the work done by the force will be 1 N m.
Here the unit of work is newton metre (N m)
or joule  (J).  Thus 1 J is the amount of work
WORK AND ENERGY 147 SCIENCE 148
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. (11.1) 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 11.1 A force of 5 N is acting on
an object. The object is displaced
through 2 m in the direction of the force
(Fig. 11.2). If the force acts on the object
all through the displacement, then
work done is 5 N × 2 m  =10 N m or
10 J.
Fig. 11.4
Consider a situation in which an object
is being displaced by the action of forces and
we identify one of the forces, F acting opposite
to the direction of the displacement s, that
is, the angle between the two directions is
180º.  In 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 _____________11.4
• Lift an object up. Work is done by the
force exerted by you on the object.  The
object moves upwards. The force you
exerted is in the direction of
displacement. However, there is the
force of gravity acting on the object.
• Which one of these forces is doing
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 11.2 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.
Fig. 11.2
uestion
1. A force of 7 N acts on an object.
The displacement is, say 8 m, in
the direction of the force
(Fig. 11.3).  Let us take it that the
force acts on the object through
the displacement. What is the
work done in this case?
Fig. 11.3
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. 11.4.  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.
Q
WORK AND ENERGY 149
raised hammer falls on a nail placed on a
piece of wood, it drives the nail into the wood.
We have also observed children winding a toy
(such as a toy car) and when the toy is placed
on the floor, it starts moving. When a balloon
is filled with air and we press it we notice a
change in its shape. As long as we press it
gently, it can come back to its original shape
when the force is withdrawn. However, if we
press the balloon hard, it can even explode
producing a blasting sound. In all these
examples, the objects acquire, through
different means, the capability of doing work.
An object having a capability to do work is
said to possess energy. The object which does
the work loses energy and the object on which
the work is done gains energy.
How does an object with energy do work?
An object that possesses energy can exert a
force on another object. When this happens,
energy is transferred from the former to the
latter. The second object may move as it
receives energy and therefore do some work.
Thus, the first object had a capacity  to do
work. This implies that any object that
possesses energy can do work.
The energy possessed by an object is thus
measured in  terms of  its  capacity of doing
work. The unit of energy is, therefore, the same
as that of work, that is, joule (J). 1 J is the
energy required to do 1 joule of work.
Sometimes a larger unit of energy called kilo
joule (kJ) is used. 1 kJ equals 1000 J.
11.2.1 FORMS OF ENERGY
Luckily the world we live in provides energy
in many different forms.  The various forms
include potential energy, kinetic energy, heat
energy, chemical energy, electrical energy and
light energy.
Think it over !
How do you know that some entity is a
form of energy? Discuss with your friends
and teachers.
Work done, W= F × s = mg × s
= 15 kg × 10 m s
-2 
× 1.5 m
= 225 kg m s
-2
 m
= 225 N m =  225 J
Work done is 225 J.
uestions
1. When do we say that work is
done?
2. Write an expression for the work
done when a force is acting on
an object in the direction of its
displacement.
3. Define 1 J of work.
4. A pair of bullocks exerts a force
of 140 N on a plough.  The field
being ploughed is 15 m long.
How much work is done in
ploughing the length of the field?
11.2 Energy
Life is impossible without energy. The demand
for energy is ever increasing.  Where do we
get energy from? The Sun is the biggest
natural source of energy to us. Many of our
energy sources are derived from the Sun.  We
can also get energy from the nuclei of atoms,
the interior of the earth, and the tides.  Can
you think of other sources of energy?
Activity _____________11.5
• A few sources of energy are listed above.
There are many other sources of
energy. List them.
• Discuss in small groups how certain
sources of energy are due to the Sun.
• Are there sources of energy which are
not due to the Sun?
The word energy is very often used in our
daily life, but in science we give it a definite
and precise meaning. Let us consider the
following examples: when a fast moving
cricket ball hits a stationary wicket, the wicket
is thrown away. Similarly, an object when
raised to a certain height gets the capability
to do work. You must have seen that when a
Q
SCIENCE 150
Fig. 11.5
• The trolley moves forward and hits the
wooden block.
• Fix a stop on the table in such a
manner that the trolley stops after
hitting the block. The block gets
displaced.
• Note down the displacement of the
block. This means work is done on the
block by the trolley as the block has
gained energy.
• From where does this energy come?
• Repeat this activity by increasing the
mass on the pan. In which case is the
displacement more?
• In which case is the work done more?
• In this activity, the moving trolley does
work and hence it possesses energy.
A moving object can do work.  An object
moving faster can do more work than an
identical object moving relatively slow. A
moving bullet, blowing wind, a rotating wheel,
a speeding stone can do work. How does a
bullet pierce the target? How does the wind
move the blades of a windmill?  Objects in
motion possess energy. We call this energy
kinetic energy.
A falling coconut, a speeding car, a rolling
stone, a flying aircraft, flowing water, blowing
wind, a running athlete etc. possess kinetic
energy.  In short, kinetic energy is the energy
possessed by an object due to its motion. The
kinetic energy of an object increases with its
speed.
How much energy is possessed by a
moving body by virtue of its motion?  By
definition, we say that the kinetic energy of a
body moving with a certain velocity is equal
to the work done on it to make it acquire that
velocity.
11.2.2 KINETIC ENERGY
Activity _____________11.6
• Take a heavy ball.  Drop it on a thick
bed of sand. A wet bed of sand would
be better. Drop the ball on the sand
bed from height of about 25 cm.  The
ball creates a depression.
• Repeat this activity from heights of
50 cm, 1m and 1.5 m.
• Ensure that all the depressions are
distinctly visible.
• Mark the depressions to indicate the
height from which the ball was
dropped.
• Compare their depths.
• Which one of them is deepest?
• Which one is shallowest?  Why?
• What has caused the ball to make a
deeper dent?
• Discuss and analyse.
Activity _____________11.7
• Set up the apparatus as shown in
Fig. 11.5.
• Place a wooden block of known mass
in front of the trolley at a convenient
fixed distance.
• Place a known mass on the pan so that
the trolley starts moving.
James Prescott
Joule was an
outstanding
British physicist.
He is best known
for his research in
electricity and
thermodynamics.
Amongst other
things, he
formulated a law
for the heating
effect of electric
current. He also
verified experimentally the law of
conservation of energy and discovered
the value of the mechanical equivalent
of heat. The unit of energy and work
called joule, is named after him.
James Prescott Joule
(1818 – 1889)
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