CLASS 8 ENERGY

WELCOME TO MY CLASS
Mr.Souvik Chatterjee
M.Sc(Physics)&B.Ed

WORK
• Work is said to be done if the force applied on
a body moves it. If no motion takes place , no
work is said to be done
• A person does no work if there is no change in
position or no motion even after application of
force.

• Thus, following two conditions must be
fulfilled for work to be done
(1) A force must act on the body
(2) A force must produce change in position i.e
motion or produce change in size or shape of
the body.

FACTORS AFFECTING THE
AMOUNT OF WORK DONE
Experimentally it is found that the amount of work
done by a force depends on the following two
factors:
(1)The magnitude of the force applied
Work done is more if the force applied to move the
body is more
(2)The distance moved by the body in the direction
of force.
Work done is more if the distance moved by the
body in the direction of force is more.

DEFINITION OF WORK
• The work done by a force on a body is equal to
the product of the force applied and the
distance moved by the body in the direction of
force i.e,
• Work done = Force X distance moved in the
direction of force.

KEY POINTS
• The work done by a force is zero if the body moves in a
direction perpendicular to the direction of force. For example,
when a stone tied at the end of a string is whirled in a
horizontal circular path, the motion of stone is always normal
to the force of tension in the string . Therefore, the work done
by the force of tension on the stone is zero.
• Similarly, in motion of earth around the sun, the force of
attraction on earth by the sun is always normal to the direction
of motion, so no work is done by the gravitational force of sun
on the earth.
• Similarly a coolie moves on a plane road with luggage on his
head, the work done by him against the force of gravity is
zero. Since distance moved by him is normal to the weight.

UNIT OF WORK
• S.I unit of force is joule.
• One joule of work is said to be done if one
newton force when acting on a body moves it
by 1 metre in the direction of force.
• 1J = 1N X 1m

ENERGY
• Energy is the capacity to do work
• The S.I unit of energy is joule(J)
• A body is said to possess energy of 1 J if it can
do work 1 J or if 1 J work is done on it.

MECHANICAL ENERGY
• The energy possessed by a body due to its state
of rest or state of motion, is called mechanical
energy.
• Mechanical energy is found in two forms
• Potential Energy
• Kinetic Energy

POTENTIAL ENERGY
• The energy possessed by a body due to its state
of rest or position is called potential energy
• The energy stored in the body when the work
has been done by a force in bringing the body
to the state of rest or position i.e work done on
the body has been stored in it in form of
potential energy.

• When a spring is compressed, the work done in
compressing the spring is stored in the spring in
the form of potential energy and the compressed
spring is said to possess potential energy. The
energy of a stretched bow or compressed spring is
also called elastic potential energy.
• When a body is taken from the earth surface to a
height, work has to be done on the body against
the force of gravity on it. This work done is stored
in the body in form of its potential energy. The
energy is also called gravitational potential
energy.

EXAMPLES
1. A wound up watch spring has potential
energy because of the wound up state of its
coils. As the spring unwinds itself, it moves
the arms of the watch.
2. A compressed spring has potential energy
stored in it which equal to the work done on
the spring in bringing it to the compressed
state.

A stretch rubber band has potential energy. It does
not work restoring itself to its original state . A
pebble placed on the stretched rubber catapult, is
thrown away when it is released to restore its
original state.
In stretching a rubber band, work is done. This
work is stored in the rubber band in the form of
potential energy.

• 4. A hammer at a height has potential energy
stored in it. The hammer has this energy
because of its position at a height. In figure,
the hammer is dropped on a nail fixed on a
piece of wood. It drives the nail into the wood
as shown in figure.

• 5.In figure a falling stone when reaches a pan,
attached at one end of a pulley, lifts up a
weight at its other end because of its potential
energy.
• 6. Water at a height has potential energy stored
in it. Falling water from a height can be used
to do work such as turning a wheel.

FACTORS AFFECTING THE
POTENTIAL ENERGY
• Potential energy of a body depends upon the following
two factors:
(a) The mass of the body: Greater the mass of the body,
greater the potential energy of the body.
(b) Its height above the ground: Higher the height of the
body, greater is its potential energy.
Formula: mass X heightX Gravity = mgh
Or
Weight X height = mg X h= mgh

KINETIC ENERGY
Kinetic energy of a body is the energy
possessed by it due to its state of
motion.
Actually it is the work done on the
body bringing it to the state of
motion

EXAMPLE
1.A fast moving stone has the capacity of
breaking when it strikes the pane. The fast
moving stone has kinetic energy.
2. A falling hammer (i.e in motion) when strikes
a nail fixed on a wooden block moves in
further into the block. Thus moving hammer
has kinetic energy and it does work on the nail.

3.In a swinging pendulum, moving to and fro,
the bob has kinetic energy.
4. In a river or sea, the flowing water has kinetic
energy which makes wooden boat or a log of
wood to move in it.
5. A bullet fired from a gun, a rolling ball, an
apple falling from a tree etc all have kinetic
energy.

FACTORS ON WHICH KINETIC
ENERGY DEPEND
Kinetic energy of a moving body depends on the
following two factors:
(a)The mass of the body: Greater the mass,
higher is the kinetic energy.
(b)The speed of the body: More the speed of the
body, more is its kinetic energy.
Formula: ½ x mass x speed2
= ½ x m x v2

CONVERSION OF POTENTIAL
ENERGY INTO KINETIC ENERGY
• Potential energy changes into kinetic energy when it is put to
use. In absence of friction the sum of potential and kinetic
energy remains same. This is THE LAW OF
CONSERVATION OF ENERGY
• Examples
(1)A hammer at a height has potential energy due to its lifted
or raised position. When a hammer is dropped from that
position, it begins to fall. The falling hammer has kinetic
energy. Thus the potential energy stored in the hammer in its
raised position changes into kinetic energy when the hammer
is falling. The kinetic energy does work on the nail as the
hammer strikes the nail and causes the nail to move into the
wood

(2)The wound up watch spring has potential energy
because of its wound up state. As the spring
unwinds itself, the potential energy changes into
kinetic energy. The kinetic energy does work in
moving the arms of the watch.
(3)A stretch bow has potential energy because of its
stretch position. When the stretch bow is released,
the potential energy of the bow changes into its
kinetic energy. This kinetic energy does work on
the arrow and makes the arrow to move

• (4) A compressed spring has potential energy
in it due its compressed state, when the
compressed spring is released, the potential
energy changes into its kinetic energy which
does work on the ball placed on it and makes
the ball to fly away.

• The water collected in a dam at a height has
potential energy stored in it. When it is made
to fall on a turbine, the potential energy
changes to kinetic energy which is transferred
to the turbine to rotate it. By connecting the
turbine to the armature of a dynamo, electricity
(called the hydroelectricity) is produced.

TRANSFORMATION OF ENERGY
• Energy can neither be created or destroyed but can
transform one form to any other form.
• While transformation of energy takes place some
energy is wasted as a un-useful form of energy such as
heat.
• The sum of useful and un-useful energy will be
constant.
• If 100J of electrical energy is converted
70 J of mechanical energy, the remaining 30 J of energy is
converted to heat and sound energy.

SOME EXAMPLE
• Mechanical to Electrical - Dynamo
• Electrical to Mechanical – Motor
• Electrical to Heat – Electrical Iron
• Heat to Electrical – Thermo couple
• Electrical to Sound – Loud Speaker
• Sound to Electrical – Microphone
• Chemical to Light – Lighted Candle/Sun
• Light to Chemical – Photosynthesis
• Electrical to Light – Electric Bulb
• Light to Electrical - Photocell

KEY POINTS
• In transformation of energy when one form of
energy changes into other form of energy, a part of
energy is converted into non-useful form which is
called dissipation of energy.
• e.g When a bulb is glowing electrical energy gets
converted to light energy. But a part of energy is
wasted as heat which is dissipation of energy.

POWER
• Power can be defined as the rate of doing work
by the body.
• Power= Work done by the body/Time taken
• P=W/t
• S.I unit of power is watt(W)
• If the work done is 1 joule in 1 second then the
power consumed is 1 watt

• If we use an electric motor of power 1.5 kW
then the electric motor can do work 1.5 X 1000
J=1500 J in 1 s.
• If this electric motor is used for 3 h the total
energy spent will be (1.5 X 3)=4.5 kWh for
which cost will be paid to the electric board.
• If we say 9W LED is glowing, we mean to say
that while glowing, LED consumes 9 J of
energy per second.

KEY POINTS
• In mechanical engineering, we often use horse
power(H.P) as the unit of power. It is related to
watt as 1 H.P.= 746 watt
• kWh (kilowatt hour) is the unit of energy
where 1 kWh= 3.6 X 10^6 J

FACTORS AFFECTING POWER
• The amount of work done by the
source
• The time taken by the source to do
the said work

NUMERICALS FROM CONCISE
PHYSICS
• 1.A force of 30 N acts on a body and moves it
through a distance of 5 m in the direction of force.
Calculate work done by the force.
• W=Force X Displacement = 30 X 5 = 150 J
• A man lifts a mass of 20 kg to a height of 2.5 m.
Assuming that the force of gravity on 1 kg mass
is 10 N, find the work done by the man.
W=m X g X h = 20 X10 X 2.5 =500 J

• 3.A boy when acted upon by a force of 10 kgf
moves to a distance 0.5 m in the direction of
force. Find the work done by the force.
• F = 10 kgf = 10 X 10 = 100 N
• W = F X S = 100 X 0.5 = 50 J
• 4.Two bodies of same masses are placed at
height h and 2h. Compare their gravitational
potential energy.
• E1 = m X g X h =mgh
• E2 = m X g X 2h = 2 mgh
• E1:E2 = mgh : 2mgh = 1 : 2

• 5. Find the gravitational potential energy of 2.5 kg
mass kept at a height of 15 m above the ground.
The force of gravity on mass 1 kg is 10 N.
• U=W = m X g X h = 2.5 X 10 X 15 = 375 J
• 6.The gravitational potential energy stored in a
box of weight 150 kgf is 1.5 X 10^4 J. Find the
height of the box. Take 1 kgf = 10
N.
• Weight = 150 kgf = 150 X 10 = 1500 N
• Height= Gravitational P.E / Weight =
(1.5 X 10^4)/1500 = 10 m

• 7. The potential energy of a body of mass 0.5
kg increases by 100 J when it is taken to the
top of a tower from ground. If the force of
gravity on 1 kg is 10 N, what is the height of
the tower?
• Change in height = Change in P.E/ Weight
• = 100/ (0.5 X 10) = 20 m
• 8. A body of mass 60 kg is moving with a
speed 50 m/s. Find the kinetic energy.
• K.E= ½ X 60 X 50^2=7.5 X 10^ 4 J

• 9. A truck of mass 1000 kg, increases its speed
from 36 km/h to 72 km/h. Find the increase in
its kinetic energy.
• Initial speed = 36 X 5/18 = 10 m/s
• Final speed = 72 X 5/18 = 20 m/s
• Increase in K.E = ½ X 1000 kg X (20^2 –
10^2)
• = ½ X 1000 X 300 = 1.5 X 10^5 J

• 10. A car is moving with a speed of 15 km/h
and another identical car is moving with a
speed 30 km/h. Compare their kinetic energy.
• Kinetic Energy proportional to speed^2
• E1/E2= (V1/V2)^2
• = (15/30)^2
• =(1/2)^2
• ¼
• E1:E2 = 1:4

• 11. A pump raises water by spending 4 X
10^5 J of energy in 10 s. Find the power of the pump.
• Power= Work done/Time = 4 X 10^5/10 = 4 X
10^4 W
• 12. It takes 20 s for a girl A to climb up the stairs while girl
B takes 15 s for the same job. Compare: (i) The work done
(ii) The power spent by them
• (i) Since both the girls climb up same no of stairs, so same
height, hence work done by them is equal. So the ratio of
work done
• W1:W2 = 1:1
• (ii) Power inversely proportional to time.
• P1/P2 = t2/t1 = 15/20 = ¾
• P1:P2 = 3:4

QUESTIONS FROM CONCISE
PHYSICS
• 1.State the kind of energy changes in the
following while in use
• (a)An Electric Bulb
• (b)A loud speaker
• (c)A electric oven
• (d)A microphone
• (e)An electric motor.

• 2.Give Reasons for the following:
• (a)No work is done if a man is pushing against
a wall
• (b)Hammer drives a nail into the wood only
when it is lifted up and then struck.
• (c)A horse and a dog are running with the
same speed. Which one of them has more
kinetic energy than the other?
• (d)A teacher moving around in the class is
doing work but a child standing and reading a
book not doing any work.

• 3.Name the type of energy (kinetic or
potential) possessed by the following:
• (i) A moving cricket ball
• (ii)A stone at rest on the top of a building
• (iii)A compressed spring
• (iv)A bullet fired from the gun
• (v)A moving bus
• (vi)Water flowing in a river
• (vii) A stretched rubber band

• 4. Can a body possess energy even when not in
motion? Explain your answer with an example.
• 5. Two toy cars A and B of masses 200 g and
500 g respectively are moving with the same
speed. Which of the two has greater kinetic
energy?
• 6.Two bodies A and B of masses 10 kg and 20
kg respectively are at the same height above
the ground. Which of the two has greater
potential energy?

• 7.The moon revolving around the earth in a
circular path. How much work is done by the
moon? Give explanation.
• 8. In which of the following cases is work
being done:
• (a)A boy pushing a heavy rock
• (b)A boy climbing up the stairs
• (c)A coolie standing with the box on his head.
• (d)A girl moving on the road.

• A bucket full of water is on the first floor of
your house and another identical bucket with
same quantity of water is kept on the second
floor. Which of the two has greater potential
energy?

CLASS 8 ENERGY

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