The document explains the concepts of energy and work, highlighting their definitions, measurements, and relationships. It describes various types of energy including mechanical, heat, light, chemical, sound, magnetic, electrical, and atomic energy, alongside examples and formulas for potential and kinetic energy. Additionally, it covers the conversion of energy between forms, conservation of energy principles, and practical applications like hydroelectricity.

1. ENERGY

3. WORK
When the force applied on a body moves it, work is
said to be done by the force on the body.
Work is measured as,
Work = Force X Displacement in the direction of
force.
The S.I unit of energy is joule(J).
1 J work is said to be done when a force of 1 N
moves a body by a distance of 1m in the direction of
force.

4. Relation between work and energy
When work is done on the body its energy
increases. In other words, the work done on
the body is stored in it in the form of energy.
But if work is done by the body its energy
decreases. In other words, energy is spent
when the body does work

5. RELATIONSHIP BETWEEN ENERGY AND
WORK
 To do more amount of work, we need more amount of energy. Hence,
We can say that there is a direct relationship between work and
energy.
 Similarly, the work done on a body is changing its state is said to be
energy possessed by the body.

6. E.g when a body is moved upwards work is done
against the force of gravity and the body at the height
is said to possess energy.
Similarly, if a body initially at rest is made to move,
work is done on the body and the body in motion is
said to posses the amount of energy equal to the work
done on the body producing motion in it.

7. ENERGY
 Energy is the capacity to do work
 A body is said to possess an energy of one joule if one joule work
is done to bring the body in that state i.e a force of 1 newton
moves the body by a distance of 1 metre in the direction of force.
 Another unit of energy is calorie.
 1 cal = 4.2 J or 4.816 J
 Bigger unit is kilo-calorie (kcal) where
 1 kcal=1000 cal

8. DIFFERENT KINDS OF
ENERGY
 Mechanical Energy
 Heat Energy
 Light Energy
 Chemical Energy
 Sound Energy
 Magnetic Energy
 Electrical Energy
 Atomic Energy or Nuclear Energy

9. MECHANICAL ENERGY
 The energy possessed by a body due to its state of rest or state
of motion is called mechanical energy
 It is found to be in the two forms
(1) Potential & (2) Kinetic Energy
 The total mechanical energy of a body is the sum of potential
and kinetic energies.

10. HEAT ENERGY
 The energy released when we burn anything like coal , oil , wood or
gas is called heat energy.
 Steam possess heat energy and it capable to do work.
 It is our common experience that if we heat some water in a kettle
with a lid on it, we notice that as the water begins to boil, the steam
makes the lid to lift up. This actually the heat energy of the steam
which moves the lid up.

11. LIGHT ENERGY
 Light is a form of energy in the presence of which other objects are
seen.
 We do not feel light energy because ordinary light does not move the
object around us.
 But strong beam of light can move small particles like electron.
 Plants convert light energy into chemical energy by the process of
photosynthesis.

12. CHEMICAL ENERGY
 The energy possessed by fuel such as coal, oil, gas etc is called
chemical energy.
 The chemical energy of petrol or diesel is capable to move a car or
truck.

13. SOUND ENERGY
 The energy responsible for the sensation of hearing is called sound
energy.
 A vibrating body is the source of sound energy
 We can hear sound when the frequency of sound is in between 20 Hz
and 20,000 Hz

14. MAGNETIC ENERGY
The energy possessed by a magnet is called magnetic
energy.
Magnets can attract an iron nail from a distance and
thus the force exerted by the magnet makes the nail to
move towards the magnet.

15. ELECTRICAL ENERGY
When two dry bodies are rubbed together they get charged
and gain electrical energy.
A comb when rubbed on dry hair can attract small bits of
paper and cause them to move towards the comb.

16. ATOMIC ENERGY
 The energy stored in atom is called atomic energy. Atomic energy cam
be used for destructive purposes.
 For peaceful purpose, atomic reactors are used to produce electrical
energy from atomic energy.

17. For destructive purpose, atom bomb used for destroying
the enemy in a war.
Actually atomic energy is the energy released due to loss
in mass when either a heavy nucleus splits into two light
nuclei or two light nuclei combine to form one nucleus
under certain circumstances. This is why atomic energy is
also called nuclear energy.

18. KEY POINTS
 The sun is a major source of energy.
 The energy which we receive on earth from the sun is called
the solar energy.
 Nowadays solar energy is used for making solar cooker,
solar cells etc.
 Efforts are being made to obtain electricity by using solar
panels from solar energy.
 The Government of India is also promoting to produce
electricity for household purposes by using solar cooker.

19. TWO FORMS OF
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 of two forms
1. Potential Energy
2. Kinetic Energy

20. POTENTIAL ENERGY
 Potential energy of a body is the energy possessed by it due to its state
of rest or position. Actually it is equal to the work done in bringing the
body to that state of rest or position.
 When a body is taken from earth’s surface to height, work has to be
done on the body against the force of gravity on it. The work done in
the body is stored in the form of potential energy. This energy also
called gravitational potential energy.

21. 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.

23. 3. 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.

25. 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.

27. 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.

29. 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 height X Gravity = mgh
Or
Weight X height = mg X h= mgh

30. 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.

31. EXAMPLE
1. A fast moving stone has the capacity of
breaking a window pane 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.

33. 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.

36. FACTORS ON WHICH KINETIC
ENERGY DEPENDS
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

37. CONVERSION OF POTENTIAL ENERGY
INTO KINETIC ENERGY
Potential energy changes into kinetic energy when it
is put to use.
 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.

38. (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.

40. (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.

42. CONVERSION OF ENERGY
Energy can neither be
created nor destroyed but
can be converted from one
form to the other

43. EXAMPLES
 (1) In a steam engine chemical energy of
coal first changes into heat of a steam.
Then the heat changes into mechanical
energy which causes train to move.
(2) In electric motor or fan the electrical
energy changes into mechanical energy.
The energy rotates the axle of motor (or
blades of a fan).

44. (3) In electric heater, oven , toaster, geyser the
electrical energy changes into heat energy.
(4) The electric cell in use, the chemical energy
changes into electrical energy.
(5) In a glowing bulb electrical energy changes
into light and heat energy.
(6) An electric bell while ringing the electric
energy changes into sound energy.

45.  (7) Electric generator or dynamo mechanical energy
changes into electrical energy.
(8) In microphone sound energy changes into electrical
energy.
(9) In loudspeaker electrical energy changes into sound
energy.
(10) Plants during photosynthesis light energy received
from sun changes into chemical energy of food.
(11) When fire crackers burst chemical energy changes
into heat sound and light energy.

50. CONSERVATION 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.
In absence of friction the sum of potential and kinetic energy
always constant i.e Total mechanical energy always constant.

51. 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.

52. ROLLER COASTER

55. FALLING BODY

57. HYDROELECTRICITY

58. 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.

59. Two bodies having masses 50 kg and 100 kg
placed at a same height. Which one should
have more potential energy?
Ans. Mass of 100 kg, more mass more
potential energy
Two bodies having same mass are placed but
placed at 50m and 100m. Which one has
greater potential energy
2nd mass , as the height is more.

60. Name the type of energy(Kinetic or potential)
possessed by the following
Moving cricket ball-
A stone at rest at the top of a building-
A compressed spring-
A moving bus-
A bullet fired from a gun-
Water flowing in a river-
A stretched rubber band-

61. State the energy change in the following:
Unwinding of watch spring-
Burning coal while operating a steam engine-
Lighting of torch bulb-
A electric generator or dynamo-

62. Give one relevant example for each of the
following transformation
Electrical to heat energy-Electric
Heater/Toaster/Geyser
Electrical to mechanical energy- Electric
Fan/Mixer-Grinder
Electrical to light energy-Electric Bulb/Tube
light
Chemical to heat energy- Burning Coal/ Fire
Craker

63. Questions from CONCISE
PHYSICS
A ball is made free to fall from a height. State
the kind/kinds of energy possessed by the ball
when it is
(i) at the highest point
(ii) just in the middle
(iii) At the ground

64. What do you mean by conservation of
mechanical energy? State the condition when
it holds.
In absence of friction the sum of potential and
kinetic energy always constant i.e Total
mechanical energy always constant.
In absence of friction it holds.

65. • Two bodies having masses 500 g and 100 g
having same speed. Which of them having higher
kinetic energy? Why?
• First body having mass 500 g will have more
kinetic energy. More the mass more the kinetic
energy if speed is constant.
• Two bodies having same mass have speed 3 m/s
and 4m/s. Which one should have more kinetic
energy? Why?
• Second body will have more kinetic energy. More
the speed more kinetic energy if mass is constant.

66. Can a body posses energy when it is not in motion?
Explain your answer with an example.
Yes, A stone which is at rest placed in a height will
have potential energy .
A bucket filled with water placed in first floor. The
same bucket with equal amount of water is placed in
second floor. Which one should have more potential
energy? Explain.
The bucket placed at second floor will have more
potential energy because it has more height than the
bucket placed at first floor and mass is constant for
both the buckets..