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1. Science
Shree Shitaladevi Community Secondary School
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Grade - 9
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2. UNIT: 4 Work, Energy & Power
• Work: The product of force acted on a body and displacement of a
body in the direction of force is called work.
It is a scalar quantity. Its SI unit is Joule and CGS unit is erg.
Work(W) = Force x displacement in the direction of force
W = F x d
Factors affecting the Work:
i) Magnitude of force ii) Direction of force
iii) Displacement of the object due to force
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3. Work
One Joule Work: One joule work is defined as the work done when
one newton (1N) force displaces an object through one meter (1m)
in its direction.
1 Joule = 1N x 1m
• One erg Work: One erg work is defined as the work done when
one dyne (1 dyne) force displaces an object through one
centimeter(1cm) in its direction. ( 1J = 107 erg)
There is two types of work: i) Work against gravity ii) Work done
against friction
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4. Energy
Ability to do work is called energy. It is scalar quantity. SI unit of
energy is Joule.
Types of Energy: i) Mechanical Energy ii) Electrical Energy iii) Heat
Energy iv) Light Energy V) Sound Energy VI) Magnetic Energy
VII) Atomic Energy VIII) Chemical Energy
Mechanical Energy: Energy possessed by a body by virtue of its
position or its motion is called mechanical energy. It is of two types:
Kinetic energy and Potential energy.
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5. Energy
Potential Energy: The energy possessed by an object due to its
position is called potential energy. For example: The water stored in
dam, the stretched rubber, the leg lifted to kick the football, a
stretched bow etc. has the potential energy.
P.E. = mgh
Kinetic Energy: The energy possessed by an object due to its
motion is called kinetic energy. For example: pebble released from
the catapult, bullet released from the gun, flowing water, wind ,
running man, moving vehicle etc. possess kinetic energy.
K.E. = ½ mv2
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6. Principle of Conservation of Energy
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“Energy can neither be created nor be destroyed, but can be
transformed from one form to another” this is called Principle of
energy.
Transformation of Energy:
Conversion of energy from one form to another is called
transformation of energy. For examples:
a) On listening to the radio
Chemical energy Electrical energy Sound Energy
(Battery) (Wire) (Radio)

7. Transformation of Energy & Power
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b) The production of electricity in hydro-stations
Potential Energy Kinetic Energy Electrical Energy Light Energy
(Water in dam) (Rotation of turbine) (Generator) (Bulb)
Power: The rate of doing work in per unit time is called power. It is scalar
quantity and SI unit is Joule per second or watt.
Power = work done / time taken
P= W/t
P= F x d/ t (W= F x d)
P= F x v ( v= d/ t)

8. Power
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One Watt Power: If one Joule work is done in one second then it is called one
watt power.
In other words, one watt is the energy conversion at the rate of 1 Joule per
second.
1 watt = 1 Joule / 1 second
Bigger Units of Power are:
1 KW = 1000 W
1 MW = 1000000 W
1 Hp = 746 W

9. Any Questions?
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10. Exercise based on Work
Short Questions
1. When a person pushes a building, even if he gets tired, he
doesn't do any work, why?
2. Is it possible that a force is acting on a body but still the work
done is zero? Explain.
3. What happens when force is applied perpendicular to the
displacement?
4. A coolie carrying a load on his head does no work, why?
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11. Exercise based on Energy
Short Questions
1. Define energy and write its SI unit.
2. What is reason behind the same SI unit of work and energy?
3. If a person sleeps for a long time, he will be in energy less
condition even if he does not do any work physically why?
4. What are the factors that affect potential energy?
5. On which case, the potential energy will be more and why?
6. A piece of stone stores potential energy only when it is lifted from
the surface, why?
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12. Exercise based on Power
Short Questions
1. What is meant by '60W written in an electric bulb’?
2. At thermal power station, one form of energy is converted
into another form, Show the energy transformation.
3. The power of machine doing work fast is more, why? Clarify
it.
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13. Numerical Problems
1. A man applied a force of 300 N to move a table by a distance of
70 cm. What is the work done by him? (Ans: 210 J)
2. A porter carrying a load of 30kg climbs up a mountain having
vertical height of 1.5km from the ground. Calculate the work done
against gravity (g = 10m/s2). (Ans: 4.5 x 105J)
3. If the work done while pushing a bus through a distance of 10m is
50KJ. Calculate the force of friction of the ground. (Ans: 5000N)
4. Calculate the amount of work done by a person while taking a
bag whose mass is 200kg to the top of a building of height 10m.
(Use g=10 m/s2). The mass of the person is 50kg. (Ans: 2.5 x
104 J)
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14. Numerical Problems
5. A stone of mass 2kg is at a height of 2m. Calculate its original
energy. Now, the stone is released. What is the kinetic energy and
velocity when it reaches the ground? (Ans: P.E. = 39.2 J,K.E.=39.2 J ,
V= 6.26m/s)
6. what happens to the mass of a body when velocity and kinetic energy
of a body is decreased by 4 times?
7. A bag of rice weighs 200kg. To what height should it be raised so that
its potential energy may be 9800 joules ? (g =9.8m/s2) (Ans: 5m)
8. The velocity & kinetic energy of a bullet are 200km/hr & 77.16 J
respectively. Find the mass of the bullet. (Ans: m= 50 gm)
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15. Numerical Problems
9. Sita weighs 40 kg. If she climbed a 3 meter high stairs in 10
seconds, what is her power? ( Ans: 120 watt)
10. Convert ; i) 5Hp into watt ii) 69 MW into Hp iii) 35KW into MW
11. A man of 50 kg mass climbs 20 steps of stair of Basantapur Durbar
in one minute. If height of each step is 15cm then find his power.
( Ans: 25W )
12. An electric motor rises 80 kg water in 3 minutes from a well 30m
deep. Find the power of the motor. (Ans: 133.33 W)
13. An engine supplies 250 J of energy. If this energy is supplied to a
weight of 400gm. What height can it be lifted to? ( Ans: 62.5 m)
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