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Chapter #6
Work & Energy
Exercise MCQs Answers
Question No.
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Exercise Short Questions
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Define work & write its SI unit?
“Work is a product of force F & displacement S in the direction of force” . SI unit of work is joule (J).
When does a force do work? Explain.
In Physics, work is said to be done when a force acts on a body and moves it in the direction of the force.
Greater is the force acting on a body and longer is the distance moved by it, larger would be the work done.
Why do we need energy?
We need energy to do work. It links almost all the natural phenomena. The energy of running water can be
used to run water mills or water turbines.
Define energy. Give two types of mechanical energy?
The ability of body to do work is called energy.
The two types of mechanical energy are :
(1) Kinetic Energy (2) Potential Energy
Define kinetic energy & write its formula.
The energy possessed by a body due to its motion is called kinetic energy.
1
K.E.= 2 mv2
Define potential energy & write its formula.
The energy possessed by a body due to its position is called potential energy.
P.E. = mgh
Why fossils fuels are called non-renewable form of energy?
The fossil fuels took millions of years for their formation. So they are called non-renewable form of energy.
Which form of energy is most preferred & why?
Energy from the Sun is most preferred because Sunlight does not pollute the environment in any way. The
sunrays are the ultimate source of life on the Earth. We are dependent on the Sun for all our food & fuels.
How is energy converted from one form to another? Explain.
Energy cannot be destroyed however it can be converted into some other forms. For example, rub our hands
together quickly, we will feel them warm. We have used our muscular energy in rubbing hands as a result
heat is produced. In the process of rubbing hands, mechanical energy is converted into heat energy.
Name the five devices that convert electrical energy into mechanical energy?
1. Electric motors 2.Washing machine 3.Fan 4. Refrigerator 5. Electric pump
Name a device that converts mechanical energy into electrical energy?
Electric generator is a device that converts mechanical energy into electrical energy.
What is meant by efficiency of system?
The ratio of output to input of any machine is called efficiency.
How can you find the efficiency of system?
% Efficiency
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required form of output
total input energy
required form of output
= total input energy
Efficiency =
x100
What is meant by term power?
W
The rate of doing work is called power. Its formula is P =
t
6.15:
Define Watt?
The power of a body is one watt if it does work at the rate of one joule per second.
Ans.
1W = 1Js-1.
Punjab Board Short Questions
Q.No.1: Write down the names of major sources of energy?
Ans.
1. Fossil Fuels
2. Nuclear Fuels
3. Energy from Water
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4. Energy from Sun
5. Wind energy
Define sound energy?
Sound is a form of energy. It is produced when a body vibrates; such as vibrating diaphragm of a drum,
vibrating strings of a sitar etc.
Which energy converts into muscular energy?
Mechanical energy is converted into muscular energy in process of rubbing hands.
Who predicted mass-energy equation? Also write equation.
Einstein predicted the interconversion of matter and energy. The relation between mass m and energy E
is given by Einstein’s mass-energy equation. E = mc2
Explain mechanical energy?
The energy possessed by a body both due to its motion or position is called mechanical energy. e.g.
1. Water running down a stream
2. Wind
3. A moving car
What is unit of power? Also define that unit of power?
SI unit of power is watt (W). It is defined as
“The power of a body is one watt if it does work at the rate of one joule per second.” 1W = 1Js-1.
Define chemical energy?
Chemical energy is energy derived from atoms and molecules contained in materials. Chemical energy
is present in food, fuels and in other substances.
Define Einstein’s Mass Energy equation?
According to him, a loss in the mass of a body provides a lot of energy. The relation between mass m
and energy E is given by Einstein’s mass-energy equation. E = mc2.
A cyclist does 12 joules of useful work while pedaling his bike from every 100 joules of food
energy which he takes. What is his efficiency?
Data:
Wo = 12J
Wi = 100J
% ɳ =?
Formula:
𝑊𝑜
%ɳ=
× 100
𝑊𝑖
Solution:
12
%ɳ=
× 100 = 12%
100
How energy is produced from Nuclear fuels? Explain.
In nuclear power plants, we get energy as a result of fission reaction. During fission reaction, heavy
atoms, such as Uranium atoms, split up into smaller parts releasing a large amount of energy. Nuclear
power plants give out a lot of nuclear radiations and vast amount of heat.
Write two parts of Solar Heating System?
A heating system consists of:
1. A collector
2. A storage device
3. A distribution system
A machine does 4 joule of work in 2 seconds. Find its power?
Data:
W = 4J
t = 2s
P=?
Formula:
𝑊
P=
𝑡
Solution:
4
P = = 2W
2
What is soil erosion?
As the rain water flows down, its kinetic energy changes into thermal energy while parts of the kinetic
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energy of flowing water is used to wash away soil particles of rocks known as soil erosion.
Q.No.14: A stone of mass 500 g strikes the ground with a velocity of 20ms-1. How much is the kinetic energy
of the stone at the time it strikes the ground?
Ans.
Data:
500
m = 500g = 1000 = 0.5kg
v = 20ms-1
K.E. = ?
Formula:
1
K. E. = 2 mv2
Solution:
1
K. E. = ×0.5×(20)2
2
1
2
200
2
K. E. = ×0.5×400
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K. E. =
= 100J
Differentiate b/w non-renewable & renewable sources of energy?
Non-renewable sources
Renewable sources
1. The natural sources of energy which 1. The natural sources of energy which are
once used up cannot be replaced over continuously replenished through number of
millions of years are known as non- natural processes & will run out are renewable
renewable sources of energy.
sources of energy.
2. Fossils Fuels & Nuclear Fuels are 2. Sun, Earth, Water, Wind, Biomass, wood are
examples of non-renewable sources of examples of renewable sources of energy.
energy.
A machine does 9 joule of work in 3 seconds. Find its power?
Data:
W = 9J
t = 3s
P=?
Formula:
𝑊
P=
𝑡
Solution:
9
P = = 3W
3
Define joule?
The amount of work is one joule when a force of one Newton displaces a body through one metre in the
direction of force. 1J = 1 N x 1m
What is meant by non-renewable sources of energy? Write any one example?
The natural sources of energy which once used up cannot be replaced over millions of years are known
as non-renewable sources of energy. Fossils Fuels & Nuclear Fuels are examples of non-renewable
sources of energy.
Define magma?
There is hot molten part, deep in the Earth called magma
A body of mass 50 kg is raised to a height of 3 m. What is its potential energy?
Data:
m = 50kg
h = 3m
g = 10ms-2
P.E. = ?
Formula:
P.E. = mgh
Solution:
P.E. = 50×10×3 = 1500J
Write down the names of any four forms of energy?
1. Mechanical energy
2. Heat energy
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3. Electrical energy
4. Sound energy
Differentiate b/w electrical energy & light energy?
The energy of moving electric charges within a circuit is called electrical energy. Light is a form of
energy which is given out by luminous objects. The Sun, bulb, candle, etc. are luminous objects.
The kinetic energy of a body of a mass 2kg is 25J. Find its speed?
Data:
m = 2kg
K.E. = 25J
v=?
Formula:
1
K. E. = 2 mv2
Solution:
1
25 = 2 ×2× v2
v2 = 25
√v 2 = √25
v = 5ms-1
A machine does 20 joule of work in 10 seconds. Find its power?
Data:
W = 20J
t = 10s
P=?
Formula:
𝑊
P=
𝑡
Solution:
20
P=
= 2W
10
Describe the importance of wind energy?
1. It has powered sailing ships across the oceans.
2. It has been used by windmills to grind grain and pump water.
3. Wind power is used to turn wind turbines.
What are environmental problems & why are they created?
Environmental problems such as pollution that consist of noise, air pollution and water pollution may
arise by using different sources of energy such as fossil fuels and nuclear energy.
What is use of solar cells?
1. Solar energy can also be converted directly into electricity by solar cells.
2. Solar cells are used in calculators, watches and toys.
3. Large numbers of solar cells are wired together to form solar panels.
How energy is converted from one form to another?
Energy cannot be destroyed however it can be converted into some other forms. For example, rub our
hands together quickly. We will feel them warm. We have used our muscular energy in rubbing hands
as a result heat is produced. In the process of rubbing hands, mechanical energy is converted into heat
energy.
Describe use of light energy?
1. Plants produce food in the presence of light.
2. We also need light to see things.
Define watt & write how much watt is equal to one horsepower?
The power of a body is one watt if it does work at the rate of 1 joule per second (1 Js-1).
1 hours power = 1 hp = 746 W
Write two disadvantages of thermal pollution?
Thermal pollution upsets the balance of life and endangers the survival of many species.
Why geothermal wells are made?
Geothermal well can be built to produce steam. The steam can be piped directly into houses and offices
for heating purposes or it can be used to generate electricity.
What is meant by nuclear energy?
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Nuclear energy is the energy released in the form of nuclear radiations in addition to heat and light
during nuclear reactions such as fission and fusion reactions.
Q.No.34: A car weighing 12KN has speed of 20ms-1. Find its kinetic energy?
Ans.
Data:
w = 12kN
w = 12×103N
v = 20ms-1
K.E. = ?
Formula:
1
K. E. = 2 mv2
Solution:
w = mg
m=
𝑤
𝑔
=
1
2
1
2
12×103
10
=1200kg
K. E. = ×1200×(20)2
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K. E. = ×1200×400
K. E. =600×400= 240000J= 240×103J = 240KJ
How mechanical energy change into heat energy?
Rub our hands together quickly. We will feel them warm. We have used our muscular energy in
rubbing hands as a result heat is produced. In the process of rubbing hands, mechanical energy is
converted into heat energy.
Write down equation of Einstein & value of c?
The relation between mass m and energy E is given by Einstein’s mass-energy equation. E = mc2.
Here c is the speed of light (3×108 ms-1)
What is meant by solar cells?
A solar cell also called photocell is made from silicon wafer. When sunlight falls on a solar cell, it
converts the light directly into electrical energy.
By defining magma, Describe geothermal energy?
There is hot molten part, deep in the Earth called magma. Water reaching close to the magma changes
to steam due to the high temperature of magma. This energy is called geothermal energy.
How we can produce electricity by using animal waste?
When animal dung, dead plants and dead animals decompose, they give off a mixture of methane and
carbon dioxide. Electricity can be generated by burning methane.
To construct such a working system whose efficiency is 100%, is impossible. Why?
To construct such a working system whose efficiency is 100%, is impossible because Every system
meets energy losses due to friction that causes heat, noise etc. These are not the useful forms of energy
and go waste.
Additional Short Questions
On what factors work depends?
It depends on
1. The force acting on a body
2. Displacement of the body
3. The angle b/w them.
A crate is moved by pulling the rope attached to it. It moves10m on a straight horizontal road by
a force of 100N.How much work will be done if
1. the rope is parallel to the road. 2. the rope is making an angle of 30°with the road.
Data:
F= 100N
S = 10m
W= ?
(a)θ = 00
(b)θ = 300
Formula:
W = FScos θ
Solution:
(a) W = 100×10cos00
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W= 1000× 1=1000J (cos00 = 1)
(b) W = 100×10cos300
W= 1000× 0.866=866J (cos300 = 0.866)
Prove that P.E. = mgh?
P.E. = F×h
P.E. = w×h
As w = mg
∴P.E. = mgh
Define elastic potential energy?
The energy present in the stretched bow is called elastic potential energy.
Define gravitational potential energy?
The energy present in a body due to its height is called gravitational potential energy. e.g. The potential
energy possessed by a hammer is due to its height.
What are fossil fuels?
Fossil fuels are non-renewable sources of energy. They are usually hydrocarbons (compounds of carbon
and hydrogen). We use fossil fuels such as coal, oil and gas to heat our houses and run industry and
transport.
What are non-renewable sources of energy?
Sun light and water power are the renewable sources of energy. They will not run out like coal, oil and
gas.
Write down two advantages of dam?
1. They help to control floods by storing water.
2. The water stored in dams is used for irrigation and also to generate electrical energy without
creating much environmental problems.
What is solar cell?
A solar cell also called photocell is made from silicon wafer. When sunlight falls on a solar cell, it
converts the light directly into electrical energy.
Define pollution & thermal pollution?
Pollution is the change in the quality of environment that can be harmful and unpleasant for living
things. A temperature rise in the environment that disturbs life is called thermal pollution.
What is energy converter?
In an energy converter, a part of the energy taken (used up) by the system is converted into useful work.
Remaining part of the energy is dissipated as heat energy, sound energy (noise) into the environment
What is ideal system?
An ideal system is that which gives an output equal to the total energy used by it. In other words, its
efficiency is 100%.
If velocity of body is doubled, what will be its effect on kinetic energy?
1
K.E.= 2 mv2
Put v = 2v in above equation
1
K.́ E.= 2 m(2v)2
1
K.́ E.= m4v2
2
1
K.́ E.= 4(2 mv 2 )
K.́ E = 4K.E.
If we double the velocity of body, K.E. increases by four times.
Q.No.13: If velocity of body is three times, what will be its effect on kinetic energy?
1
Ans.
K.E.= 2 mv2
Put v = 3v in above equation
1
K.́ E.= 2 m(3v)2
1
K.́ E.= 2 m9v2
1
K.́ E.= 9( mv 2 )
2
K.́ E = 9K.E.
If the velocity of body become three times, K.E. increases by nine times.