NCERT Solutions for Class 9 Science -
“WORK AND ENERGY” :
NCERT Solutions for Class 9
Science Chapter 11 “WORK AND ENERGY”: by M.
Farrukh Asif
NCERT Solutions for Class 9 Science Chapter
11 “WORK AND ENERGY”, Updated.
Introducing: Our Science experts have
inteligently crafted "Motion" to align with the updated CBSE 2024-25 syllabus
for class 9. By condensing complex textbook content into straightforward
solutions, students can easily tackle NCERT questions. You can access all the
NCERT Solutions for Class 9 Science Chapter 11 on our “WORK AND ENERGY” (cbsencert21.blogspot.com) website. Furthermore,
students can also find NCERT Solutions for Class 9 Science.
NCERT SOLUTIONS FOR CLASS 9 SCIENCE
CHAPTER-11 WORK AND ENERGY
NCERT Solutions for class 9 Science chapter 8
Motion have been prepared and uploaded for reference by our team of academic
experts at NCERT Solution cbsencert21.blogspot.com.
Access solutions to all chapters of NCERT class 9 Science from our website for
comprehensive guidance. Utilize the following NCERT solutions of chapter 11,
crafted by our team, as a valuable reference. Take the time to review the chapter 8 on Motion before heading into the
exam.
What we have learnt?
Ø Work done on an object
is defined as the magnitude of the force multiplied by the distance moved by
the object in the direction of the applied force. The unit of work is joule: 1
joule = 1 newton × 1 metre
Ø Work done on an object
by a force would be zero if the displacement of the object is zero.
Ø An object having
capability to do work is said to possess energy. Energy has the same unit as
that of work.
Ø An object in motion
possesses what is known as the kinetic energy of the object. An object of mass,
m moving with velocity v has a kinetic energy of 2 mv 1 2 .
Ø The energy possessed by
a body due to its change in position or shape is called the potential energy.
The gravitational potential energy of an object of mass, m raised through a
height, h from the earth’s surface is given by m g h.
Ø According to the law of
conservation of energy, energy can only be transformed from one form to
another; it can neither be created nor destroyed. The total energy before and
after the transformation always remains constant.
Ø Energy exists in nature
in several forms such as kinetic energy, potential energy, heat energy,
chemical energy etc. The sum of the kinetic and potential energies of an object
is called its mechanical energy.
Ø Power is defined as the
rate of doing work. The SI unit of power is watt. 1 W = 1 J/s.
Ø
The energy used in one hour at the rate of 1kW is called 1 kW h.
Multiple Choice Questions (MCQs) on Work and
Energy
5.
THE
FUNDAMENTAL UNIT OF LIFE
6.
TISSUES
7.
MOTION
9.
GRAVITATION
10.
WORK
AND ENERGY
11.
SOUND
12.
IMPROVEMENT
IN FOOD RESOURCES
- What is the SI unit of work?
a) Joule
b) Newton
c) Watt
d) Pascal
Answer: a) Joule
- Which of the following best
describes work done?
a) Force multiplied by the time
b) Force multiplied by the displacement in the direction of force
c) Force multiplied by the velocity
d) Force multiplied by the mass
Answer: b) Force
multiplied by the displacement in the direction of force
- A force of 10 N is applied to
move a block 5 meters. How much work is done?
a) 50 J
b) 5 J
c) 500 J
d) 15 J
Answer: a) 50 J - What type of energy is stored
in a stretched or compressed spring?
a) Kinetic energy
b) Thermal energy
c) Gravitational potential energy
d) Elastic potential energy
Answer: d) Elastic potential energy - What is the kinetic energy of
an object with a mass of 2 kg moving at a velocity of 3 m/s?
a) 3 J
b) 6 J
c) 9 J
d) 18 J
Answer: c) 9 J - The potential energy of an
object at height hhh is directly proportional to:
a) Its velocity
b) Its mass
c) The acceleration due to gravity
d) Both its mass and height
Answer: d) Both its mass and height - What happens to the work done
when the angle between the force and displacement is 90 degrees?
a) Work is maximum
b) Work is zero
c) Work is negative
d) Work is positive but less than the maximum value
Answer: b) Work is zero - Power is defined as:
a) The rate of doing work
b) The product of force and displacement
c) The energy stored in a system
d) The capacity to do work
Answer: a) The rate of doing work - Which of the following is a
form of mechanical energy?
a) Chemical energy
b) Thermal energy
c) Nuclear energy
d) Kinetic energy
Answer: d) Kinetic energy - If a 60 W light bulb is turned
on for 2 hours, how much energy does it consume?
a) 120 J
b) 120 W
c) 7200 J
d) 432,000 J
Answer: d) 432,000 J
Question 1. A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Answer :
Since Work (W)= Force (F) x displacement (d)
= 7 N x 8 m = 56 J
Question 2. When do we say that work is done?
Answer : Work is done when a force
acts on an object and displacement occurs in the direction of force.
Question 3. Write an expression for the work done when a
force is acting on an object in the direction of its displacement.
Answer :
Work (W)= Force (F) x displacement (d)
W = F x d
Question 4. Define 1 J of work.
Answer : 1J is that work which is
done when a force of 1N is applied on an object and it moves a distance of 1m
in the direction of force
Question 5. 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?
Answer :
Work done W = F x d
= 140 N x 15 m = 2100 J
Question 6. What is the kinetic energy of an object?
Answer : Kinetic energy of an object
is a kind of mechanical energy and it is present in that object due to its
state of motion(movement)
Question 7. Write an expression for the kinetic energy of an
object.
Answer :
KE (Kinetic energy ) =
where, m = mass of the object,
v = velocity of the object(motion)
Question 8. The kinetic energy of an object of mass, m
moving with a velocity of 5 m s-1 is 25 J. What will be its kinetic energy
when its velocity is doubled? What will be its kinetic energy when its velocity
is increased three times?
Answer :
K.E. of the object
KE (Kinetic energy ) =
m = (25 x 2) /25 = 2 kg
If velocity is doubled
If velocity is increased three times
Question 9. What is power?
Answer : Power is the rate of work
done. It is denoted by P.
power = work/time
P = W/t
Question 10. Define 1 watt of power.
Answer : If an agent works at the
rate of 1 J/s then the power of that agent is 1 watt (where watt is the unit of
power).
Question 11. A lamp consumes 1000 J of electrical energy in
10 s. What is its power?
Solution :
P = W/t = 1000J/10s
= 100 W
Question 12. Define average power.
Answer : An agent may not be able to
perform same amount of work in a given period of time always. In other words,
power of that agent may vary with time. Hence we can take average power in such
situations. Average power is defined as average amount of work done by a body
per unit time (i.e. total energy consumed divided by total time).
Question 13. Look at the activities listed below. Reason out
whether or not work is done in the light of your understanding of the term
‘work’.
• Suma is swimming in a pond.
• A donkey is carrying a load on its back.
• A wind-mill is lifting water from a well.
• A green plant is carrying out photosynthesis.
• An engine is pulling a train.
• Food grains are getting dried in the sun.
• A sailboat is moving due to wind energy.
Answer : Work will
be done if a force acts on an object and displacement occurs in the direction
of force. According to this explanation work is done in following activities:
• Suma is swimming in a pond.
• A donkey is carrying a load on its back.
• A wind-mill is lifting water from a well.
• An engine is pulling a train.
• A sailboat is moving due to wind energy.
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Question 14. An object thrown at a certain angle to the
ground moves in a curved path and falls back to the ground. The initial and the
final points of the path of the object lie on the same horizontal line. What is
the work done by the force of gravity on the object?
Answer : For calculating work done
there must be displacement but since in this example vertical displacement is
zero (as initial and final points lie on the same horizontal line) so the work
done by force of gravity is zero.
Question 15. A battery lights a bulb. Describe the energy
changes involved in the process.
Answer : Chemical energy of battery
is changed into heat energy and light energy.
Question 16. Certain force acting on a 20 kg mass changes
its velocity from y 5 m s-1 to 2 m s-1.
Calculate the work done by the force.
Answer :
Work done by the force = change in kinetic energy
= 210 J
Question 17. A mass of 10 kg is at a point A on a table. It
is moved to a point B. If the line joining A and B is horizontal, what is the
work done on the object by the gravitational force? Explain your answer.
Answer : Since work done on the
object by gravitational force depends upon change in the vertical height of the
object. Vertical height of the object is not changing as the joining A and B is
horizontal at the same height hence the work done is zero.
Question 18. The potential energy of a freely falling object
decreases progressively. Does this violate the law of conservation of energy?
Why?
Answer : It doesn’t violate the law
of conservation of energy because the potential energy of a freely falling
object decreases progressively since it keeps changing into kinetic energy of the
free falling object therefore the total energy to the object remains conserved.
Question 19. What are the various energy transformations
that occur when you are riding a bicycle?
Answer : It is the transformation of
our muscular energy into mechanical energy to ride the bicycle.
Question 20. Does the transfer of energy take place when you
push a huge rock with all you might and fail to move it? Where is the energy
you spend going?
Answer : While we push a huge rock
with all our might(power) but fail to move it no energy transfer occurs as
cellular energy simply wastes out in muscle contraction and relaxation even
heat generation(sweating).
Question 21. A certain household has consumed 250 units of
energy during a month. How much energy is this in joules?
Answer :
Energy consumed by a certain household = 250 kWh
since 1 kWh = 3.6 x 106 J
therefore 250 kWh
= 250 x 3.6 x 106
= 9 x 108 J
Question 22. An object of mass 40 kg is raised to a height
of 5 m above the ground. What is its potential energy? If the object is allowed
to fall, find its kinetic energy when it is half way down.
Answer :
Potential energy
(P) = m x g x h = 40 x 5 x 10
= 2000 J
When the object is half way down the height of the object is
= 2.5 m
initial velocity (u) = 0 (thrown from ground/rest)
since, v2 = u2 + 2gh
= 0 + 2 x 10 x 2.5
= 50
Kinetic energy
= 1000 J
Question 23. What is the work done by the force of gravity
on a satellite moving round the earth? Justify your answer.
Answer : The satellite is moving on a
round path, displacement in the object is perpendicular to the direction of
force.
W = F x s Cosq
= F x s Cos 900
= F x 0 = 0
Therefore, work done is zero.
Question 24. Can there be displacement of an object in the
absence of any force acting on it? Think. Discuss this Question with your
friends and teacher.
Answer : No.
Question 25. A person holds a bundle of hay over his head
for 30 minutes and gets tired. Has he done some work or not? Justify your
answer.
Answer : When a person holds a bundle
of hay over his head for 30 minutes and gets tired he applies force in upward
direction and displacement of bundle of hay is in forward direction which is
perpendicular to the direction of force applied therefore displacement is zero
W = F x s Cosq
= F x s Cos 900
= F x 0 = 0
No work done.
Question 26. An electric heater is rated 1500 W. How much
energy does it use in 10 hours?
Answer :
Electric heater’s power (p)
= 1500 W
= 1.5 kW
Energy = power x time
= 1.5 KW x 10 h
= 15 kWh
Question 27. Illustrate the law of conservation of energy by
discussing the energy changes which occur when we draw a pendulum bob to one
side and allow it to oscillate. Why does the bob eventually come to rest? What
happens to its energy eventually? Is it a violation of the law of conservation
of energy?
Answer :
The bob eventually comes to rest due to the frictional force
offered by the air and the rigid support holding the thread.
It is not a violation of the law of conservation of energy since mechanical
energy can get converted into another form of energy which cannot be utilised
for useful work. This loss of energy is called dissipation of energy.
Question 28. An object of mass,m is moving with a constant
velocity,v. How much work should be done on the object in order to bring the
object to rest?
Answer :
In order to bring the object to rest the work done must be
equal to the kinetic energy of the moving object
Question 29. Calculate the work required to be done to stop
a car of 1500 kg moving at a velocity of 60 km/h?
Answer :
Initial velocity of the car (u)
= 60 km/h
= (60 x 1000) / 60 x 60
= 50 / 3 ms-1
Final velocity (v) = 0 (object has to be stopped)
Initial kinetic energy
= 208333.30 J
Final kinetic energy
= 0
Therefore, work done = change in kinetic energy
= 208333.30 ― 0 = 208333.30 J
Question 30. In each of the following a force,F is acting on
an object of mass,m. The direction of displacement is from west to east shown
by the longer arrow. Observe the diagrams carefully and state whether the work
done by the force is negative, positive or zero.
Answer :
Case I
In this case, the direction of force acting on the block is perpendicular to
the direction of displacement. Therefore, work done by force on the block will
be zero.
Case II
In this case, the direction of force acting on the block and the direction of
displacement is same. Therefore, work done by force on the block will be
positive.
Case III
(i) Since in this diagram displacement is perpendicular to the direction of
force, so work done is zero.
(ii) Since in this diagram displacement is in the direction
of force, so work done is positive.
(iii) Since in this diagram displacement is in the opposite
direction of the force applied hence work done is negative.
Question 31. Soni says that the acceleration in an object
could be zero even when several forces are acting on it. Do you agree with her?
Why?
Answer : Yes
we agree with her statement. Because when many balanced forces act on the
object its displacement becomes zero.
Question 32. Find the energy in kW h consumed in 10 hours by
four devices of power 500 W each.
Answer :
Since Energy = power x time
= 4 x 500 x 10= 20000 Wh
= 20 kWh
Question 33. A freely falling object eventually stops on
reaching the ground. What happens to its kinetic energy?
Answer : A
free falling object eventually stops on reaching the ground since on striking
the ground its kinetic energy is transmitted to the
Activities Solution:
Q1. Look at the activities listed below. Reason out whether
or not work is done in the light of your understanding of the term ‘work’.
(a) Suma is swimming in a pond.
(b) A donkey is carrying a load on its back.
(c) A wind mill is lifting water from a well.
(d) A green plant is carrying out photosynthesis.
(e) An engine is pulling a train.
(f) Food grains are getting dried in the sun.
(g) A sailboat is moving due to wind energy.
Work is done whenever the given conditions are satisfied:
(i) A force acts on a body.
(ii) There is a displacement of the body.
(a) While swimming, Suma applies a force to push the water backwards.
Therefore, Suma swims in the forward direction caused by the forward reaction
of water. Here, the force causes a displacement. Hence, work is done by Suma
while swimming.
(b) While carrying a load, the donkey has to apply a force in the upward
direction. But, displacement of the load is in the forward direction. Since,
displacement is perpendicular to force, the work done is zero.
(c) A wind mill works against the gravitational force to
lift water. Hence, work is done by the wind mill in lifting water from the
well.
(d) In this case, there is no displacement of the leaves of the plant.
Therefore, the work done is zero.
(e) An engine applies force to pull the train. This allows the train to move in
the direction of force. Therefore, there is a displacement in the train in the
same direction. Hence, work is done by the engine on the train.
(f) Food grains do not move in the presence of solar energy. Hence, the work
done is zero during the process of food grains getting dried in the Sun.
(g) Wind energy applies a force on the sailboat to push it
in the forward direction. Therefore, there is a displacement in the boat in the
direction of force. Hence, work is done by wind on the boat.
Q2. An object thrown at a certain angle to the ground
moves in a curved path and falls back to the ground. The initial and the final
points of the path of the object lie on the same horizontal line. What is the
work done by the force of gravity on the object?
Since the body returns to a point which is on the same horizontal line through
the point of projection, no displacement has taken place against the force of
gravity, therefore, no work is done by the force due to gravity.
Q3. A battery lights a bulb. Describe the
energy changes involved in the process.
Within the electric cell of the battery the chemical energy changes into
electrical energy. The electric
energy on flowing through the filament of the bulb, first changes into heat
energy and then into the light energy.
Q4. Certain force acting on a 20 kg mass
changes its velocity from 5 m s-1 to 2 m s-1. Calculate the work done by
the force.
Work done by the force is equal to the change in kinetic
energy produced in the body.
Now, m = 20 kg, u = 5 ms-1 , v= 2 ms-1
, W=?
Using the expression W = 1/2 mv2 – 1/2 mu2,
we have
W= 1/2 m(v2-u2)
= 1/2 x 20 x (2)2 – (5)2
= - 210 J
W= -210J , the negative sign indicates that work has been
done in slowing the body.
Q5. A mass of 10 kg is at a point A on a table. It is moved to a
point B. If the line joining A and B is horizontal, what is the work done on
the object by the gravitational force? Explain your answer.
The work done is zero. This is because the gravitational force and displacement
are perpendicular to each other.
Q6. The potential energy of a freely falling
object decreases progressively. Does this violate the law of conservation of
energy? Why?
It does not violate the law Of conservation of energy. Whatever, is the
decrease in PE due to loss of height, same is the increase in the KE due to
increase in velocity of the body.
Q7. What are the various energy
transformations that occur when you are riding a bicycle?
The chemical energy of the food changes into heat and then to muscular energy.
On paddling, the muscular energy changes into mechanical energy
Q8. Does the transfer of energy take place
when you push a huge rock with all your might and fail to move it? Where is the
energy you spend going?
Energy transfer does not take place as no displacement takes place in the
direction of applied force. The energy spent is used to overcome inertia of
rest of the rock.
Q9. A certain household has consumed 250
units of energy during a month. How much energy is this in joules?
Energy consumed in a month = 250 units
= 250 kWh
= 250 kW x 1 h
= 250 x 1000 W x 60 x 60 s
= 900,000,000 J
= 9.0 x 108J
Q10. An object of mass 40 kg is raised to a height of 5 m
above the ground. What is its potential energy?
If the object is allowed to fall, find its kinetic energy when it is half-way
down.
Q11. What is the work done by the force of
gravity on a satellite moving round the earth? Justify your answer.
When a satellite moves round the Earth, then at each point of its path, the
direction of force of gravity on the satellite (along the radius) is
perpendicular to the direction of its displacement (along the tangent). Hence,
the work done on the satellite by the force of gravity is zero.
Q12. Can there be displacement of an object in the
absence of any force acting on it? Think. Discuss this question with your
friends and teacher.
The answer is both Yes and No. Yes because when an object moves in deep space
from one point to another point in a straight line, the displacement takes
place, without the application of force. No, because force cannot be zero for
displacement on the surface of earth. Some force is essential.
Class 9 Science NCERT Textbook – Page 159
Q13. A person holds a bundle of hay over his
head for 30 minutes and gets tired. Has he done some work or not? Justify your
answer.
The person does not do work because no displacement takes place in the
direction of applied force as the force acts in the vertically upward
direction.
Q14. An electric heater is rated 1500 W. How
much energy does it use in 10 hours?
Energy consumed by an electric heater can be obtained with the help of the
expression,
P=W/t
where,
Power rating of the heater, P = 1500 W = 1.5 kW
Time for which the heater has operated, t = 10 h
Work done = Energy consumed by the heater
Therefore, energy consumed = Power × Time
= 1.5 × 10 = 15 kWh
Hence, the energy consumed by the heater in 10 h is 15 kWh or 15 units.
Q15. Illustrate the law of conservation of
energy by discussing the energy changes which occur when we draw a pendulum bob
to one side and allow it to oscillate. Why does the bob eventually come to
rest? What happens to its energy eventually? Is it a violation of the law of
conservation of energy?
When the pendulum bob is pulled (say towards left), the energy supplied is
stored in it is the form
Of PE on account of its higher position. When the pendulum is released so that
it starts moving towards right, then its PE changes into KE such that in mean
position, it has maximum KE, and Zero PE. As the pendulum moves towards extreme
right, its KE changes into PE such that at the extreme position, it has maximum
PE and zero KE. When it moves from this extreme position to mean position, its
PE again changes to KE. This illustrates the law Of conservation of energy.
Eventually, the bob comes to rest, because during each oscillation a part of
the energy possessed by it transferred to air and in overcoming friction at the
point of suspension. Thus, the energy of the pendulum is dissipated in air.
The law of conservation of energy is not violated because the energy merely
changes its form and is not destroyed.
Q16. An object of mass, m is moving with a
constant velocity, v. How much work should be done on the object in order to
bring the object to rest?
Kinetic energy of an object of mass m moving with a velocity v is given by the
expression 1/2mv². To bring the object to rest, an equal amount of work i.e.
1/2mv² is required to be done on the object.
Q17. Calculate the work required to be done
to stop a car of 1500 kg moving at a velocity of 60 km/h.
Mass of car, m = 1500 kg
Velocity of ar is , v = 60 km/h = 60 x 5/10 m/s
Kinetic Energy Ek = ½ mv2
Ek = 1/2
x 1500 x (60 x 5/10 )2
= 208 x 104
To stop the car , an amount of work equal to Ek
is required to be done.
Hence, 20.8 x 104 J of work is required to stop
the car.
Q18. In each of the following a force, F is
acting on an object of mass, m . The direction of
displacement is from west to east shown by the longer arrow. Observe the
diagrams carefully and state whether the work done by the force is negative,
positive or zero.
Case I
In this case, the direction of force acting on the block is perpendicular to
the direction of displacement. Therefore, work done by force on the block will
be zero.
Case II
In this case, the direction of force acting on the block and the direction of
displacement is same. Therefore, work done by force on the block will be
positive.
Case III
In this case, the direction of force acting on the block is opposite to the
direction of displacement. Therefore, work done by force on the block will be
negative.
Q19. Soni says that the acceleration in an
object could be zero even when several forces are acting on it. Do you agree
with her? Why?
Yes, acceleration in an object could be zero even when several forces are
acting on it. This happens when all the forces cancel out each other i.e., the
net force acting on the object is zero.
Q20. Find the energy in kWh consumed in 10
hours by four devices of power 500 W each.
Power rating of each device, P = 500 W = 0.50 kW
Time for which each device runs, t = 10 h
Work done = Energy consumed by each device (E)
We know, power = Energy consumed / Time
Energy consumed by each device= Power × Time
E = P x t
= 0.50×10 = 5 kWh
Hence, the energy consumed by four devices of power 500 W each in 10 h will be
4 × 5 kWh = 20 kWh = 20 units
Q21. A freely falling object eventually stops
on reaching the ground. What happens to its kinetic energy?
As the object hits the hard ground, its kinetic energy gets converted into
(i) heat energy (the object and the ground become slightly warm)
(ii) sound energy (sound is heard when the object hits the ground)
(iii) potential energy of configuration of the body and the ground (the object
and the ground get deformed a little bit at the point of collision).
Frequently Asked Questions (FAQs) on Work and
Energy
1. What is
work in physics?
Answer: In physics, work is defined
as the transfer of energy that occurs when a force is applied to an object,
causing it to move. Work is calculated as the product of the force and the
displacement in the direction of the force.
2. How is
work calculated?
Answer: Work is calculated using the
formula W=F⋅d⋅cos(θ)W = F \cdot d \cdot
\cos(\theta)W=F⋅d⋅cos(θ), where WWW is work, FFF is the magnitude of the
force, ddd is the displacement, and θ\thetaθ is the angle between the force and
the displacement.
3. What is
energy?
Answer: Energy is the capacity to do
work. It exists in various forms, such as kinetic energy, potential energy,
thermal energy, chemical energy, and more.
4. What is
Kinetic Energy?
Answer: Kinetic energy is the energy
possessed by an object due to its motion. It is given by the formula KE=12mv2KE
= \frac{1}{2}mv^2KE=21mv2, where mmm is the mass of the object and vvv is its
velocity.
5. What is
Potential Energy?
Answer:
Potential
energy is the energy stored in an object due to its position or state. For
example, gravitational potential energy is stored in an object due to its
height above the ground, and elastic potential energy is stored in a stretched
or compressed spring.
6. What is
the principle of conservation of energy?
Answer: The principle of conservation
of energy states that energy cannot be created or destroyed in an isolated
system. It can only be transformed from one form to another, and the total
energy remains constant.
7. How is
power related to work and energy?
Answer: Power is the rate at which
work is done or energy is transferred. It is measured in watts (W) and is given
by the formula P=WtP = \frac{W}{t}P=tW, where WWW is work and ttt is time.
8. What is
the difference between work and energy?
Answer: Work is the process of energy
transfer, while energy is the capacity to do work. Work is done when a force
causes displacement, whereas energy can exist without any immediate transfer.
9. Can
work be negative?
Answer: Yes, work can be negative if
the force and displacement are in opposite directions. For example, when an
object slows down due to friction, the work done by friction is negative.
10. What
is mechanical energy?
Answer: Mechanical energy is the sum
of kinetic and potential energy in a system. It is the energy associated with
the motion and position of an object.
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