# TEXTBOOK ANSWERS AND SOLUTIONS OF CBSE CLASS IX SCIENCE Chapter 11 Work And Energy

1) 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:-

(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 Seema 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.

2) 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:-W= mgh

Where,

h= Vertical displacement = 0

W = mg x 0 = 0 J

Therefore, the work done by
gravity on the given object is zero joule.

3) A battery lights a bulb.
Describe the energy changes involved in the process.

ANSWER:-When a bulb is connected
to a battery, then the chemical energy of the battery is transferred into electrical
energy. When the bulb receives this electrical energy, then it converts it into
light and heat energy.

4) Certain force acting on a 20
kg mass change its velocity from 5 m s −1
to 2 m s −1.Calculate the work done by
the force.

ANSWER:-Kinetic energy is given
by the expression,(E k ) v = 1/2 mv2

Where,

E k = Kinetic energy of the
object moving with a velocity, v

(i) Kinetic energy when the
object was moving with a velocity 5 m s-1

(E k ) 5= 1/2 x 20 x (5) 2 = 250
J

(ii) Kinetic energy when the
object was moving with a
velocity 2m s-1

(E k ) 2= 1/2 x 20 x (2) 2 = 40
J.

5) 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:-W= mgh

Where,

Vertical displacement, h = 0

∴W= mg x 0 = 0

Hence, the work done by gravity on the body is zero.

6) The potential energy of a freely
falling object decreases progressively. Does this violate the
law of conservation of energy? Why?

ANSWER:-No. The process does not
violate the law of conservation of energy. This is because when the body falls
from a height, then its potential energy
changes into kinetic energy
progressively. A decrease in the potential energy is equal to an increase in
the kinetic energy of the body. During the process, total mechanical energy of
the body remains conserved. Therefore, the law of conservation of energy is not
violated.

7) What are the various energy transformations
that occur when you are riding a bicycle?

ANSWER:-While riding a bicycle,
the muscular energy of the rider gets
transferred into heat energy and kinetic energy of the bicycle. Heat energy heats
the rider's body. Kinetic energy provides a velocity to the bicycle

8) 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?

ANSWER:-When we push a huge rock,
there is no transfer of muscular
energy to the stationary rock. Also, there is no loss of energy because
muscular energy is transferred into heat energy, which causes our body to
become hot.

9) A certain household has
consumed 250 units of energy during a month. How much energy
is this in joules?

ANSWER: 1 unit of energy is equal
to 1 kilowatt hour (kWh).

1 unit = 1 kWh

1 kWh = 3.6 x 10 6 J

Therefore, 250 units of energy

= 250 x 3.6 × 106 = 9 x 10 8 J.

10) 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:-W = mgh

Where,

h = Vertical displacement = 5m

m = Mass of the object = 40 kg

g = Acceleration due to gravity

= 9.8 m s −2

∴ W = 40 x 5 x 9.8 = 1960 J.

At half-way down, the potential
energy of the object will be
1960 / 2 = 980 J.

11) What is the work done by
the force of gravity on a satellite moving round the earth? Justify your
answer.

ANSWER:-Work is done whenever

A force acts on the body and
there is a displacement of the body by the application of force in or opposite
to the direction of force. When a satellite moves around the Earth, then the
direction of force of gravity on the satellite is perpendicular to its displacement. Hence, the work
done on the satellite by the
Earth is zero.

12) 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:-Yes. For a uniformly
moving object. Suppose an object is moving with constant velocity. The net
force acting on it is zero. But, there is a displacement along the motion of
the object. Hence, there can be a displacement without a force

13) 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:-Work is done whenever the
given two conditions are satisfied:

(i) A force acts on the body.

(ii) There is a displacement of
the body by the application of
force in or opposite to the direction of force. When a person holds a bundle of
hay over his head, then there is no displacement in the bundle of hay. Although, force of gravity
is acting on the bundle, the person is not applying any force on it. Hence, in
the absence of force, work done by the person on the bundle is zero.

14) An electric heater is rated
1500 W. How much energy does it use in 10 hours?

ANSWER:-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 x Time

= 1.5 x 10 = 15 kWh

15) 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 law of conservation
of energy states that energy can be neither created nor destroyed. It can only
be converted from one form to another. Consider the case of an oscillating
pendulum. When a pendulum moves from its mean position P to either of its
extreme positions A or B, it rises through a height h above the mean level P.
At this point, the kinetic energy of the bob changes completely into potential
energy. The kinetic energy becomes zero, and the bob possesses only potential
energy. As it moves towards point P, its potential energy decreases
progressively. Accordingly, the kinetic energy increases. As the bob reaches
point P, its potential energy
becomes zero and the bob possesses only kinetic energy. This process is
repeated as long as
the pendulum oscillates. The bob does not oscillate forever. It comes to rest
because air resistance resists its motion. The pendulum loses its kinetic
energy to overcome this friction
and stops after some time. The law of conservation of energy is not violated
because the energy lost by the pendulum to overcome friction is gained by its
surroundings. Hence, the total
energy of the pendulum and the surrounding system remain conserved.

16) 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:-Kinetic energy of an
object of mass, moving with a velocity, v is given by the expression,

E k = 1/2 mv 2

To bring the object to rest, 1/2
mv2 amount of work is required to
be done on the object.

17) Calculate the work required
to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?

ANSWER:-Kinetic energy, Ek= 1/2
mv2

Where,

Mass of car, m= 1500 kg

Velocity of car, v= 60 km/h=60 x
5 / 18 ms-1

20.8 x 10 4 J of work is required
to stop the car.

18) 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 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 is in the direction of displacement. 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.

19) 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:-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,
the net force acting on the
object is zero. For a uniformly
moving object, the net force acting on the object is zero. Hence, the
acceleration of the object is zero. Hence, Soni is right.

20) Find the energy in kW h
consumed in 10 hours by four devices of power 500 W each

ANSWER:-P = W / T

Where,

Power rating of the device, P=500
W = 0.50 kW

Time for which the device runs,
T= 10 h

Work done = Energy consumed
by the device

Therefore, energy consumed =Power
x Time= 0.50 x 10 = 5 kWh

Hence, the energy consumed by
four equal rating devices in 10 h will be 4 x 5 kWh = 20kWh = 20 Units.

21) A freely falling object
eventually stops on reaching the ground. What happens to its kinetic energy?

ANSWER:-When an object falls
freely towards the ground, its potential energy decreases and kinetic energy
increases. As the object touches the ground, all its potential energy gets
converted into kinetic energy. As the object
hits the hard ground, all its kinetic
energy gets converted into heat energy and sound energy. It can also deform the
ground depending upon the nature of the ground and the amount of kinetic energy
possessed by the object.