Class 9 LAKHMIR SINGH AND MANJIT KAUR Solutions Physics Chapter 4 - Work and energy
Work and energy Exercise 145
Solution 1
Mass = m
Height above the ground = h
Work done = Potential energy acquired by the body = m x g x h
where g is acceleration due to gravity
Solution 2
SI unit of work is Joule (J).
Solution 3
Work is a scalar quantity.
Work, as a physical quantity, requires only magnitude to be represented. Hence, it is scalar quantity.
Solution 4
When a force of 1 newton moves a body through a distance of 1 metre in its own direction, then the work done is known as 1 joule.
Solution 5
The condition for a force to do work on a body is that it should produce motion in the body.
Solution 6
Energy is a scalar quantity. It has only magnitude but no direction.
Solution 7
a) Unit of work is joule.
b) Unit of energy is joule.
Solution 8
The work done against gravity is zero when a body is moved horizontally along a frictionless surface because force of gravity acts perpendicular to the direction of motion.
Solution 9
Kinetic energy will become four times when the speed is doubled because kinetic energy is directly proportional to square of speed of the body
Solution 10
Mass = m
Velocity = v
Solution 11
Kinetic energy will become one-fourth when the speed is halved because kinetic energy is directly proportional to square of speed of the body
Solution 12
The kinetic energy of a body depends on
a) Mass of the body, m
b) Square of the velocity of the body, v2
Solution 13
Doubling the velocity would have a greater effect on kinetic energy.
Solution 14
Solution 15
a) Both kinetic and potential energy
b) Both kinetic and potential energy
c) Only kinetic energy
d) Only potential energy
e) Only potential energy
Note: In all the above cases we take ground as reference level where potential energy is zero.
Solution 16
Let masses of body A and B be m
Height of body A = h
Height of body B = 2h
Potential energy for body A, PEA = m x g x h
Potential energy for body B, PEB = m x g x 2h
Ratio = PEA : PEB = 
= 
Solution 17
Mass = 1 kg
Velocity = 2 m/s
K.E.
= 
= 
= 2 J
Solution 18
Potential energy is a scalar quantity as it has magnitude only and it does not require any specification of direction.
Solution 19
Mass = 100 kg
Height = 5 m
g = 9.8 m/s2
P.E. = m x g x h = 100 x 5 x 9.8 = 4900 J
Work done is equal to PE acquired by the body.
Solution 20
False.
PE = m x g x h
= 1 x 9.8 x 1 = 9.8 J
Solution 21
The potential energy is doubled when the height is doubled since potential energy is directly proportional to height, h to which body is raised.
PE 
h
Solution 22
a) Potential energy
b) Kinetic energy
c) Potential energy
d) Potential energy
e) Potential energy
Solution 23
a)Force ; distance
b)Zero
c)newton; metre; force
d)Energy; kinetic energy
e)mechanical
Solution 24
The work done by a force on a body depends on two factors
a) magnitude of force
b) distance through which the body moves
Work done is directly proportional to the force applied and the distance through which the body moves.
W = F x s
where W is work done, F is force applied and s is distance through which the body moves.
Solution 25
Yes, it is possible that a force is acting on a body but still work done is zero. For example, in the case of a man pushing a wall, the work done is zero despite of non-zero force, since there is no displacement of the wall.
Work and energy Exercise 146
Solution 26
a) Work done by force applied by the boy is positive because this force is in the direction of motion of the body.
b) Work done by the gravitational force is negative because this force is against the direction of motion of the body.
Solution 27
Solution 28
Kinetic energy is directly proportional to the mass of the body, m.
KE
m
Kinetic energy is directly proportional to square of speed of the body, v
KE
v2
Solution 29
(a) Positive work: Work done by the force applied by a person on a ball that is thrown upwards.
(b) Negative work: Work done by gravitational force of earth on a ball thrown upwards.
(c) Zero work: Work done by gravitational force of earth on a box that is sliding horizontally on the ground.
Solution 30
Solution 31
KE = 20 J
Mass = 100 g = 0.1 kg
KE
= 
20
= 
v2 = 400
v = 20 m/s
Momentum = m x v = 0.1 x 20 = 2 kg.m/s
Solution 32
Solution 33
Mass of body = 2 kg
Initial velocity u = 0
Time taken = 2 s
Acceleration due to gravity, g = 10 m/s2
Final velocity v
Using first equation of motion
v = u + gt
= 0 + 10 x 2 = 20 m/s
KE
= 
= 
= 400 J
Solution 34
Mass of scooter+scooterist = 150 kg
Initial velocity u = 10 m/s
Final velocity v = 5 m/s
Retardation = a
Distance covered= s
Using third equation of motion
v2 - u2 = 2as
52 - 102 = 2as
as = -75/2 ------(i)
Work done W = F x s
But F = m x a
So, W= m x a x s
Put the value of 'as' from eq(i)
W = 150 x (-75/2) = -5625 J
Neagtive sign implies that force of brakes acts opposite to the direction of motion.
Solution 35
Mass of rock = 10 kg
Height of ladder, h = 5 m
Initial velocity of rock, u = 0
Final velocity v
g = 10 m/s2
using third equation of motion
v2 - u2 = 2gh
v2 - 02 = 2 x 10 x 5
v = 10 m/s
Solution 36
Mass of car = 1000 kg
Initial velocity u = 20 m/s
Final velocity v = 10 m/s
Retardation = a
Distance covered = s
Using third equation of motion
v2 -u2 = 2as
102- 202 = 2as
as = -150 ------(i)
Work done W = F x s
But F = m x a
So, W= m x a x s
Put the value of 'as' from equation (i)
W = 1000 x -150 = -150000 = -150 kJ
Neagtive sign implies that force of brakes acts opposite to the direction of motion.
Solution 37
Mass of body, m = 100 kg
Height , h= 10 m
Acceleration due to gravity, g = 10 m/s2
i) Work done, W = m x g x h = 100 x 10 x 10 = 10000 = 10 kJ
ii) Potential energy of the body = work done = 10 kJ
Solution 38
Mass of the boy, m = 50 kg
Height, h = 100 m
Acceleration due to gravity, g = 9.8m/s2
Work done by the boy, W = m x g x h = 50 x 9.8 x 100 = 49000 J = 49 kJPotential energy gained by the boy = work done by the boy = 49 kJ
Solution 39
Work done by a force applied on a body is
i) positive when the force acts in the direction of motion of the body.
ii) negative when the force acts in the direction opposite to the direction of motion of the body.
iii) zero when the force acts at right angle to the direction of motion of the body.
Solution 40
Mass of the box, m = 150 kg
PE = 7350 J
Acceleration due to gravity, g = 9.8m/s2
PE = m x g x h
7350 = 150 x 9.8 x h
h= 
= 5 m
Solution 41
Mass of the body, m = 2 kg
Initial velocity, u = 20 m/s
Acceleration due to gravity, g = 10 m/s2
Height reached = h
Time, t = 2 s
Using second equation of motion
PE after 2 s = m x g x h = 2 x 10 x 20 = 400 J
Solution 42
Force, F = 1 N
Distance, s = 1 m
Work done W = F x s
= 1 x 1 = 1 J
Solution 43
Force, F = 2.5 x 1010 N
Velocity, v = 5 m/s
Time, t = 2 minutes = 120 s
Distance, s = v x t = 5 x 120 = 600 m
Work done, W = F x s = 2.5 x 1010 x 600 = 15 x 1012 J
Solution 44
A stretched rubber band is an example of a body possessing energy while it is not in motion. The rubber band contains potential energy due to the change in its shape or configuration.
Solution 45
a) Gravitational potential energy of a body depends on:
i) mass of the body, m
ii) height to which the body is lifted, h
iii) acceleration due to gravity, g
b)
i. A moving cricket ball has kinetic energy
ii. A stretched rubber band has potential energy
Solution 46
Two examples where a body possesses both kinetic energy as well as potential energy are
i) a man climbing up a hill
ii) a flying aeroplane
Solution 47
Mass of man, m
Height of tree, h = 5 m
Work done, W = 2500 J
Acceleration due to gravity, g = 10 m/s2
W = m x g x h
2500 = m x 10 x 5
Solution 48
Work done, W =24.2 J
Distance, s = 20 cm = 0.2 m
Force, F
W = F x s
24.2 = F x 0.2
F = 24.2/0.2 = 121 N
Solution 49
Mass of boy, m = 40 kg
Height, h = 1.5 m
Acceleration due to gravity, g = 10 m/s2
i) At highest point, velocity, v = 0
Therefore KE = 0
ii) PE = m x g x h = 40 x 10 x 1.5 = 600 J
Solution 50
a) Potential energy
b) Both potential and kinetic energy
Work and energy Exercise 147
Solution 51
Mass, m = 200 kg
Height, h = 2 m
Acceleration due to gravity, g = 9.8 m/s2
i) Potential energy = m x g x h = 200 x 9.8 x 2 = 3920 J
ii) Work done is against gravity = Potential energy gained by the weights, therefore
Work done W = m x g x h = 200 x 9.8 x 2 = 3920 J
Solution 52
(a) Work is done when a force applied on a body produces motion in it.
Formula for work done:
W = F x s
where W is the work doneF is force applied
S is the displacement of the body in the direction of force
(b) Mass of the person, m = 50 kgHeight of tower, h = 72 m
Acceleration due to gravity = 9.8 m/s2
Work done W = m x g x h
= 50 x 9.8 x 72 = 35280 J
Solution 53
(a) Work is said to be done when the force applied on a body produces motion in it.
Work done by a body in moving up is given by
W = m x g x h
where W is the work done against the gravity
m = mass of the body
g = acceleration due to gravity
h = height through which the body is lifted above the ground
(b) Force, F = 2 N
Distance, s = 10 cm = 0.1 m
Work done W = F x s = 2 x 0.1 = 0.2 J
Solution 54
(a) When the displacement of a body is at right angles to the direction of foce acting on it, then work done is zero.
(b) Force, F = 50 N
Distance, s = 4 m
Angle between direction of force and direction of motion, 
= 60o
Work done, W = F cos x s
= 50 x cos60o x 4 = 50 x 0.5 x 4= 100 J
Solution 55
(a) Energy is the ability to do work. SI unit of energy is Joule.
(b) Various forms of energy are:
1. Kinetic energy
2. Potential energy
3. Chemical energy
4. Heat energy
5. Light energy
6. Sound energy
7. Electrical energy
8. Nuclear energy
(c) Let masses of bodies be m
Velocity of one body, v1= v
Velocity of another body, v2 = 2v
Solution 56
(a) The energy of a body due to its motion is called kinetic energy
(b) When the velocity becomes zero, the kinetic energy also becomes zero since kinetic energy is directly proportional to square of the velocity.
(c)
Solution 57
(a) The energy of a body due to its position or change in its shape is known as its potential energy. E.g. a stretched rubber has potential energy due to change in its shape and water in the overhead tank has potential energy due to its height above the ground.
PE = m x g x h
where, PE is the potential energy of the body,
m is the mass of the body,
g is the acceleration due to gravity,
h is the height above the surface ofearth.
(b) Kinetic energy of a body is due to motion of the body while potential energy is due to position or change in shape of the body.
Kinetic energy is zero for a still body, while potential energy may or may not be zero for a still body.
Kinetic energy of a body is directly proportional to its speed while potential energy is directly proportional to the height to which the body is above the ground.
(c) Mass of ball, m = 0.5 kg
Speed v1 = 5 m/s
Speed v2 = 3 m/s
Solution 58
(a) The potential energy due to the position of the body above the ground is gravitational potential energy and the potential energy due to change in shape and size of the body is elastic potential energy. E.g. a stretched rubber has elastic potential energy due to change in its shape while water in the overhead tank has gravitational potential energy due to its height above the ground.
(b) Work done, W = 784 J
Mass, m = 20 kg
g = 9.8 m/s2
W = m x g x h
784 = 20 x 9.8 x h
Work and energy Exercise 148
Solution 71
Work done is more on the bicycle because the truck does not move at all and the bicycle moves through a certain distance. And work is said to be done only when applied force produces motion in the body.
Work and energy Exercise 149
Solution 72
The work done will decrease as the angle between the direction of force and direction of motion is increased gradually because
And 
decreases as the angle is increased.
Solution 73
The work done will be zero when angle between the direction of force and direction of motion is 90o because
Solution 74
Work done will be maximum when angle between the direction of force and direction of motion is 0o because
Solution 75
The work done is zero because the gravitational force acts along the radius of the circular path, at right angles i.e. 90o to the motion of satellite.
Solution 76
Weight of man = Mg = 800 N
Weight of package = mg = 200 N
Total weight of man and package = Mg + mg = (M+m)g = 1000 N
Height of the summit, h = 1200 m
i) Work done = (M+m) x g x h = 1000 x 1200 = 12 x 105 J
ii) Potential energy of the package = m x g x h = 200 x 1200 = 2.4 x 105 J
Solution 78
The work done by both X and Y are equal because irrespective of whether they reach the top of building by using a spiral or slanted ladder, the vertical distance moved by them against the gravity is same.
Solution 79
Yes, the kinetic energy of the ball thrown inside a moving bus depends on the speed of the bus because the speed of the bus adds up to the speed with which the ball is thrown inside the moving bus.
Solution 80
Solution 77
Work and energy Exercise 162
Solution 1
The commercial unit of energy is kilowatt-hour.
Solution 2
One kilowatt-hour is the amount of electrical energy consumed when an electrical appliance having power of 1 kilowatt is used for 1 hour.
Solution 3
Megawatt and kilowatt are the units of power bigger than watt.
Solution 4
1 watt is the power of an appliance which does work at the rate of 1 joule per second.
Solution 5
1 horse power = 746 watt
Solution 6
Power has watt as its physical unit
Solution 7
1 watt
Solution 8
Work done = 1200 J
Time taken = 2 minutes = 2 x 60 = 120 s
Solution 9
One kilowatt = 3.6 x 106 J
Solution 10
a) Power
b) Electrical Energy
Solution 11
1 kW-h of electrical energy is commonly known as unit of electricity.
Solution 12
A cell converts chemical energy into electrical energy.
Solution 13
Electric motor
Solution 14
a) Electric generator
b) Cell
c) Electric iron
d) Solar cell
e) Electric bulb
Solution 15
a) Speaker
b) Steam engine
c) Car engine
d) Gas stove
e) Solar water heater
Solution 16
a) Work
b) Joule ; second
c) kWh
d) conservation; transformed; created; destroyed
e) kinetic; potential
Work and energy Exercise 163
Solution 17
Force, F = 400 N
Distance, s = 60 m
Time taken, t = 1 minute = 60 s
Work done, W = F x s = 400 x 60
Solution 18
At a hydroelectric power station, the potential energy of water is transformed into kinetic energy and then into electrical energy.
Solution 19
At a coal-based thermal power station, the chemical energy of coal is transformed into heat energy, whhich is further converted into kinetic energy and electrical energy.
Solution 20
Weight of the man = 500 N
Weight of the load = 100 N
Total weight = 600 N
Height of stairs = 4 m
Time taken = 5 s
Work done = mg x h = weight X h = 600 x 4
Solution 21
Power = 3 kW
Time = 20 s
Power
=
Work done = power x time = 3 x 20 kWs = 60 kJ
Solution 22
Energy consumed = 600 kJ
Time taken = 5 minutes = 300 s
Power
=
=
= 2 kW
Solution 23
Power = 100 W
a)time = 1 s
energy = power x time = 100 J
b)time = 1 minute = 60 s
energy = power x time= 100 x 60 = 6 kJ
Solution 24
Power of 1 fan = 120 W
Power of 5 fans = 5 x 120 = 600 W = 0.6 kW
Time = 4 hours
Electrical energy = 0.6 x 4 = 2.4 kWh
Solution 25
A radio first converts electrical energy into kinetic energy and then into sound energy
Solution 26
In an electric bulb, electrical energy is first converted into heat energy and then into light energy
Solution 27
Fan, washing machine, mixer grinder, water pump, hair dryer use electric motor
Solution 28
i) chemical energy to electrical energy
ii) electrical energy to heat and light energy
Solution 29
i) Maximum potential energy is present in the bob at point C
as at point C bob is at maximum height.
ii) Maximum kinetic energy is present in the bob at point A as at point A bob is at maximum speed
Solution 30
Weight of the car = 20000 N= 20 kN
Speed = 8 m/s
Distance s= 120 m
Time = 100 s
a) Work done W = f x s = 20 x 120 = 2400 kJ
b)
Power
=
=
= 24 kW
Solution 31
a) The change of one form of energy into another form of energy is known as transformation of energy, e.g. in a cell chemical energy is transformed into electrical energy
b)
a. When a ball is thrown upwards its kinetic energy gradually converts into potential energy and potential energy becomes maximum at the maximum height attained by the ball
b. When a stone is dropped from the roof of the building its potential energy gradually converts into kinetic energy and kinetic energy becomes maximum when the stone is just above the ground
Solution 32
a)Law of conservation of energy states that whenever energy changes from one form to another form, the total amount of energy remains constant. Energy can never be created nor destroyed, it transforms from one form to another. For example, when electrical energy is converted into light energy in an electric bulb, then some energy is wasted as heat during conversion but the total energy remains the same.
b)Initially the pendulum is at rest. The bob is pulled to one side to position B to give it potential energy due to higher position of B and then released, the bob starts swinging
i.When the bob is at position B, it has potential energy but no kinetic energy
ii.As the bob starts moving down from position B to A, its potential energy starts decreasing and kinetic energy starts increasing
iii.When the bob is at position A, it has maximum kinetic energy and zero potential energy
iv.As the bob starts moving down from position A to C, its kinetic energy starts decreasing and potential energy starts increasing
v.On reaching the extreme position C, the bob stops for a very small instant of time and bob maximum potential energy and zero kinetic energy.
Therefore at extreme positions B and C bob has only potential energy and at A it has only kinetic energy and at other intermediate positions bob has both kinetic and potential energy. Thus the total energy of the pendulum is same for any instant of time (conserved)
Solution 33
a) The unit kWh stands for kilowatt-hour. One kilowatt-hour is the amount of electrical energy consumed when an electrical appliance having power of 1 kilowatt is used for 1 hour. It represents the amount of electrical energy consumed in 1 hour.
b) Power = 1000 W = 1 kW
Time = 60 minutes = 1 hour
Energy(kWh) = 1 x 1 = 1 kWh
Solution 34
a) 1 kilowatt-hour = 1 kW for 1 hour
= 1000 W for 1 hour
But
1 watt = 
1
kilowatt- hour = 
for 1 hour
= 
for 3600 seconds
= 1000 x 3600 = 3.6 x 106 joule
b) Energy consumption = 650 units = 650 kWh
1 kWh = 3.6 x 106 J
650 kWh = 3.6 x 650 x 106 =2.34 x 109 J
Solution 35
a) Power is the rate of doing work.
Power
= 
SI unit of power is watt(W)
b) Mass of the boy = 40 kg
Mass of the box = 20 kg
Total mass = 60 kg
Height h= 15 m
g = 10 m/s2
time taken = 25 s
work done = m x g x h = 60 x 15 x 10 = 9000 J
Power
= = 
= 360 W
Work and energy Exercise 164
Solution 46
Mass of body = 1 kg
g = 10 m/s2
PE = mgh = 1 x 10 x h = 10 x h
KE = 
=
= 
i. At h = 5 m; v = 0 m/s
PE = 10 x 5 = 50 J
KE = 0 J
Total energy = PE + KE = 50 J
ii. At h = 3.2 m; v = 6 m/s
PE = 10 x 3.2 = 32 J
KE = 
= 18 J
Total energy = PE + KE = 32+ 18 = 50 J
iii. At h = 0 m; v = 10 m/s
PE = 0 J
KE = 
= 50 J
Total energy = PE + KE = 50 J
The total energy in all three cases is constant. This proves the law of conservation of energy
Exercise
Solution
Work and energy Exercise 165
Solution 48
No. of steps = 28
Height of each step = 28 cm
Total height = 20 x 28 = 560 cm = 5.6 m
Mass of student = 55 kg
g = 9.8 m/s2
time = 5.4 s
Work done = m x g x h = 55 x 9.8 x 5.6 = 3018.4 J
Power = 
=
= 559 W
Solution 49
Weight of box = 100 N
Height = 1.5 m
i. work done = m x g x h = 100 x 1.5 = 150 J
ii. potential energy = m x g x h= 100 x 1.5 = 150 J
iii. weight of 4 boxes = 400 N
time = 1 minute = 60 s
work done = 400 x 1.5 = 600 J
power = 
= 
= 10 W
Solution 50
a) Electrical energy to sound energy
b) Sound energy to electrical energy
c) Electrical energy to light (and heat) energy
d) Chemical energy to electrical energy to light energy(and heat energy)
