NCERT Solutions for Chapter 11 Work and Energy Class 9 Science
Book Solutions1
Answer
W = F × S
Where,
F = 7 N
S = 8 m
Therefore, work done, W = 7 × 8
= 56 Nm
= 56 J
1
Answer
Work is done whenever the given conditions are satisfied:→ A force acts on the body.
→ There is a displacement of the body caused by the applied force along the direction of the applied force.
2
Answer
When a force F displaces a body through a distance S in the direction of the applied force, then the work done W on the body is given by the expression:Work done = Force × Displacement
W = F × s
3
Answer
1 J is the amount of work done by a force of 1 N on an object that displaces it through a distance of 1 m in the direction of the applied force.4
Answer
Work done by the bullocks is given by the expression:Work done = Force x Displacement
W= F ×d
Where,
Applied force, F = 140 N
Displacement, d = 15 m
W= 140 × 15 = 2100 J
Hence, 2100 J of work is done in ploughing the length of the field.
1
Answer
The energy possessed by a body by the virtue of its motion is called kinetic energy. Every moving object possesses kinetic energy. A body uses kinetic energy to do work. Kinetic energy of hammer is used in driving a nail into a log of wood, kinetic energy of air is used to run wind mills, etc.2
Answer
If a body of mass mis moving with a velocity v, then its kinetic energy Ekis given by the expression,Ek= 1/2 mv2
Its SI unit is Joule (J).
3
Answer
K.E. of the object= 25 JVelocity of the object, v= 5 m/s
∵ K.E.= 1/2 mv2
⇒ m = 2 × K.E./v2
⇒ m = 2 × 25/25 = 2 kg
If velocity is double, v = 2 × 5= 10 m/s
∴ K.E. (for v = 10 m/s) = 1/2 mv2 = 1/2 × 2 × 100= 100 J
If velocity is tripled, v = 3 × 5 = 15 m/s
∴ K.E. (for v = 10 m/s) = 1/2 mv2 1/2 × 2 × 225 = 225 J
1
Answer
Power is the rate of doing work or the rate of transfer of energy. If W is the amount of work done in time t, then power is given by the expression,Power= Work/Time = Energy/Time
P = W/T
It is expressed in watt (W).
2
Answer
A body is said to have power of 1 watt if it does work at the rate of 1 joule in 1 s, i.e.,1 W= 1J / 1s3
Answer
Power = Work Done/TimeWork done = Energy consumed by the lamp = 1000 J
Time = 10 s
Power= 1000 / 10
= 100 Js-1
=100 W
4
Answer
The average Power of an agent may be defined as the total work done by it in the total time taken.Average Power = Total Work Done/Total time taken
1
• 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 is done whenever the given two conditions are satisfied:
→ A force acts on the body.
→ There is a displacement of the body by the application of force in or opposite to the direction of force.
(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.
(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
Answer
Work done by the force of gravity on an object depends only on vertical displacement. Vertical displacement is given by the difference in the initial and final positions/heights of the object, which is zero.Work done by gravity is given by the expression,
W= mgh
Where,
h= Vertical displacement = 0
W = mg × 0 = 0 J
Therefore, the work done by gravity on the given object is zero joule.
3
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. Hence, the transformation of energy in the given situation can be shown as:Chemical Energy → Electrical Energy → Light Energy + Heat Energy
4
Answer
Kinetic energy is given by the expression,(Ek)v= 1/2 mv2Where,
Ek= 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
(Ek)5= 1/2 × 20 × (5)2 = 250 J
Kinetic energy when the object was moving with a velocity 2 m s-1
(Ek)2= 1/2 × 20 × (2)2 = 40 J.
5
Answer
Work done by gravity depends only on the vertical displacement of the body. It does not depend upon the path of the body. Therefore, work done by gravity is given by the expression,W= mgh
Where,
Vertical displacement, h = 0
∴W= mg x 0 = 0
Hence, the work done by gravity on the body is zero.
6
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
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. The transformation can be shown as:Muscular Energy → Kinetic Energy + Heat Energy
During the transformation, the total energy remains conserved.
8
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
Answer
1 unit of energy is equal to 1 kilowatt hour (kWh).1 unit = 1 kWh
1 kWh = 3.6 × 106 J
Therefore, 250 units of energy = 250 x 3.6 × 106 = 9 × 108 J.
10
Answer
Gravitational potential energy is given by the expression,W = mgh
Where,
h = Vertical displacement = 5 m
m = Mass of the object = 40 kg
g = Acceleration due to gravity = 9.8 m s−2
∴ W = 40 × 5 × 9.8 = 1960 J.
At half-way down, the potential energy of the object will be 1960/2 = 980 J.
At this point, the object has an equal amount of potential and kinetic energy. This is due to the law of conservation of energy. Hence, half-way down, the kinetic energy of the object will be 980 J.
11
Answer
Work is done whenever the given two conditions are satisfied:
→ A force acts on the body.
→ There is a displacement of the body by the application of force in or opposite to the direction of force.
If the direction of force is perpendicular to displacement, then the work done is zero.
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
Answer
Yes. For a uniformly moving objectSuppose 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
Answer
Work is done whenever the given two conditions are satisfied:→ A force acts on the body.
→ 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
Answer
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.
15
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
Answer
Kinetic energy of an object of mass, moving with a velocity, v is given by the expression,
Ek= 1/2 mv2
17
Answer
Kinetic energy, Ek = 1/2 mv2
Where,
Mass of car, m = 1500 kg
Velocity of car, v = 60 km/h = 60 × 5/18 ms-1
Hence, 20.8 x 104 J of work is required to stop the car.
18
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
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 i.e., 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
Answer
Energy consumed by an electric device can be obtained with the help of the expression for power,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 × Time
= 0.50 × 10 = 5 kWh
Hence, the energy consumed by four equal rating devices in 10 h will be 4 × 5 kWh = 20 kWh = 20 Units.
21
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.