CBSE Class 9 Science Work and Energy VBQs

Question. 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 : Work done = Force (F) x Displacement (S)
= 7 x 8 = 56 J (joules).

Question. Write an expression for the kinetic energy of an object.
Answer : Kinetic Energy = 1/2 mv². {where m= mass and v is velocity of a body.}

Question. Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Answer : Work done = Force (F) x Displacement (S)
W = F x S

Question. What is power?
Answer : Rate of doing work is known as power. SI unit of power is watt (W).
Power = Work/Time

Question. 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 = Force (F) x Displacement (S)
= 140 x 15 = 2100 J (joules).

Question. A lamp consumes 1000 J of electrical energy in 10 s. What is its power?
Answer : Energy = W = 1000 J
Time = t = 10 sec
Power = W = 1000 J
Time = t = 10 sec
Power = P = W/t = 1000/10 = 100 watt (W).

Question. When do we say that work is done?
Answer : Work is said to be done if:
(i) A force acts on the body.
(ii) There is a displacement of the body in the direction (or opposite) of applied force.

Question. What is the kinetic energy of an object?
Answer : The energy possessed by a body by the virtue of its motion is termed as kinetic energy.

Question. Define average power.
Answer : Average power is the ratio of total work done to total time taken.
Average power = Total work done/Total time taken

Question. The kinetic energy of an object of mass, m moving with a velocity of 5 ms-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 : As K.E. = 1/2 mv². So by doubling velocity K.E. will become 4 times i.e. 25 x 4 = 100 J.
Similarly K.E. will become 9 times if velocity is increased 3 times, i.e. 25 x 9 = 225 J.

Question. Define 1 watt of power.
Answer : Power of an object is said to be 1 W if it does 1 joule of work in 1 second.

Question. Define 1 J of work.
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.

Exercise Question-Answers

Question. 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.
Answer : Suma is doing work. She is applying force to move horizontally.

(b) A donkey is carrying a load on its back.
Answer : Donkey is not doing any work. Here, the displacement and the force are at 90°.

(c) A wind-mill is lifting water from a well.
Answer : Work is done by the windmill. The water is lifted against force of gravity.

(d) A green plant is carrying out photosynthesis.
Answer : No work is done by a green plant during photosynthesis.

(e) An engine is pulling a train.
Answer : The engine applies a pulling force on the train, and the train moves in the direction of this force.
Therefore, engine is doing work.

(f) Food grains are getting dried in the sun.
Answer : During drying of food grains in the sun no work is done.

(g) A sailboat is moving due to wind energy.
Answer : Work is done by the wind. The sailboat moves in the direction of the force exerted by wind.

Question. What are the various energy transformations that occur when you are riding a bicycle?
Answer : During riding a bicycle, the muscular energy of the rider is transformed into heat and mechanical energy. Kinetic energy provides motion to the bicycle and warmth energy heats our body.
Muscular energy → mechanical energy + heat

Question. A battery lights a bulb. Describe the energy changes involved in the process.
Answer : A battery converts chemical energy into electrical energy. This electrical energy is converted into heat and then light energy. The sequence of energy changes is as follows :
Chemical energy —> Electrical energy —> Heat Energy —>Light energy.

Question. 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. In the case of uniform motion in a straight line, a body keeps in motion even without any force.

Question. 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 : Zero. Because here angle between force of gravity and displacement is 90 degree (90°).
Now Work = W = F S Cosθ = FS Cos90° = 0 ( because Cos90°= 0)

Question. 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 : Zero. Because here, angle between force of gravity and displacement is 90 degree (90°).
Now, Work = W = F S Cosθ = FS x Cos90° = 0 ( because Cos90°= 0)

Question. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Answer : As the potential energy of the freely falling object decreases, its kinetic energy increases. The sum total of the potential energy and the kinetic energy of the object during its free fall remains the same. Thus, the law of conservation of energy is not violated.

Question. Certain force acting on a 20 kg mass changes its velocity from 5 ms-1 to 2 ms-1 . Calculate the work done by the force.
Answer : Mass = m = 20 kg
Initial velocity = u = 5 m/s
Final velocity = v = 2 m/s
Work done by force = change in Kinetic energy = (KE)f – (KE)i 
W = 1/mv² - (1/2) mu² = 1/2 x 20 x (2)² - (1/2) x 20 x (5)² = 10 x (4-25) = 10 x (-21) = -210 J
Here negative sign indicates the direction of force is opposite to the direction of motion

Question. 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 : No transfer of energy takes place when we push a huge rock unsuccessfully. The energy is spent for the physical activity of muscles.

Question. 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 halfway down.
Answer : Mass = m = 40 kg
Height = h = 5 m
Acceleration due to gravity = g = 9.8 ms-2
Potential Energy = mgh = 40 x 9.8 x 5 = 1960 J
Kinetic energy in halfway = PE/2 = 1960/2 = 980 J.

Question. 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 :The acceleration of an object can be zero even if several forces are acting on it provided the resultant force (F) is zero.

Question. 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 : Not any work is done by the person, because his displacement is zero.

Question. Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.
Answer : Time = t = 10 hours
Power = P = 500 W = 0.5 kW
Energy consumed by 4 devices = 4 x P x t = 4 x 0.5 x 10 = 20 kWh.

Question. A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Answer : Energy = 250 kWh (units) = 250 x 3.6 x 106 Joules (because 1 kWh = 3.6 x 106 J )
= 9 x 108 J

Question. 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 : Consider the case of following pendulum:

The bob is at its highest position at the extreme positions and at lowest at its mean position. So it has maximum potential energy at extreme positions with zero kinetic energy, and maximum kinetic energy with zero potential energy at mean position. At every point sum of kinetic and potential energy remains constant. So it is not the violation of law of conservation of energy. Bob comes to rest eventually due to fraction of air.

Question. 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 : Same as its kinetic energy i.e. 1/2 mv².

Question. What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Answer : Zero. Because, here angle between force of gravity and displacement of satellite is 90 degree (90°).
Now Work = W = F S Cosθ = FS Cos90° = 0 ( because Cos90°= 0)

Question. Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Answer : Mass = m = 1500 kg
Velocity = v = 60 km/h = 60 x 5/18 m/s = 50/3 m/s
Work required to be done to stop a car = Kinetic energy of car = 1/2 mv² = 1/2 X 1500 X (50/3)2
= 750  x 2500/9 = 208333.3 J.

Question. An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Answer : Power = P= 1500 W = 1500/1000 kW = 1.5 kW
Time = t = 10 hours
Energy used = P x t = 1.5 x 10 kWh = 15 kWh.

Question. 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- (1) Here, the force acts at right angle to the displacement. So, work done by the force = 0
(2) Here, the direction of displacement is the same as that of the force. So, work done by the force is positive.
(3) Here, the body moves in a direction opposite to the direction of the force. So, work done by the force is negative.

Question. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
Answer : When a freely falling body eventually stops on reaching the ground, its kinetic energy gets converted into the heat energy, sound produced due to collision, potential energy of configuration of the body and the ground.