NCERT Solutions for Class 9th Science
Chapter 11 - WORK AND ENERGY
(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)
Q.1 A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force (Fig. 11.3). Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Q.2 When do we say that work is done?
Ans Two conditions need to be satisfied for work to be done:
(i) A force should act on an object, and
(ii) The object must be displaced in the direction of force.
Q.3 Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Ans Let the force applied = F
Displacement in the direction of force = S
Hence, work done, W = F x S
Q.4 Define 1 J of work.
Ans 1 J of work is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.
W (1 J) = F (1 N) x S (1 m)
Q.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?
Q.6 What is the kinetic energy of an object?
Q.7 Write an expression for the kinetic energy of an object.
Q.8 The kinetic energy of an object of mass, m moving with a velocity of 5 m/s 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?
Q.9 What is power?
Q.10 Define 1 watt of power.
Q.11 A lamp consumes 1000 J of electrical energy in 10 s. What is its power?
Q.12 Define average power.
Ans The power of an agent may vary with time i.e. the agent may be doing work at different rates at different intervals of time. Thus the concept of average power arises. Average power is obtained by dividing the total energy consumed with the total time taken.
Average power = Total energy consumed / total time taken
EXERCISES
Q.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.
Ans Two conditions need to be satisfied for work to be done:
(i) A force should act on an object, and
(ii) The object must be displaced in the direction of force.
Suma is swimming in a pond – Work is done as for swimming Suma must have applied force backwards. The reaction applied by water is in the forward direction and so as the displacement caused by this force. Hence work is done in this case.
A donkey is carrying a load on its back – No work is done as the force applied by donkey (upward direction) is in the perpendicular direction to the displacement (forward direction).
A wind-mill is lifting water from a well – Work is done as force applied by wind mill (upward direction opposite to gravitational pull) caused displacement in the same direction (upward direction).
A green plant is carrying out photosynthesis – No work is done as there is no displacement of leaves.
An engine is pulling a train – Work is done as pull force applied by engine ( forward direction) caused displacement of train in the same direction (forward direction).
Food grains are getting dried in the sun – No work is done as there is no displacement of food grains during drying process.
A sailboat is moving due to wind energy – Work is done as the force applied by wind (forward direction) caused displacement of sailboat in the same direction (forward direction).
Q.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?
Ans Let the mass of object = m
Force of gravity = mg (downward direction)
Displacement (vertical direction) = 0 (as initial and final point lies on the same horizontal line)
Hence, work done = Force x Displacement = mg x 0 = 0 J
Q.3 A battery lights a bulb. Describe the energy changes involved in the process.
Q.4 Certain force acting on a 20 kg mass changes its velocity from 5 m/s to 2 m/s. Calculate the work done by the force.
Q.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.
Q.6 The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Q.7 What are the various energy transformations that occur when you are riding a bicycle?
Q.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?
Q.9 A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Ans 1 unit = 1 kWh and 1 kWh = 3.6 x 106 J
Hence, Energy consumed for 250 units = 250 x 3.6 x 106 = 9 x 108 J
Q.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.
Ans mass, m = 40 kg; height, h = 5 m; g = 10 m/s2
» Potential Energy, P.E. = mgh = 40 x 10 x 5 = 2000 J
At half-way down,
P.E. will be 2000 / 2 = 1000 J
Due to the law of conservation of energy,
At half-way down, Kinetic energy, K.E. = P.E. = 1000 J
Q.11 What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Q.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.
Q.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.
Q.14 An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Q.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?
Q.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?
Q.17 Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Q.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.
Q.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?
Q.20 Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.
Q.21 A freely falling object eventually stops on reaching the ground. What happenes to its kinetic energy?