# Work and Energy (NCERT, Class 9) Page149

(1) When do we say that work is done?

Ans- We say that work is done when an object displaced after applying force on it.

(2) Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Ans- The expression for the work done when a force is acting on an object in the direction of its displacement is given below –

W = Fs

Where, W = work done on the object

F = force applied on the object

s = displacement of the object after apply force

(3) Define 1 joule of work.

Ans-

1 joule is the amount of the work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.

Joule can be denoted by symbol (j).

If F = 1 N and s = 1 m Then,

W = 1 Nm

W = 1 joule       Exercise

(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-

 SN Activities Work done Work not done 1 Suma is swimming in a pond. Yes work done, because an force is applied by her on water in backward direction and she displaced in forward direction. 2 A donkey is carrying a load on its back. Work not done because monkey is standing at a place and not showing displacement. 3 A wind – mill is lifting water from a well. Work done because wind – mill lift water (displacement of the water) and applied force on the water. 4 A green plant is carrying out photosynthesis. Work is not done because green plant does not apply force and show displacement. 5 An engine is pulling a train. Work done because the engine applied force on the train and train shows displacement. 6 Food grains are getting dried in the sun. Work is not done because the Sun  does not apply force on food grains and grains do not show displacement. 7 A sailboat is moving due to wind energy. Work is done because wind energy applies force on sail boat and it shows displacement. (3) A battery lights a bulb. Describe the energy changes involved in the process.

Ans- Change of energy is tabulated below–

 SN Connection Energy Change 1 Battery connected to bulb Chemical energy changes to electrical energy 2 When bulb lighten up Light and heat energy  (6) The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Ans- The potential energy of a freely falling object decreases progressively. But kinetic energy of the object increases simultaneously. Therefore, the law of conservation of energy is not violate.

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

Ans- The various energy transformations that occur when you are riding a bicycle are given below –

 SN Conditions of bicycle riding Transformation of Energy 1. Rider move bicycle Muscular energy transformed in kinetic energy and heat energy.

(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?

Ans- When we push rock, it not begins to move. Because the muscular energy generated by pushing the rock does not transfer to the stationary rock. Therefore transfer of energy will not take place when we push a huge rock with all our energy.

The muscular energy we spend is going in to producing heat energy.   (11) What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.

Ans- The work done by the force of the gravity on a satellite moving round the earth is zero.

When a force applied on the object is perpendicular to the displacement of the object, the work done on the object is zero.

Satellite is moving round the earth in circular path. Therefore, force applied on the object is perpendicular to the displacement.

Hence, the work done by the force of gravity on a satellite moving round 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.

Ans- Yes, there can be displacement in the absence of any force acting on it. If object moves with constant velocity, the net force acting on the object is zero. Therefore, net force acting on the object is zero, but object is moving with uniform velocity. Hence, the object shows displacement in the absence of net force.  (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?

Ans-(i) When bob is at the middle at the rest, it has only potential energy.

Total energy at point A = Potential energy + Kinetic energy  ..(1)

(ii) Before reaching Point B the potential energy of the bob changes to kinetic energy. It means potential energy of the bob decreases and kinetic energy increases simultaneously.

Total energy from point A to B = Potential energy + Kinetic energy …(2)

This equation always balances because potential energy decreases kinetic energy increases.

(iii) Now bob reaches to point B. Its kinetic energy changes to potential energy because at a moment the bob stay at point B before moving toward point A.

Total energy from point B = Potential energy + Kinetic energy …(3)

(iv) The bob releases from point B and approaches towards point A. In this way, potential energy decreases and kinetic energy increases simultaneously.

Total energy from point B to A = Potential energy + Kinetic energy …(4)

(v) When the bob reaches to point A, it has kinetic energy only and move towards point C.

Total energy at point A = Potential energy + Kinetic energy ..(5)

(vi) From point A to C, kinetic energy decreases and potential energy increases again. The process of oscillation of the bob continues till it comes to rest.

In all above equation, total energy remains equal. Therefore, law of conservation of energy holds good.

The bob eventually come to rest because of air resistance or inertia provided by the air.

It is not a violation of the law of conservation of energy because enrgy lost by the pendulum or bob gain by the surrounding. Therefore, law of conservation of energy maintained.  (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. Ans-

(i) In fig (i)

Force F is perpendicular to the object. Therefore, work done on the object is zero.

(ii) In fig (b)

The work done on an object is called positive if displacement of the object is in the direction of the applied force. The work done on the object is positive because displacement is in the direction of force applied.

(iii) In fig (c),

The work done on an object is called negative if displacement of the object is in the opposite direction of the applied force. The work done on the object is negative because displacement is in the opposite direction of force applied.

(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?

Ans- Yes, acceleration in an object could be zero if no force is applied on it. (21) A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?

Ans-  A freely falling object eventually stops on reaching the ground. The kinetic energy transform to heat energy and sound energy.

Helping Topics

Work and energy

Practice sheet, class 9 