# Chapter 11 Work and Energy

From very previous class we have learnt about work, that is the ability to do work. But these are many situation where work done is said to be zero. Satellites revolving around earth is the best example. In this chapter we will discuss about when the work done is positive, negative and zero and various forms of energy.

Download pdf of NCERT Solutions for Class Science Chapter 11 Work and Energy

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### Exercise 1

•  Q1 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? Ans: Force = 7 N Displacement = 8 m Work done = Force × Displacement W = F × S W = 7 × 8 = 56 Nm = 56 J Therefore, work done is 56 J.

### Exercise 2

•  Q1 When do we say that work is done? Ans: A work is said to be done when a force is applied on a body and makes the body move then we said work is done. Q2 Write an expression for the work done when a force is acting on an object in the direction of its displacement. Ans: When a force is acting on a body in the direction of the applied force, then the work done on the body is given by the expression: Work done = Force × Displacement W = F × s Q3 Define 1 J of work. Ans: 1 J is defined as the amount of work done when a force of one newton is applied over a displacement of one meter. Q4 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? Ans: Applied force, F = 140 N Displacement, d = 15 m Work done = force x displacement W = 140 × 15 = 2100 J Work done is 2100 J.

### Exercise 3

•  Q1 What is the kinetic energy of an object? Ans: The energy of a body due to its motion is known as kinetic energy. It is a scalar quantity, i.e it does not depend on direction. Q2 Write an expression for the kinetic energy of an object. Ans: Kinetic energy possessed by a body of mass m is moving with a uniform velocity v, is given by the expression, Its SI unit is Joule (J). Q3 The kinetic energy of an object of mass, m moving with a velocity of 5 m s-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? Ans: Mass of the object = m Velocity of the object = v = 5 m/s Kinetic energy, EK = 25 J (i) If the velocity of an object is doubled, then v = 5 × 2 = 10 m/s Therefore, its kinetic energy becomes 4 times because it is proportional to the square of the velocity. Hence, kinetic energy = 25 × 4 = 100 J. (ii) If velocity is increased three times, then its kinetic energy becomes 9 times because it is proportional to the square of the velocity. Hence, kinetic energy = 25 × 9 = 225 J.

### Exercise 4

•  Q1 What is power? Ans: Power is the amount of work done per unit time. It is a scalar quantity. It is expressed in watt (W). Q2 Define 1 watt of power. Ans: 1 watt of power is defined as the power produced when 1 joule of work is done for 1 second Q3 A lamp consumes 1000 J of electrical energy in 10 s. What is its power? Ans: Energy consumed by the lamp ( work done ) = 1000 J Time = 10 s = 100 W Q4 Define average power. Ans: The average power is defined as the ratio of total work done and the total time taken.