Class Nine Science Chapter 11 NCERT Solutions

Chapter 11: Work, energy, and power 

Intext Questions (PAGE 148)

1. 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?

Ans. Force = 7 N; Displacement = 8 m; Work done = Force × Displacement = 7 N × 8 m = 56 J

Intext Questions (PAGE 149)

1. When do we say that work is done?

Ans. When a force causes displacement of an object in the direction of applied force, work is said to be done.

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

Ans. Work done (W) =

3. Define 1 J of work.

Ans. When a force of 1 N causes a displacement of 1 m, in its own direction, the work done is said to be one joule.

4. 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. Work done = Force × Displacement = 140 N × 15 m = 2100 J.

Intext Questions (PAGE 152)

1. What is the kinetic energy of an object?

Ans. The energy possessed by an object on account of its motion is known as the kinetic energy of that object.

2. Write an expression for the kinetic energy of an object.

Ans. When ‘m’ is the mass of an object and ‘ν’ the velocity, then

Kinetic energy of an object (K.E) =1/2 mv2.

3. 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 the velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

Ans. Mass of an object (m) = ?; Velocity of an object (v) = 5 ms–1

Kinetic energy of an object (K.E.) = 25 J

Now, K.E. =1/2mv2

 ⇒  25 J = ½ × m × (5 ms–1)2

m = 50 J/25 m2s–2 = 2 kg

When the velocity doubles:

K.E. = ½ mv2 = ½ × 2 kg × (10 ms–1)2 = 100 J

When the velocity triples:

K.E. = ½ mv2 = ½ × 2 kg × (15 ms–1)2 = 225 J

Intext Questions (PAGE 156)

1. What is power?

Ans. Rate of doing work is called power.

2. Define 1 watt of power.

Ans. When a work of 1 joule is done in 1 second, the power is said to be one watt.

3. A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

4. Define average power.

Ans. When a machine/person does different amounts of work or consumes energy at different rates at different intervals of time, then the ratio between the total work or energy consumed and the total time is known as average power.

Average power = Total work done or energy consumed/Total time