Class 9th Science Chapter-10 In-Text Questions

Chapter 10: Work and Energy

Page 115 – NCERT Class 9 Science

Questions with Answers

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Question 1

A force of 7 N acts on an object. The displacement is 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

$$\text{Work done}=\text{Force}\times \text{Displacement}$$

$$W=7\times 8=56J$$

Work done = 56 joules

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Page 116 – NCERT Class 9 Science

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Question 1

When do we say that work is done?

Answer:
Work is said to be done when a force acts on an object and the object gets displaced in the direction of the force.

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Question 2

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

Answer:

$$W=F\times s$$

where $F$ is force and $s$ is displacement.

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Question 3

Define 1 J of work.

Answer:
1 joule is the work done when a force of 1 newton displaces an object by 1 metre in the direction of the force.

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Question 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?

Answer:

Given Force = 140 N, Distance covered = 15 m

$$W=F\times s=140\times 15=2100J$$

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Page 119

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Question 1

What is the kinetic energy of an object?

Answer:
The energy possessed by an object due to its motion is called kinetic energy.

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Question 2

Write an expression for the kinetic energy of an object.

Answer:
The kinetic energy of an object of mass $m$ moving with velocity $v$ is given by:

$$E_k=\frac{1}{2}m{v}^2$$

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Question 3

The kinetic energy of an object of mass $m$ moving with a velocity of $5m{s}^{-1}$ is $25J$. 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:

Since kinetic energy is proportional to the square of velocity:

• When velocity is doubled:

$$E_k=4\times 25=100J$$

• When velocity is tripled:

$$E_k=9\times 25=225J$$

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Page 123

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Question 1

What is power?

Answer:
Power is defined as the rate of doing work or the rate of transfer of energy.

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Question 2

Define 1 watt of power.

Answer:
1 watt is the power when 1 joule of work is done in 1 second.

$$1W=1J{s}^{-1}$$

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Question 3

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

Answer:

$$P=\frac{W}{t}=\frac{1000}{10}=100W$$

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Question 4

Define average power.

Answer:
Average power is defined as the total work done divided by the total time taken.

$$\text{Average power}=\frac{\text{Total work done}}{\text{Total time taken}}$$

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