NCERT Questions

Class 9
Chapter 11 Class 9 - Work and Energy

## 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

Work is done when

1. Force is applied
2. Displacement takes place
3. The angle between the force and displacement is not 90 ° .

Let’s look at each of these examples one by one

(a) Suma is swimming in a pond.

### Swimming in a pond Image

When Suma is swimming in a swimming pool, she applies force in the forward direction.

And moves in the forward direction. So, displacement is in the forward direction.

Since displacement and force are in the same direction,

Hence, work is done.

(b) A donkey is carrying a load on its back.

When a donkey is carrying a load on his back, it is applying a force in the upward direction.

And he is moving in the forward direction.

So, angle between force and displacement is 90 degrees.

Hence, no work is done

(c) A wind-mill is lifting water from a well.

### wind-mill is lifting water from a well. Image - Add heading and arrows of force and displacement

When a wind-mill is lifting water from well, it is applying a force in the upward direction.

And it is moving water in the upward direction.

Since displacement and force are in the same direction,

Hence, work is done.

(d) A green plant is carrying out photosynthesis.

When a green plant is carrying photosynthesis, there is no force involved.

And the plant does not move.

Since there is no force or displacement.

Hence, no work is done

(e) An engine is pulling a train.

### Engine pulling the trail Image - Add heading and arrows of force and displacement

When an engine is pulling a train, it is applying a force in the forward direction.

And it is moving in the forward direction.

Since displacement and force are in the same direction,

Hence, work is done.

(f) Food grains are getting dried in the sun.

When a food grains are getting dried in the sun, there is no force involved.

And the grain does not move.

Since there is no force or displacement.

Hence, no work is done

(g) A sailboat is moving due to wind energy

### Sailboat moving Image - Add heading and arrows of force and displacement

When a sailboat is moving due to wind energy, force is applied by the boat in the forward direction.

And it is moving in the forward direction.

Since displacement and force are in the same direction,

Hence, work is done.

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