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Class Nine NCERT Solution Science Chapter 11

Chapter 11: Work, energy, and power

Textbook Exercises

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’.

(a) Suma is swimming in a pond.

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

(c) A windmill is lifting water from a well.

(d) A green plant is carrying out photosynthesis.

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

(f) A sailboat is moving due to wind energy.

Ans. (a) Work is done because the displacement of swimmer takes place in the direction of the applied force.

(b) If the donkey is not moving, no work is done as the displacement of the load does not take place in the direction of applied force.

(c) Work is done, as the displacement takes place in the direction of the force.

(d) No work is done, as the displacement does not take place.

(e) No work is done, as no displacement takes place.

(f) Work is done, as displacement takes place in the direction of applied force.

2. An object was thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

Ans.

Consider an object at point A, thrown with a force F at an

angle ‘θ’ such that it lands at point B. If ‘s’ is the horizontal

distance between the points A and B, then :

Force of gravity of Earth = mg

Work is done by the force of gravity = mgs

3. A battery lights a bulb. Describe the energy changes in the process.

Ans. (i) Within the electric cell of the battery, the chemical energy changes into the electric energy.

(ii) The electric energy on flowing through the filament of the bulb first changes into heat energy and then into the light energy.

4. The certain force acting on 20 kg mass changes its velocity from5 ms–1 to 2 ms–1. Calculate the work done by the force.

Ans. Work is done by the force when velocity is 5 ms–1 = ½ mv2

= ½ × 20 kg × (5 ms–1)2 = 250 J

Work is done by the force when velocity is 2 ms–1 = ½ mv2

= ½ × 20 kg × (2 ms–1)2

 = 40 J

∴ Resultant work is done by force = 250 J – 40 J = 210 J.

5. A mass of 10 kg is at a point A on a table. It is moved to point B. If the line joining the A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.

Ans. Zero, because displacement does not take place in the direction of gravitational force.

6. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Ans. It does not violate the law of conservation of energy. Whatsoever, is a decrease in potential energy due to loss of height, same is the increase in the kinetic energy due to gain in velocity.

7. What various energy transformations occur when you are riding a bicycle?

Ans. The chemical energy of the food changes into heat and then to muscular energy. The muscular energy changes to mechanical energy of cycle on paddling.

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 spent?

Ans. Energy transfer does not take place as no displacement takes place in the direction of applied force. The energy spent is used to overcome the inertia of the rest of the rock.

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