NCERT Solution | Work and Energy | Class IX

Q1. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.

(i) Suma is swimming in a pond.

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

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

(iv) A green plant is carrying out photosynthesis.

(v) An engine is pulling a train.

(vi) Food grains are getting dried in the Sun.

(vii) A sailboat is moving due to wind energy. (NCERT)

Ans. (i) Suma is swimming in a pond: In this case, Suma pushes the water in the backward direction. It is, known as action. However, due opposite to reaction, the water, pushes the person in the forward direction. In this situation, the force and the displacement are in the same direction. The work is done by Suma and the work done by the force is positive.

(ii) A donkey is carrying a load on its back: If the displacement of an object is perpendicular to the force acting on it, the work done by the force on the object is zero. In the given situation, the force of gravity on the load is in the downward direction, whereas the displacement is in the horizontal direction, i.e., the force and the displacement are perpendicular to each other. There is no displacement in the direction of the force of gravity, and therefore, work done is zero.

(iii) A wind-mill is lifting water from a well: In this situation, the object (bucket of water from a well) moves upwards. The force exerted by wind-mill is in the direction of displacement. Therefore, work is done. This work done is Positive as the force and the displacement are in same direction.

(iv) A green plant is carrying out photosynthesis: Since there is no force applied or displacement, no work is clone in this case.

(v) An engine is pulling a train: In this situation. an engine is pulling a train parallel to the ground. The force exerted by the engine is in the direction of displacement of the train. Thus. the force and the displacement are in the same direction. Therefore. work is done. This work done is positive.

(vi) Food grains are getting dried in the Sun: Since there is no displacement, no work is done in this case.

(vii) A sailboat is moving due to wind energy: The force exerted by wind on the sail moves the boat in the direction of force, hence, positive work is done by wind energy.