Question

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 wind-mill is lifting water from a well.
(d) A green plant is carrying out photosynthesis.
(e) An engine is pulling a train.
(f) Food grains are getting dried in the sun.
(g) A sailboat is moving due to wind energy.

Answer 1

Work is done only when force is applied and a body moves under the influence of this force. Work is done only when the direction of displacement is not perpendicular to the direction of force applied.

Complete Step by Step Answer
Work is done whenever the given two conditions are satisfied:
-A force acts on the body.
-There is a displacement of the body by the application of force in or opposite to the direction of the force.
The activities listed and whether or not work is done has been stated below.
(a) Suma is swimming in a pond.
While swimming, Suma applies a force to push the water backwards. Therefore, Suma swims in the forward direction caused by the forward reaction of water. Here, the force causes a displacement. Hence, work is done by Suma while swimming.
(b) A donkey is carrying a load on its back.
While carrying a load, the donkey has to apply a force in the upward direction. But, displacement of the load is in the forward direction. Since displacement is perpendicular to force, the work done is zero.
(c) A wind-mill is lifting water from a well.
A windmill works against the gravitational force to lift water. There is motion when water is lifted. Hence, work is done by the windmill in lifting water from the well.
(d) A green plant is carrying out photosynthesis.
Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize nutrients from carbon dioxide and water. Photosynthesis in plants generally involves the green pigment chlorophyll and generates oxygen as a by-product.In this case, there is no displacement of the leaves of the plant. Therefore, the work done is zero.
(e) An engine is pulling a train.
An engine applies force to pull the train. This allows the train to move in the direction of the force. Therefore, there is a displacement in the train in the same direction. Hence, work is done by the engine on the train.
(f) Food grains are getting dried in the sun.
While they dry, food grains do not move in the presence of solar energy. Hence, the work done is zero during the process of food grains getting dried in the Sun.
(g) A sailboat is moving due to wind energy.
Wind energy applies a force on the sailboat to push it in the forward direction. Therefore, there is a displacement in the boat in the direction of a force. Hence, work is done by wind on the boat.

Note
Force is an external agent capable of changing the state of rest or motion of a particular body. It has a magnitude and a direction.
Displacement is defined as the act of moving someone or something from one position to another or the measurement of the volume replaced by something else.