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'.
• 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. [5 Marks]

Answer 1

Work is done whenever the given two conditions are satisfied:
$\to$ A force acts on the body.
$\to$ There is a displacement of the body by the application of force in or opposite to the direction of force.
(a) 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. [1 Mark]
(b) 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. [1 Mark]
(c) A wind mill works against the gravitational force to lift water. Hence, work is done by the wind mill in lifting water from the well. [1 Mark]
(d) In this case, there is no displacement of the leaves of the plant. Therefore, the work done is zero. [1 Mark]
(e) An engine applies force to pull the train. This allows the train to move in the direction of force. Therefore, there is a displacement in the train in the same direction. Hence, work is done by the engine on the train. [1 Mark]