Work is done whenever the given two conditions are satisfied:
i) a force acts on the body.
ii) There is a displacement of the body by the application of force in or opposite to the direction of the 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 Seema while swimming.
(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.
(c) A windmill works against the gravitational force to lift water. Hence, work is done by the windmill in lifting water from the well.
(d) In this case, there is no displacement of the leaves of the plant. Therefore, the work done is zero.
(e) 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) Foodgrains 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)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.