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.

Work done
W=FScosθ
where θ is the angle between F and S
(A) : While swimming, Suma pushes the water in the backward direction and as a result of Newton's 3rd law of motion, the water applies an equal force on Suma in forward direction. Hence Suma swims in the forward direction.
∴In this case, θ=0° ⇒W=FScos0°=FS
(B) : Donkey applies a force in the vertical direction to carry the load and its displacement is in the horizontal direction, thus θ=90° in this case.
⇒W=FScos90°=0
(C) : Wind-mill applies a force in the vertical direction to lift the water from well and the water also moves in the vertical direction.
∴In this case, θ=0° ⇒W=FScos0°=FS
(D) : During photosynthesis, all the metabolic reactions take place inside the leaf of the plant, but there is no displacement either of the plant or the leaves.
∴S=0 ⇒W=0
(E) : While pulling, the engine applies a force in the forward direction on the train and the train also moves in the forward direction.
∴In this case, θ=0° ⇒W=FScos0°=FS
(F) : When the food grains are getting dried in the sun, they remain still at their position i.e the food grains do not get displaced.
∴S=0 ⇒W=0
(G) : Wind applies a force on the sailboat in the forward direction making the boat to move in the direction of force.
∴In this case, θ=0° ⇒W=FScos0°=FS