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 done $W=FS$ $cos\theta$ where $\theta$ 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.

$\therefore$ In this case, $\theta =0^o$ $⟹W=FScos{0}^o=FS$

(B) : Donkey applies a force in the vertical direction to carry the load and its displacement is in the horizontal direction, thus $\theta ={90}^o$ in this case.

$⟹W=FScos{90}^o=0$

(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.

$\therefore$ In this case, $\theta =0^o$ $⟹W=FScos{0}^o=FS$

(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.

$\therefore$ $S=0$ $⟹W=0$

(E) : While pulling, the engine applies a force in the forward direction on the train and the train also moves in the forward direction.

$\therefore$ In this case, $\theta =0^o$ $⟹W=FScos{0}^o=FS$

(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.

$\therefore$ $S=0$ $⟹W=0$

(G) : Wind applies a force on the sailboat in the forward direction making the boat to move in the direction of force.

$\therefore$ In this case, $\theta =0^o$ $⟹W=FScos{0}^o=FS$