Discuss whether or not work is done in the following cases:
(i) When we press a football.
(ii) When we push a table.
(iii) When a person holds a book in his hand and keeps it stationary.
(iv) When a wire is twisted.
(v) When a girl carries a hand bag and walks on a levelled road.
(i) In pressing a football, we change its volume.
Therefore, work is done against the pressure of the gas inside the bladder of the football.
(ii) When we push a table, we do work against the force of friction between the legs of the table and the floor.
(iii) Here work done is zero, because there is no
displacement, though the force is constantly being applied.
(iv) When a wire is twisted, its shape is changed, in this manner, work is done against internal forces of cohesion between the molecules of wire.
(v) Here, no work is done because the force, which the girl is applying against the weight of the bag is acting vertically upwards and her motion is in the horizontal direction. The angle between the direction of force and the displacement is 90°. Therefore, no work is done.
As, W = Fs cos 90° = Fs x 0 = 0