State whether work is done or not by writing yes or no, in the following cases:
(a) A man pushes a wall.
(b) A coolie stands with a box on his head for 15 min.
(c) A boy climbs up 20 stairs.
(a) No, as there is no displacement.
(b) No, as there is no displacement.
(c) Yes, work is done against gravity.