Which of the following examples represent periodic motion?
(a) A swimmer completing one return trip from one bank of a river to other bank. (b) A freely suspended bar magnet displaced from its `N-S` direction and released. (c) A hydrogen molecule rotating about its centre of mass. (d) An arrow released from a bow.

Explanation

(i) The motion of a freely-suspended magnet , if displaced from its N-S direction and released is periodic. This is because the magnetic oscillates about its position with a definte period of time.
(ii) An arrow released from a bow moves only in the forward direction. It does not come backward. Hence, this motion is not a periodic.