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

(a) It is not a periodic motion. Though the motion of a swimmer is to and fro but will not have a definite period.
(b) Since a freely suspended magnet if once displaced from N.S direction and released it oscillated about this position, it is a periodic motion
(c) The rotating motion of a hydrogen molecule about its centre of mass is periodic.
(d) Motion of an arrow released from a bow is non-periodic.