Which of the following examples represents periodic motion ?
(i) A swimmer completing onė (return) trip from one bank of a river to the other bank.
(ii) A freely suspended bar magnet displaced form its N-S direction and released.
(iii) Halley's comet
(iv) A hydrogen molecules rotating about its centre of mass.
(v) 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.