Which of the following examples represent periodic motion?
(i) A swimmer completing one (return) trip from one bank of a river to the other and back.
(ii) A freely suspended bar magnet displaced from its N-S direction and released.
(iii) A hydrogen molecule rotating about its centre of mass.
(iv) An arrow released from a bow.
(i) There is no repetition of the motion as the swimmer just completes one trip hence not periodic.
(ii) The motion is repeated after a certain interval of time,hence periodic. In fact , the bar magnet oscillates about its mean position with a definite period of time.
(iii) Rotatary motion is periodic as repeating after fixed time-interval.
(iv) There is no repetition, hence not periodic.