Which of the following examples represents periodic motion? (a) A swimmer completing one (return) trip from one bank of a river to the other and back. (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.
When a hydrogen molecule rotates about its centre of mass, it comes to the same position again and again after an equal interval of time. Such a motion is periodic. (b) The motion of a freely-suspended magnet, if displaced from its N-S direction and released, is periodic because the magnet oscillates about its position with a definite period of time. (c) The swimmer's motion is not periodic. Though the motion of a swimmer is to and fro but will not have a definite period. (d) An arrow released from a bow moves only in the forward direction. It does not come backward. Hence, this motion is not a periodic.
