Question

Among the following which are the examples of periodic motion?
A. A hydrogen molecule rotating about its centre of mass.
B. A freely suspended bar magnet displaced from its N-S direction and released.
C. A swimmer completing one (return) trip from one bank of a river to the other and back.
D. An arrow releases from a bow.

Answer 1

Hint: Periodic motion is the motion that repeats at regular intervals. All periodic motions need not be oscillatory. However, all the oscillatory motions are periodic. The motion of the body can be analysed by the movement of the particles along certain positions.

Complete step by step answer:
In the given question when we consider option A, the hydrogen molecule that rotates about its centre of mass comes to the same position again and again and this happens at regular intervals of time. Hence we can say that this motion is periodic.
In option B, the freely suspended magnet oscillates about its position in a definite period of time, when displaced from the north-south direction. Since this is oscillatory, it is also periodic.
In option C, the swimmer need not have a definite period, even when he is swimming to and fro. Hence we can say that the motion is not periodic.
In option D, the arrow released from the bow will not come backwards. It is projected in the forward direction only. Hence, it is not periodic.

Hence, option A and B is the correct answers among the four.

Note:
Period is the smallest time interval after the motion repeats. It is denoted by t and the SI unit is seconds. The reciprocal of the time period gives frequency , that is the number of oscillations in one second. The SI unit of frequency is Hertz (Hz)