Which of the following examples represent (nearly) simple harmonic motion and which represent periodic but not simple harmonic motion?
(a) The rotation of earth about its axis. (b) Motion of an oscillating mercury column in a U tube,
(c) Motion of a ball bearing inside a smooth curved bowl, when released from a point slightly above the lower most position. (d) General vibrations of a polyatomic molecule about its equilibrium position.

Explanation

(i) During its rotation about its axis, earth comes to the same position again and again in equal intervals of time. Hence, it is a periodic motion. However, this motion is not simple harmonic. This is because earth does not have a to and fro motion about its axibr.
(ii)An oscillating mercury column in a U mercury moves to and fro on the same path, about the fixed position, with a certain period of time.