Which of the following examples 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

(a) It is periodic but not S.H.M. because it is not to and fro motion about a fixed point. (b) It is S.H.M. (c) It is S.H.M.
(d) It is a periodic but not S.H.M. A polyatomic gas molecule has a number of natural frequencies and its general motion is the resultant of S.H.Ms. of a number of different frequencies. The resultant motino is periodic but not S.H.M.