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

(a) Since the rotation of earth is not a and fro motion about a fixed point, thus it is periodic but not S.H.M.
(b) It is S.H.M.
(c) It is S.H.M
(d) General vibrations of a polyatomic molecule about its equilbrium position is periodic but no SHM. in fact, it is a result of superposition of SHMs executed by individual vibrations of atoms of the molecule.