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) The rotation of the earth about its axis represents periodic but not S.H.M because it is not following to and fro motion.
(b) The motion of an oscillatory mercury column in a U-tube is S.H.M.
(c ) The motion of a ball-bearing inside a smooth curved bowl when released from a point slightly above the lowermost position is S.H.M.
(d) A polyatomic molecule has a number of natural frequencies, so its motion is superposition of S.H.M.s of a number of different frequencies. Therefore, the resultant motion is periodic but not S.H.M.