Which of the following examples represent (nearly) simple
harmonic motion and which represent periodic but not simple
(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 point.
(d) general vibrations of a polyatomic molecule about its
Answer: (b) and (c) are SHMs
(a) and (d) are periodic, but not SHMs
(a) 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 axis.
(b) An oscillating mercury column in a U-tube is simple
harmonic. This is because the mercury moves to and fro on the
same path, about the fixed position, with a certain period of
(c) The ball moves to and fro about the lowermost point of
the bowl when released. Also, the ball comes back to its initial
position in the same period of time, again and again. Hence, its
motion is periodic as well as simple harmonic.
(d) A polyatomic molecule has many natural frequencies of
oscillation. Its vibration is the superposition of individual
simple harmonic motions of a number of different molecules.
Hence, it is not simple harmonic, but periodic.
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