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

To determine which examples represent simple harmonic motion (SHM) and which represent periodic but not SHM, we need to analyze each example based on the definitions of periodic motion and SHM.

### Definitions:
1. **Periodic Motion**: Motion that repeats itself after a fixed interval of time.
2. **Simple Harmonic Motion (SHM)**: A type of periodic motion where the restoring force is directly proportional to the displacement from the equilibrium position and acts in the opposite direction.

### Analysis of Each Example:

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