Which of the following examples represents (nearby) shm and which represents periodic but not shm.
(i) The rotation of earth about its axis.
(ii) Motion of an oscillating mercury column in a U-tube.
(iii) Motion of a ball bearing inside a smooth curved bowl when released from a point slightly above the lowermost position.
(iv) General vibration of a polyatomic molecules about its equilibrium configuration.

Explanation

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

### Step-by-Step Solution:

1. **Understanding SHM**:
- Simple Harmonic Motion (SHM) is defined as 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.

2. **Analyzing Each Example**:
...