Question

Every SHM is a periodic motion. But all periodic motions are not SHM why? Explain with an example

Answer 1

Periodic motions are any type of repeated motion. A simple harmonic motion is a one in which the object moves to and fro about a mean position, and the restoring force on the moving object is dirctly proportional to its displacement in either direction.
In tSHM, the speed of the moving object is zero at the extreme positions, and maximum at the mean position. Just like in a pendulum, where the bob stops at the extreme positions (zero speed), and attains maximum speed at the mean position (at the Bottom most position of the bob).
All the other periodic motions which do not satisfy this condition are not simple harmonic. Like the example earth revolving around the sun,uniform rotation of fan etc
Angular speed of earth or a fan is constant, not zero or maximum at any position. Also the earth or a fan aren't moving back and forth, they're just moving in one direction.