Question

What are the similarities and differences between periodic motion and vibratory motion?

Answer 1

Periodic motion is that motion which repeats itself after equal intervals of time. The interval of time is called the time period of periodic motion. Examples:
(i) motion of planets around the sun
(ii) motion of the pendulum of a wall clock
(iii) motion of the minute hand of a clock

If a body is moving back and forth repeatedly about a mean position, it is said to possess oscillatory motion. Examples:
(i) motion of the pendulum of a clock
(ii) vibrations of the string of a sitar etc.