If the motion of a moving particle is such that it passes through a definite point along the path of its motion in the same direction in a definite interval of time, this type of motion is called periodic motion. The hands of the watch and the electric fans are the examples of circular periodic motion. The contraction and expansion of spring is the linear periodic motion. The time required for a complete oscillation of an object executing periodic motion is called its time period.
Oscillatory or harmonic motion:
If a body executing periodic motion moves in a definite direction for one half of its time period and exactly for the other half in the opposite direction then this motion is called oscillatory or harmonic motion. If any spring is hung from a strong support and an object is attached at the lower end and released after stretching, the spring will vibrate up and down. This type of motion is known as oscillatory or harmonic or vibratory motion. Motion of a simple pendulum and motion of a vibrating tuning fork is harmonic motion.