(1) A motion, which repeats itself over and over again after a regular interval of time is called a periodic motion
Revolution of earth around the sun (period one year), Rotation of earth about its polar axis (period one day), Motion of hour’s hand of a clock (period 12-hour) etc are common example of periodic motion.
(2) Oscillatory or vibratory motion is that motion in which a body moves to and fro or back and forth repeatedly about a fixed point in a definite interval of time. In such a motion, the body is confined with in well-defined limits on either side of mean position. Oscillatory motion is also called as harmonic motion.
(i) Common examples are
(a) The motion of the pendulum of a wall clock
(b) The motion of a load attached to a spring, when it is pulled and then released.
(c) The motion of liquid contained in U-tube when it is compressed once in one limb and left to itself.
(d) A loaded piece of wood floating over the surface of a liquid when pressed down and then released executes oscillatory motion.
Simple Harmonic Motion
Some Important Definitions