If the value and direction of a variable ball is the same at certain time intervals, then this ball is called periodic ball. When this periodic force is applied to an object, the object first tries to vibrate at its own normal frequency. But after vibrating irregularly for a while, the object continues to vibrate at the applied periodic ball. As long as the periodic force is acting on the object, the object continues to vibrate at the frequency of the applied ball. This type of vibration is called aftershock or imposed vibration.