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When a body repeats its motion along a definite path after regular interval of time, its motion is said to be periodic motion and interval of time is called time period (T). The path of periodic motion may be linear, circular, elliptical or any other curve. For example, the revolution of the earth about the sun.
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To and fro type of motion is called an oscillatory motion. It needs to periodic and has fixed extreme positions. For example, the motion of pendulum of a wall clock.
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The force/torque directed towards equilibrium point acting in oscillatory motion is called restoring force/torque.
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If the restoring force/torque acting on the body in oscillatory motion is directly proportional to the displacement of body and is always directed towards equilibrium position, then the motion is called simple harmonic motion (SHM). It is the simplest form of oscillatory motion.
