You need to know the following physical magnitudes to describe simple harmonic movement:
Elongation (y): is the distance from the moving body to the origin (O) of the movement at every instant.
Amplitude (A): is the maximum elongation it achieves.
Period (T): is the time it takes to finish one complete vibration.
Phase difference , initial phase or phase angle (φ): its value determines the position of a body at the initial instant.
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