the phase of a pasticular point in oscillation or vibration is the fraction of the oscillation measured from an initial chosen point.
oscillation motion is closely related to circular motion. in figure 4.8 one complete revolution of the radius vactor OP propaged one cycle of a sinsoidal wave.
in general, the phase at point S of a sinusoidal wave measured from point R is given by phase, where
from figure 4.8,R and U are in phase with each other and R and T are 180 or radians out of phase with each other