Simple Harmonic Motion 

The red
particle follows a circular path with radius R in anticlock
wise direction (considered positive) with a uniform speed. Its x
and y projections are shown in lightblue and orange
respectively. If the angular position of the red
particle is denoted by q then its x
and y coordiantes are R cos
φ and R sin φ (with x to the right, y upward,
and origin at the center of the circle). Since the motion of the particle
round the circle is uniform, q = ωt + φ (where ω is the
angular velocity and φ is the initial angular position of the red
particle). In the applet, φ = 0 and q is shown by the small,
bluefilled arc at the center. The phase is
reset to zero after every revolution. In practice it would increase by
2π radians for every revolution.
The
motion of the lightblue particle can be represented as 
Applet Menu 