Until now we have always considered that the real moving body begins its movement (at t = 0 s) at the coordinate origin (y = 0 m) and moves towards the positive ordinates (upwards). However, the initial situation may be different, and to reflect this fact we introduce the concept of the phase difference "φ".

Remember that the complete equation of an S.H.M. is:

The function of the phase difference"φ" is to indicate what the position of the moving body is at the start (when t= 0) and in which direction it is moving.

To determine its value we must "imagine" what position the auxiliary body must be in at the beginning so that, once the movement has started, its shadow continues to be on the real body, as we saw when we studied the relation between the S.H.M. and the U.C.M.

Go to the next page to practise identifying the phase difference.