
76
Fig. 5.4 Starting position of an alternative
method of representing sinusoidal
variation
which by mathematical convention is the positive direction of
rotation. Then the height of the end Ρ above a horizontal line
through Ο varies in exactly the same way as a sine wave. Assuming
OP starts, as shown in Fig. 5.4, at '3 o'clock', Ρ is actually on the
horizontal line, so its height above it is, of course, nil. That
represents the starting point of a sine-wave cycle. As OP rotates, its
height above the line increases at first rapidly and then more slowly,
until, after a quarter of a revolution, it is at right angles to its original
position.