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Consider, for concreteness, the system to be a mass on a
horizontal spring. The mass then takes
to go
from the far right to the far left, since at the end points of
the motion. Thus the mass has moved through *half* of it's periodic
motion. Thus the period will be twice this amount of time or
. The frequency is the reciprocal of the period,
or 2 Hz.
Finally, the amplitude of the motion is the distance from equilibrium
to on of the end points. Since the distance between end points is 36
cm, the amplitude is half this amount, or 18 cm.

Dan Cross
2006-10-18