In my usage of the words, oscillation requires a system in which a
restoring force is acting. The oscillating object has an equilibrium
position, and displacement from that position gives rise to a force that
brings the object back through the equilibrium position. As the object
passes through the equilibrium position the restoring force becomes
zero. This is also periodic because "periodic" simply means the system
repeats its motion at a definite time interval.
A object such as a satellite is periodic, but is not acting under the
influence of a restoring force. There is no equilibrium position in the
mostion where a restoring force reduces to zero.
It is interesting that a "simple pendulum" system can exhibit both
behaviors. If displaced from equilibrium and just released, it
oscillates as a simple pendulum. If the "release" also imparts a push
that is not in line with the restoring force, the pendulum becomes a
conical pendulum that is periodic, but not oscillating (it does not pass
through an equilibrium position). Indeed, for the same pendulum the two
types of periodic motion do not have the same periods.
One of the lab experiments my students do is show that the simple
pendulum, when undergoing oscillation, has a period that is amplitude
dependent, which is one aspect that results from the fact that the
simple pendulum is not "simple harmonic motion" (i.e. the restoring
force is not linear). One of the difficult aspects of the experiment
(which requires very accurate timing of the period) is releasing the
pendulum so it is an oscillatory pendulum rather than a conical
pendulum. Using photogate timers it is a piece of cake to measure the
period with sufficient accuracy to demonstrate the amplitude dependence.
However, keeping the oscillation from becoming a conical pendulum
requires a better release than many students can obtain.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu