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The planet Saturn and its well-known rings as viewed by telescope from Earth. The rings consists of particles, some very fine, some the size of boulders, which orbit the planet. Although their motion is not perfectly circular, all the important characteristics of the motion can be described by mathematics developed for describing circular motion.
It is reasonable to apply the same terminology to circular motion that we applied to oscillations in the first week. The period, T, for example, is the time needed to complete one full circle. The frequency, f, is then just 1/T, or the number of full circles completed per unit time. The amplitude, A, of the motion is given by the radius of the circular motion since this defines the maximum distance from the origin along either the horizontal or vertical axis. In fact, the connection, as implied earlier, is even deeper since the y and x position of an object in circular motion is given by x = A cos(2*pi*ft) and y = A sin(2*pi*ft) as we discovered when considering the parametric representation of a circle. However, these same sine and cosine functions describe the position of a mass attached to a spring which oscillates back and forth.