Distance, time and mass are known as fundamental physical quantities. Distance is a measure of space in one dimension. Two other physical quantities that are closely related to distance are area and volume. The measure of time is based on periodic phenomena, (processes that repeat over and over and over itself). The rotation of the earth was originally used to establish the universal unit of time, which is the second. (the time it takes for one rotation was set equal to 86,400 seconds) A period is the time for one complete cycle of a process that repeats. The unit for period are seconds, minutes, hours and so forth. Frequency is the number of cycles of a periodic process that occur per unit of time. The standard unit of frequency is hertz (hz) The relationship between the period and frequency is as follows: period = 1/ frequency. (1 divided by frequency)