Frequency and Period

Frequency tells you how many complete cycles happen each second, while period tells you how long one complete cycle takes. Because these are two ways of describing the same repeating motion, they are reciprocals: if one cycle takes more time, fewer cycles fit into a second, and vice versa. The units reflect this inverse relationship: period is measured in seconds, so frequency is measured in hertz, meaning "per second".

Use this formula for any repeating motion or wave, such as vibrating strings, sound waves, rotating wheels, or oscillating pendulums. It is especially useful when you are given the time for one cycle and need the rate of repetition, or the other way around.

• Using period and frequency with the same units instead of seconds and hertz
• Multiplying frequency and period instead of taking the reciprocal
• Forgetting that a smaller period means a larger frequency