**Frequency** is a quantity that measures the number of repetitions per unit time of any phenomenon or periodic event.

To calculate the frequency of an event, a number of occurrences s of the given time interval are recorded, then these repeats are divided by the elapsed time. According to the International System (SI), the frequency is measured in Hertz (Hz), in honor of Heinrich Rudolf Hertz. One hertz is the frequency of an event or phenomenon repeated once per second. Thus, a phenomenon with a frequency of two Hertz is repeated two times per second. This unit was originally called "cycle per second" (cps). Other units to indicate the frequency are revolutions per minute (rpm). The heart beats and musical tempo are measured in "beats per minute" (bpm).

An alternative method for calculating the frequency is to measure the time between two repeats (period) and then calculate the frequency (f).

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- decinewton
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- dekanewton
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- yoctonewton
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