Frequency is the number of events per unit of time where an event is repeating at regularly spaced time intervals.

Frequency's units are per second, written s^{-1} or hertz, Hz. Hertz and per second are identical units, i.e. 1 s^{-1} = 1 Hz.

**Example 1**

When describing a moving wave, frequency means the number of peaks which pass a stationary point in a given amount of time.

**Example 2**

When describing a heartbeat, frequency means the number of beats per second.

**Example 3**

When describing a rotating wheel, frequency means the number times the wheel rotates through 360° per second.

**Worked Example 1**

A fast drummer beats a drum 150 times in 30 seconds keeping a regular rhythm. What is the frequency of the beats?

^{-1}or 5 Hz

**Worked Example 2**

A humming bird's wings go up and down again 440 times in 8 seconds. What is the frequency of the wing beats?

**Worked Example 3**

On the beach, you count waves as they pass a rock. You notice that 6 wave peaks pass the rock in a minute. What is the frequency of the waves?

Frequency is the inverse of period, T. The period is the time taken between events that are repeating at regular intervals.

**Worked Example 4**

The frequency of a fan's rotation is quoted as 100 Hz. How long does a complete 360° rotation of the fan take?

⇒ T = 1/f

⇒ T = 1/100 s

^{-1}

⇒ T = 0.01 s