## Compound Measures

**Compound Measures**

A Compound measurement is a measurement that uses more than one quantity.

Examples include density measurements, speed measurements and rates of pay.

**Density**

Density is calculated by mass ÷ volume. So density is therefore written as mass per volume.

The most common units used to measure density are kg/m^{3} (alternatively kgm^{-3}) kilograms divided by metres cubed, or g/cm^{3} (gcm^{-3}) grams divided by centimetres cubed.

The density of pure water is always 1000kgm-^{3}. So anything with a lower density will float in water.

**Speed **

Speed is measured using the formula distance ÷ time. So this will therefore be written as distance per time.

The most common units used to measure speed are mph (miles per hour) or km/h (kilometres per hour).

Calculating compound measures should be done in stages. So to change 60km/h into m/s you can do the following:

60km in 1 hour

60,000m in 1 hour

60,000m in 60 minutes

1,000m in 1 minute

16.66m in 1s

So 60km/h is 16.66m/s (to 4.s.f.)

**Rates of Pay**

Rates of pay are given for a unit of time for example £20/h (twenty pounds per hour) or £750/week or £3,250/month or £39,000/annum.