## 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/m3 (alternatively kgm-3) kilograms divided by metres cubed, or g/cm3 (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.