Density is an example of a compound measure, it is a measure of the mass of an object per unit volume. Density is usually measured in g/cm³ or kg/m³ etc.
For example: A density of 1.5 g/cm³ means that every 1 cm³ of the material has a mass of 1.5 grams.
The density of a given material is calculated using the following formula:
A change in constant rate is when one measurement changes in direct proportion with another. For example, speed is measured in km/h or mph and if calculated as follows:
As in some circumstances, speed is not always constant, the average speed may need to be calculated. The average speed is calculated as follows:
Converting compound units
When converting compound units, it is usually carried out in several steps. For example:
50 mph stands for 50 miles per hour.
If 50 miles is being converted into metres and seconds, the following steps are taken:
50 miles = 50 x 1.6 km (1 mile = 1.6 km)
50 miles = 80 km
50 miles = 80,000 metres
1 hour = 60 minutes
1 hour = 3,600 seconds
To convert 50 mph to m/s the following steps are carried out:
50 mph = 80,000 / 3,600
50 mph = 22.22 m/s (to 2 d.p.)