Compound Units

Compound units are quantities that are a combination of different units, and are found when we multiply and divide measurements together.

Speed is a good example of this. Depending on where you are in the world, you will use either miles per hour (mph) or kilometres per hour (kph).

If you are travelling at 60mph, this means that for every hour of travel, you will travel 60 miles.

The units come from the formula for speed which is $\dfrac{distance}{time}$. Therefore, when we say miles per hour, we are really thinking of $\dfrac{miles}{hour}$

Sometimes, you may find speed recorded in metres per second. In this case, it is conventional to give the units as $ms^{-1}$. Remember that a negative power indicates a reciprocal, so this is the same as $\dfrac{m}{s}$ or metres per second.

Another example of compound units is pressure, which is defined as $\dfrac{force}{area}$. The standard units for pressure are Pascals (Pa).

1 Pascal = 1 Newton/metre$^2$ (N/m$^2$)

You also need to be able to calculate individual quantities from compound units. You can do this by rearranging the formulae.