Fluid mechanics deals with the measurement of many variables of many different types of units. Hence we need to be very careful to be consistent.

Dimensions and Base Units

The dimension of a measure is independent of any particular system of units. For example, velocity may be in metres per second or miles per hour, but dimensionally, it is always length per time, or

The dimensions of the relevant base units of the Systeme International (SI) system are:

Derived Units

From these we have some relevant derived units (shown on the next page).

Checking the dimensions or units of an equation is very useful to minimize errors. For example, if when calculating a force and you find a pressure then you know you’ve made a mistake.

SI Prefixes

SI units use prefixes to reduce the number of digits required to display a quantity. The prefixes and multiples are:

Be very particular about units and prefixes. For example:

·         kN means kilo-Newton, 1000 Newtons;

·         Kn is the symbol for knots – an imperial measure of speed;

·         KN has no meaning;

·         kn means kilo-Nano – essentially meaningless.