When we first look at numerical problem in Physics then we need to be able to recognise what quantities we are given in the question. This can be made a lot easier if we know what quantity corresponds to the units given in the question. For example, if a question says someone’s speed changes at a rate of 5 m s^-2, you need to be able to recognise that m s^-2 is the unit of acceleration and so we know that we have been given an acceleration (even though the word "acceleration" wasn’t used in the question).

You also need to watch out for implied values - you might be told for instance, that a car starts from rest. In other words u = 0 m s^-1.

We can classify physical quantities as either

1. Basic: These are fundamental which are defined as being independent
2. There are seven basic quantities defined by the Systeme International d’Unites (SI Units). They have been defined for convenience not through necessity (charge could have been chosen instead of current). Once defined we can make measurements using the correct unit and measure with direct comparison to that unit.

1. Derived: These are obtained by multiplication or division of the basic units without numerical factors. For example:

Some derived SI units are complicated and are given a simpler name with a unit defined in terms of the base units.

farad (F) is given as m^-2 kg^-1 s⁴ A²

watt (W) is given as m² kg s^-3

A table of quantities with their units is shown on the next page along with the most commonly used symbols for both the quantities and units.

Note that in GCSE we wrote units like metres per second in the format of m/s but in A-level it is written as m s^-1, and this is the standard way units are written at university.