Terminology of Units
| Name | Examples | Explanation |
| Derived unit |
metres per second |
A derived unit measures a quantity made from a combination of other quantities. For example, an object has a speed of 1 km per hour (derived unit) if it travels 1 kilometre in one hour. Many everyday and scientific quantities require derived units, eg velocity and speed (distance per unit time); acceleration (velocity change per unit time); force (mass unit times acceleration unit); interest rate (money per year). |
| SI unit or metric unit |
metre |
Any unit in the "metric system" . These are all base-ten compatible (except where they involve time). SI stands for 'international system' in French (Système Internationale d'Unités) |
| Standard unit | metre (m) kilogram (kg) second (s) |
There are 7 units which are used to make the agreed SI units for all known physical quantities. For example, the newton, the unit for force is defined in terms of three of the standard units: kilograms, metres and seconds. There are only three standard units which are common in school mathematics - kg, m, s. Other standard units are for electric current (ampere), temperature (degrees kelvin), amount of substance (mole)and light intensity (candela). |
| Formal unit |
hour |
Any unit with an agreed definition across society. They include metric and imperial units. |
| Informal unit |
the length of my foot |
A term used by teachers for impermanent units used to teach students the principles of measurement. |
| Imperial unit |
foot |
Any unit from the old British system (used in Australia before 1980s). |
Abbreviations
| quantity to measure | SI unit | approved abbreviation |
| length | metre | m |
| mass | kilogram | kg |
| time | second | s |
NOTE: Other abbreviations which are strictly not correct are used occasionally.
Teachers should adhere to the standard abbreviations but accept other common forms.
Common prefixes for SI units
We can combine these basic units with prefixes to form a multiple unit of more convenient size. See the table below for some of these commonly used prefixes.
|
prefix and |
value and meaning
|
example
|
| mega (M) |
1 000 000
|
a megalitre is one million litres |
| kilo (k) | 1 000 one thousand |
a kilogram is one thousand grams |
| deci (d) | 0.1 one tenth |
a decimetre is one tenth of a metre |
| centi (c) | 0.01 one hundredth |
a centimetre is one hundredth of a metre |
| milli (m) | 0.001 one thousandth |
a milligram is one thousandth of a gram |
| micro ( µ) | 0.000 0001 one millionth |
a micrometre is one millionth of a metre. µ is a Greek letter, pronounced as 'mu'. |
Non SI-Units
Some units not in the SI system have been retained because of their practical importance. In Australia, there is common usage of the following non-SI units:
| Unit | Definition | Value |
| minute | 1 min = 60 s | 60 s |
| hour | 1 hour = 3600 s | 3600 s |
| temperature | 1 degree Celsius = 1 kelvin | 1° C |
Strictly speaking, 'tonne, 'litre' and 'hectare' are metric but not SI units. (The alternative SI terms, which are not commonly used, are, Mkg, 1 dm3 and 10000 m2, respectively.)