SI Units
For any measurement of a quantity such as distance, time, mass, force, etc. it is important to specify the units of the measurement. A distance of 3 is meaningless unless we indicate whether we mean 3 metres, 3 kilometres or 3 Megaparsecs.
The most widely used set of units is known variously as the Systéme Internationale, International System of Units or SI Units. These units are a modernised version of the metric system, and make use of 7 base units:
| Measurement | Name | Symbol |
|---|---|---|
| Length | Metre | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Current | Ampere | A |
| Thermodynamic Temperature | Kelvin | K |
| Quantity of substance | Mole | mol |
| Luminous Intensity | Candela | cd |
The SI units are complemented by two angular units:
| Measurement | Name | Symbol |
|---|---|---|
| Plane angle | radian | rad |
| Solid angle | steradian | sr |
It should be noted that two of the SI units are somewhat redundant. The mole is a measure of the number of atoms or molecules, so it is not really a unit. Candelas can be specified in terms of m, kg and s (1 Cd = 1/683 W sr-1= 1/683 kg m2 s-3 sr-1 at 5.40 × 1014 Hertz).
Commonly derived units include:
| Measurement | Name | Symbol | Derived Unit |
|---|---|---|---|
| Force | Newton | N | m kg s-2 |
| Pressure | Pascal | Pa | N m-2 |
| Energy | Joule | J | N m |
| Power | Watt | W | J s-1 |
| Electric charge | Coloumb | C | A s |
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