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 m^{2} s^{-3} sr^{-1} at 5.40 × 10^{14} 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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