# British Imperial System

## A principal system of units

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English units forms the basic for the Imperial system formerly used in the Commonwealth countries and the Customary system used in the US.

The basic difference between dimensional systems are not the units employed but the primary dimensions.

- The SI-system use mass, length and time as primary quantities (mechanics). Other units quantities are secondary. The SI-system is a MLT (mass-length-time) system
- The English system use force, mass, length and time as primary quantities. Other units are secondary. The British Imperial System is a FMLT (force-mass-length-time) system

Note that the use of four, three, two or even one(!) primary quantities is a human choice and not necessary basic nature.

### Imperial Primary Quantities

Imperial primary quantities are expressed in the table below:

Primary Quantity | Dimensions | Common Units |

Force | F | pound-force |

Mass | M | ounce, pound, slug, ton |

Length | L | inch, foot, yard rod, chain, furlong, mile |

Time | T | second, minute hour, day |

### Imperial Secondary Quantities

Some common imperial secondary quantities are expressed in the table below

Primary Quantity | Dimensions | Common Units |

Acceleration | L / T^{2} | foot per second squared |

Area | L^{2} | square foot, square yard acre, square mile |

Density | M / L^{3} | pounds per cubic foot |

Frequency | 1/T | cycle per second |

Impulse | F T | pound-force second |

Momentum | ML / T | pound foot per second |

Power | LF / T | foot pound-force per second horsepower |

Pressure | F / L^{2} | pound-force per square inch |

Velocity | L / T | foot per second mile per hour, knot |

Volume | L^{3} | pint, quart, gallon cubic foot, cubic yard |

Work | L F | foot pound-force |

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