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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 QuantityDimensionsCommon 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 QuantityDimensionsCommon Units
Acceleration L / T2 foot per second squared
Area L2 square foot, square yard
acre, square mile
Density M / L3 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 / L2 pound-force per square inch
Velocity L / T foot per second
mile per hour, knot
Volume L3 pint, quart, gallon
cubic foot, cubic yard
Work L F foot pound-force
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