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The pound or pound-mass is a unit of mass used in both the British imperial and United States customary systems of measurement.Various definitions have been used; the most common today is the international avoirdupois pound, which is legally defined as exactly 0.453 592 37 kilograms, and which is divided into 16 avoirdupois ounces. [1]
Units for other physical quantities are derived from this set as needed. In English Engineering Units, the pound-mass and the pound-force are distinct base units, and Newton's Second Law of Motion takes the form = where is the acceleration in ft/s 2 and g c = 32.174 lb·ft/(lbf·s 2).
The pound avoirdupois, which forms the basis of the U.S. customary system of mass, is defined as exactly 453.59237 grams by agreement between the U.S., the United Kingdom, and other English-speaking countries in 1959. Other units of mass are defined in terms of it.
The table of imperial avoirdupois mass is the same as the United States table up to one pound, but above that point, the tables differ. The imperial system has a hundredweight, defined as eight stone of 14 lb each, or 112 lb ( 50.802 345 44 kg ), whereas a US hundredweight is 100 lb ( 45.359 237 kg ).
In some contexts, the term "pound" is used almost exclusively to refer to the unit of force and not the unit of mass. In those applications, the preferred unit of mass is the slug, i.e. lbf⋅s 2 /ft. In other contexts, the unit "pound" refers to a unit of mass. The international standard symbol for the pound as a unit of mass is lb. [8]
Force exerted on a mass of one pound due to gravity 4.448 N Mass slug: BGS mass which, when subjected to a force of one pound-force, accelerates by 1 ft/sec 2: 14.59 kg Mass pound-mass: lbm EEU mass which, when subjected to a force of one pound-force, accelerates by g ft/sec 2 (32.17 ft/sec 2) 0.4536 kg Power horsepower: hp EEU
The unit of substance in the FPS system is the pound-mole (lb-mol) = 273.16 × 10 24. Until the SI decided to adopt the gram-mole, the mole was directly derived from the mass unit as (mass unit)/(atomic mass unit). The unit (lbf⋅s 2 /ft)-mol also appears in a former definition of the atmosphere.
The avoirdupois weight system is thought to have come into use in England around 1300. [citation needed] It was originally used for weighing wool. In the early 14th century several other specialized weight systems were used, including the weight system of the Hanseatic League with a 16-ounce pound of 7200 grains and an 8-ounce mark.