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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]
This is convenient because one pound mass exerts one pound force due to gravity. Note, however, unlike the other systems the force unit is not equal to the mass unit multiplied by the acceleration unit [11] —the use of Newton's second law, F = m ⋅ a, requires another factor, g c, usually taken to be 32.174049 (lb⋅ft)/(lbf⋅s 2).
The avoirdupois pound is legally defined as a measure of mass, [25] but the name pound is also applied to measures of force. For instance, in many contexts, the pound avoirdupois is used as a unit of mass, but in some contexts, the term "pound" is used to refer to "pound-force". The slug is another unit of mass derived from pound-force.
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 both systems, 20 hundredweights make a ton ...
Systems of measure either define mass and derive a force unit or define a base force and derive a mass unit [1] (cf. poundal, a derived unit of force in a mass-based system). A slug is defined as a mass that is accelerated by 1 ft/s 2 when a net force of one pound (lbf) is exerted on it. [2]
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).
In 1959, by international agreement, the definitions of the pound and ounce became standardized in countries which use the pound as a unit of mass. [3] The International Avoirdupois Pound was then created. It is the everyday system of weights used in the United States.
The troy pound (373.241 7216 g) was made the primary unit of mass by the Weights and Measures Act 1824 and its use was abolished in the UK on 1 January 1879, [30] with only the troy ounce (31.103 4768 g) and its decimal subdivisions retained. [31] The Weights and Measures Act 1855 made the avoirdupois pound the primary unit of mass. [32]