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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 ).
Conventional mass is defined as follows: "For a mass at 20 °C, 'conventional mass' is the mass of a reference standard of density 8,000 kg/m 3 which it balances in air with a density of 1.2 kg/m 3." The effect is a small one, 150 ppm for stainless steel mass standards, but the appropriate corrections are made during the manufacture of all ...
When a distinction is necessary, the term pound-force may refer to a unit of force rather than mass. 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]
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).
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]
the slug (sl), an Imperial unit of mass (about 14.6 kg) the pound (lb), a unit of mass (about 0.45 kg), which is used alongside the similarly named pound (force) (about 4.5 N), a unit of force [note 3] the Planck mass (about 2.18 × 10 −8 kg), a quantity derived from fundamental constants
The pound per square inch (abbreviation: psi) or, more accurately, pound-force per square inch (symbol: lbf/in 2), [1] is a unit of measurement of pressure or of stress based on avoirdupois units and used primarily in the United States. It is the pressure resulting from a force with magnitude of one pound-force applied to an area of one square ...
Together with the fact that the term "weight" is used for the gravitational force in some technical contexts (physics, engineering) and for mass in others (commerce, law), [10] and that the distinction often does not matter in practice, the coexistence of variants of the FPS system causes confusion over the nature of the unit "pound".