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The electrostatic and electromagnetic systems are derived from units of length and force, mainly. As such, these are ready extensions of any system of containing length, mass, time. Stephen Dresner [7] gives the derived electrostatic and electromagnetic units in both the foot–pound–second and foot–slug–second systems. In practice, these ...
One slug is a mass equal to 32.17405 lb (14.59390 kg) based on standard gravity, the international foot, and the avoirdupois pound. [3] In other words, at the Earth's surface (in standard gravity), an object with a mass of 1 slug weighs approximately 32.17405 lbf or 143.1173 N. [ 4 ] [ 5 ]
Since a pound of force (pound force) accelerates a pound of mass at 32.174 049 ft/s 2 (9.80665 m/s 2; the acceleration of gravity, g), we can scale down the unit of force to compensate, giving us one that accelerates 1 pound mass at 1 ft/s 2 rather than at 32.174 049 ft/s 2; and that is the poundal, which is approximately 1 ⁄ 32 pound force.
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 ]
Mass 1 slug = 1 lbf⋅s 2 /ft ≈ 14.59390 kg; Force 1 poundal = force to accelerate 1 pound mass 1 foot/second/second ≈ 0.138 newtons. 1 kip = 1000 lbf ≈ 4.44822 kN; Energy 1 foot-pound ≈ 1.356 J; 1 British thermal unit (Btu) ≈ 1.055 kJ (1,054–1,060 J, depending on which of several definitions of BTU is used) 1 Quad 10 15 BTU, one ...
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).
Avoirdupois is a system of mass based on a pound of 16 ounces, while Troy weight is the system of mass where 12 troy ounces equals one troy pound. The symbol g 0 is used to denote standard gravity in order to avoid confusion with the (upright) g symbol for gram.
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).