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The foot-pound force (symbol: ft⋅lbf, [1] ft⋅lb f, [2] or ft⋅lb [3]) is a unit of work or energy in the engineering and gravitational systems in United States customary and imperial units of measure. It is the energy transferred upon applying a force of one pound-force (lbf) through a linear displacement of one foot. The corresponding SI ...
The British imperial units and U.S. customary units for both energy and work include the foot-pound force (1.3558 J), the British thermal unit (BTU) which has various values in the region of 1055 J, the horsepower-hour (2.6845 MJ), and the gasoline gallon equivalent (about 120 MJ).
The foot–pound–second system (FPS system) is a system of units built on three fundamental units: the foot for length, the (avoirdupois) pound for either mass or force (see below), and the second for time.
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 ).
As the unit name suggests, it is the product of pounds for the unit of force and feet for the unit of displacement. One joule is approximately equal to 0.7376 ft-lbs. [16] [17] Non-SI units of work include the newton-metre, erg, the foot-pound, the foot-poundal, the kilowatt hour, the litre-atmosphere, and the horsepower-hour.
Other units of mass are also in use. Historical units include the stone, the pound, the carat, and the grain. For subatomic particles, physicists use the mass equivalent to the energy represented by an electronvolt (eV). At the atomic level, chemists use the mass of one-twelfth of a carbon-12 atom (the dalton).
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 United States customary units, the pound can be either a unit of force or a unit of mass. [24] Related units used in some distinct, separate subsystems of units include the poundal and the slug. The poundal is defined as the force necessary to accelerate an object of one-pound mass at 1 ft/s 2, and is equivalent to about 1/32.2 of a pound-force.