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A pound-foot (lb⋅ft), abbreviated from pound-force foot (lbf · ft), is a unit of torque representing one pound of force acting at a perpendicular distance of one foot from a pivot point. [2] Conversely one foot pound-force (ft · lbf) is the moment about an axis that applies one pound-force at a radius of one foot.
Both energy and torque can be expressed as a product of a force vector with a displacement vector (hence pounds and feet); energy is the scalar product of the two, and torque is the vector product. Although calling the torque unit "pound-foot" has been academically suggested, both are still commonly called "foot-pound" in colloquial usage.
In the US, torque is most commonly referred to as the foot-pound (denoted as either lb-ft or ft-lb) and the inch-pound (denoted as in-lb). [ 17 ] [ 18 ] Practitioners depend on context and the hyphen in the abbreviation to know that these refer to torque and not to energy or moment of mass (as the symbolism ft-lb would properly imply).
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. [13] [14] 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.
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).
Torque 1 pound-foot ≈ 1.356 N⋅m; Insulation 1 R-value (ft 2 ⋅°F⋅h/Btu) ≈ 0.1761 R SI (K⋅m 2 /W) Various combination units are in common use; these are straightforwardly defined based on the above basic units. Sizing systems are used for various items in commerce, several of which are U.S.-specific: US standard clothing size
Torque; system unit code symbol or abbrev. notes conversion factor/N⋅m combinations Industrial: SI: Newton-metre: Nm N⋅m 1 Nm lbft; Nm lbfft; Non-SI metric: kilogram-metre: kgm kg·m 9.80665 Imperial & US customary: pound-foot: lbft lb⋅ft Pound-inch (lb.in) is also available 1.3558 Scientific: SI: newton metre: Nm N⋅m 1 Nm lbft; Nm ...
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