The pound of force or pound-force (symbol: lbf,[1] sometimes lbf,[2]) is a unit of force used in some systems of measurement, including English Engineering units[a] and the foot–pound–second system.
[3] Pound-force should not be confused with pound-mass (lb), often simply called "pound", which is a unit of mass; nor should these be confused with foot-pound (ft⋅lbf), a unit of energy, or pound-foot (lbf⋅ft), a unit of torque.
The pound-force is equal to the gravitational force exerted on a mass of one avoirdupois pound on the surface of Earth.
Since the 18th century, the unit has been used in low-precision measurements, for which small changes in Earth's gravity (which varies from equator to pole by up to half a percent) can safely be neglected.
[4] The 20th century, however, brought the need for a more precise definition, requiring a standardized value for acceleration due to gravity.
The pound-force is the product of one avoirdupois pound (exactly 0.45359237 kg) and the standard acceleration due to gravity, approximately 32.174049 ft/s2 (9.80665 m/s2).
[5][6][7] The standard values of acceleration of the standard gravitational field (gn) and the international avoirdupois pound (lb) result in a pound-force equal to 32.174049 ft⋅lb/s2 (4.4482216152605 N).
A pound-force is the amount of force required to accelerate a slug at a rate of 1 ft/s2, so:
In those applications, the preferred unit of mass is the slug, i.e. lbf⋅s2/ft.
The international standard symbol for the pound as a unit of mass is lb.
[8] In the "engineering" systems (middle column), the weight of the mass unit (pound-mass) on Earth's surface is approximately equal to the force unit (pound-force).
The SI is an "absolute" metric system with kilogram and meter as base units.
The term pound of thrust is an alternative name for pound-force in specific contexts.
It is frequently seen in US sources on jet engines and rocketry, some of which continue to use the FPS notation.
For example, the thrust produced by each of the Space Shuttle's two Solid Rocket Boosters was 3,300,000 pounds-force (14.7 MN), together 6,600,000 pounds-force (29.4 MN).
[12][13] The value adopted in the International Service of Weights and Measures for the standard acceleration due to Earth's gravity is 980.665 cm/s2, value already stated in the laws of some countries.This value was the conventional reference for calculating the kilogram-force, a unit of force whose use has been deprecated since the introduction of SI.