| Pound-force | |
|---|---|
| Unit system | English Engineering units,British Gravitational System |
| Symbol | lbf |
| Conversions | |
| 1 lbfin ... | ... is equal to ... |
| SI units | 4.448222 N |
| CGS units | 444,822.2 dyn |
| Absolute English System | 32.17405 pdl |
Thepound of force orpound-force (symbol:lbf,[1] sometimeslbf,[2]) is aunit offorce used in somesystems of measurement, includingEnglish Engineering units[a] and thefoot–pound–second system.[3]
Pound-force should not be confused withpound-mass (lb), often simply called "pound", which is a unit ofmass; nor should these be confused withfoot-pound (ft⋅lbf), a unit ofenergy, orpound-foot (lbf⋅ft), a unit oftorque.
The pound-force is equal to the gravitational force exerted on amass of oneavoirdupois pound on the surface ofEarth. Since the 18th century, the unit has been used in low-precision measurements, for which small changes inEarth'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 oneavoirdupois pound (exactly 0.45359237 kg) and thestandard 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 theinternational avoirdupois pound (lb) result in a pound-force equal to32.174049 ft⋅lb/s2 (4.4482216152605 N).[b]
This definition can be rephrased in terms of theslug. A slug has a mass of 32.174049 lb. A pound-force is the amount of force required to accelerate a slug at a rate of1 ft/s2, so:
| newton | dyne | kilogram-force, kilopond | pound-force | poundal | |
|---|---|---|---|---|---|
| 1 N | ≡ 1 kg⋅m/s2 | = 105 dyn | ≈ 0.10197 kp | ≈ 0.22481 lbF | ≈ 7.2330 pdl |
| 1 dyn | = 10−5 N | ≡ 1 g⋅cm/s2 | ≈ 1.0197×10−6 kp | ≈ 2.2481×10−6 lbF | ≈ 7.2330×10−5 pdl |
| 1 kp | = 9.80665 N | = 980665 dyn | ≡ gn × 1 kg | ≈ 2.2046 lbF | ≈ 70.932 pdl |
| 1 lbF | ≈ 4.448222 N | ≈ 444822 dyn | ≈ 0.45359 kp | ≡ gn × 1 lb | ≈ 32.174 pdl |
| 1 pdl | ≈ 0.138255 N | ≈ 13825 dyn | ≈ 0.014098 kp | ≈ 0.031081 lbF | ≡ 1 lb⋅ft/s2 |
| The value ofgn (9.80665 m/s2) as used in the official definition of the kilogram-force is used here for all gravitational units. | |||||
In some contexts, the term "pound" is used almost exclusively to refer to the unit of force and not the unit of mass. In those applications, the preferred unit of mass is the slug, i.e. lbf⋅s2/ft. In other contexts, the unit "pound" refers to aunit of mass. The international standard symbol for the pound as a unit of mass is lb.[8]
| Base | Force | Weight | Mass | |||||
|---|---|---|---|---|---|---|---|---|
| 2nd law of motion | m =F/a | F =W ⋅a/g | F =m ⋅a | |||||
| System | BG | GM | EE | M | AE | CGS | MTS | SI |
| Acceleration (a) | ft/s2 | m/s2 | ft/s2 | m/s2 | ft/s2 | Gal | m/s2 | m/s2 |
| Mass (m) | slug | hyl | pound-mass | kilogram | pound | gram | tonne | kilogram |
| Force (F), weight (W) | pound | kilopond | pound-force | kilopond | poundal | dyne | sthène | newton |
| Pressure (p) | pound per square inch | technical atmosphere | pound-force per square inch | standard atmosphere | poundal per square foot | barye | pieze | pascal |
In the "engineering" systems (middle column), theweight of the mass unit (pound-mass) on Earth's surface is approximately equal to the force unit (pound-force). 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 ofNewton's second law,F =m ⋅a, requires another factor,gc, usually taken to be 32.174049 (lb⋅ft)/(lbf⋅s2)."Absolute" systems arecoherent systems of units: by using the slug as the unit of mass, the "gravitational" FPS system (left column) avoids the need for such a constant. TheSI is an "absolute" metric system with kilogram and meter as base units.
The termpound 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 theSpace Shuttle's twoSolid 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 is980.665 cm/s2, value already stated in the laws of some countries.
This value was the conventional reference for calculating thekilogram-force, a unit of force whose use has been deprecated since the introduction of SI.
Thrust: combined thrust 29.36 MN SL (maximum thrust at launch reducing by 1/3 after 50 s)