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| {{wiktionary|pound-force|pound}}
| | Good to meet up you, I'm Solomon Cosner. Circulating production is my day job today but I anticipate changing it. For years I Have been surviving in Tennessee. My partner doesn't like it the way I do but what I actually like doing is always to barrier and I'm attempting to make it a career.<br><br>Here is my website [http://www.linkedin.com/pub/jordan-kurland/a/618/581 Jordan Kurland] |
| {{redirect|Klbf|the airport in North Platte, Nebraska, assigned the ICAO code KLBF|North Platte Regional Airport}}
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| {{Two other uses| |the unit of mass|Pound (mass)|the basis weight of paper|Paper density}}
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| The '''pound''' or '''pound force''' (symbol: '''lb, lb<sub>f</sub>, lbf''') is a [[Units of measurement|unit]] of [[force]] in some [[systems of measurement]] including English engineering units and British gravitational units.<ref>[http://www.engineeringtoolbox.com/mass-weight-d_589.html The Engineering ToolBox]</ref>
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| == Definitions ==
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| The pound-force is equal to the gravitational force exerted on a [[mass]] of one [[Pound_(mass)#Avoirdupois_pound|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 [[gravity (Earth)|Earth's gravity]] (which varies from place to place by up to half a percent) can safely be neglected.<ref>Acceleration due to gravity varies over the surface of the Earth, generally increasing from about 9.78 m/s<sup>2</sup> (32.1 ft/s<sup>2</sup>) at the equator to about 9.83 m/s<sup>2</sup> (32.3 ft/s<sup>2</sup>) at the poles.</ref>
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| The 20th century, however, brought the need for a more precise definition. A standardized value for acceleration due to gravity was therefore needed. Today, in accordance with the [[General Conference on Weights and Measures]], [[standard gravity]] is usually taken to be 9.80665 m/s<sup>2</sup> (32.174 049 ft/s<sup>2</sup>).<ref> In 1901 the [http://www1.bipm.org/en/CGPM/db/3/2/ third CGPM] declared (second resolution) that:
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| <blockquote>The value adopted in the International Service of Weights and Measures for the standard acceleration due to Earth's gravity is 980.665 cm/s<sup>2</sup>, value already stated in the laws of some countries.</blockquote>
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| 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.</ref><ref>Barry N. Taylor, ''[http://physics.nist.gov/Pubs/SP811/sp811.html/appenB8.html#P Guide for the Use of the International System of Units (SI)]'', 1995, [[NIST]] Special Publication 811, Appendix B note 24</ref>
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| The acceleration of the [[Standard gravity|standard gravitational field]] (''g''<sub>n</sub>) and the [[Pound (mass)#International pound|international avoirdupois pound]] (lbm) define the pound-force as:<ref>The international avoirdupois pound is defined to be exactly 0.45359237 kg.</ref>
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| :<math>\begin{align}
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| 1\,\mathrm{lbf} &= 1\,\mathrm{lbm} \times g_{\rm n} \\
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| &= 1\,\mathrm{lbm} \times 32.174049\,\mathrm{\tfrac{ft}{s^2}}\\
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| &= 32.174049\,\mathrm{\tfrac{ft {\cdot} lbm}{s^2}}\end{align}</math>
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| :<math>\begin{align}
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| or\end{align}</math>
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| :<math>\begin{align}
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| 1\,\mathrm{lbf} &= 0.45359237\,\mathrm{kg} \times 9.80665\,\mathrm{\tfrac{m}{s^2}}\\
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| &= 4.4482216152605\,\mathrm{N} \text{ (exact)}\end{align}</math>
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| This definition can be rephrased in terms of the [[slug (mass)|slug]]. A slug has a mass of 32.174049 lbm. A pound-force is the amount of force required to accelerate a slug at a rate of 1 ft/s<sup>2</sup>, so:
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| :<math> 1\,\mathrm{lbf} = 1\,\mathrm{slug} \times 1\,\mathrm{\tfrac{ft}{s^2}} </math>
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| :::<math> = 1\,\mathrm{slug}\,\mathrm{\tfrac{ft}{s^2}} </math>
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| ==Conversion to other units==
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| {{Units of force}}
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| == Foot-pound-second (FPS) systems of units ==
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| 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 (mass)|slug]], i.e. lb<sub>''f''</sub>·s<sup>2</sup>/ft. In other contexts, the unit "pound" refers to a [[Pound (mass)|unit of mass]]. In circumstances where there may otherwise be ambiguity, the symbols "lb<sub>''f''</sub>" and "lb<sub>''m''</sub>" and the terms "pounds-force" and "pounds-mass" can be used to distinguish.
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| {{GravEngAbs}}
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| In the gravitational systems, the [[weight]] 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<ref>The acceleration unit is the distance unit divided by the time unit squared.</ref>—the use of [[Newton's laws of motion|Newton's Second Law]], ''F = m·a'', requires another factor, ''g<sub>c</sub>'', usually taken to be 32.174049 lb<sub>m</sub>·ft/lb<sub>f</sub>·s<sup>2</sup> = 32.174049 lb<sub>m</sub>/slug.
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| "Absolute" systems are ''coherent'' systems of units: by using the slug as the unit of mass, the "engineering" FPS system avoids the need for such a constant. The [[SI]] is an "absolute" metric system with kilogram and meter as base units...
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| == See also ==
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| {{div col|2}}
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| * [[Foot-pound (energy)]]
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| * [[Ton-force]]
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| * [[Kip (unit)]]
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| * [[Mass in general relativity ]]
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| * [[Mass in special relativity ]]
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| * [[Mass versus weight]] for the difference between the two physical properties
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| * [[Newton (unit)|Newton]]
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| * [[Poundal]]
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| * [[Pounds per square inch]], a unit of [[pressure]]
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| {{div col end}}
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| == Notes and references ==
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| * Obert, Edward F., “THERMODYNAMICS”, D.J. Leggett Book Company Inc., New York 1948; Chapter I, Survey of Dimensions and Units, pages 1-24.
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| {{reflist}}
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| {{reflist|group=ref}}
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| [[Category:Units of force]]
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| [[Category:Imperial units]]
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| [[Category:Customary units of measurement in the United States]]
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Good to meet up you, I'm Solomon Cosner. Circulating production is my day job today but I anticipate changing it. For years I Have been surviving in Tennessee. My partner doesn't like it the way I do but what I actually like doing is always to barrier and I'm attempting to make it a career.
Here is my website Jordan Kurland