Editing Pound (force) (section)

== Definitions ==
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 of Earth|Earth's gravity]] (which varies from equator to pole 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 32.1&nbsp;ft/s<sup>2</sup> (9.78&nbsp;m/s<sup>2</sup>) at the equator to about 32.3&nbsp;ft/s<sup>2</sup> (9.83&nbsp;m/s<sup>2</sup>) at the poles.</ref>

The 20th century, however, brought the need for a more precise definition, requiring a standardized value for acceleration due to gravity.  

=== Product of avoirdupois pound and standard gravity ===
The pound-force is the product of one [[Avoirdupois system|avoirdupois]] pound ([[International yard and pound|exactly]] {{cvt|1|lb|kg|sigfig=8|disp=out}}) and the [[standard gravity|standard acceleration due to gravity]], approximately {{cvt|32.174049|ft/s2|m/s2|sigfig=6}}.<ref name="BS350">{{cite book|title=BS 350 : Part 1: 1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors|date=1974|publisher=British Standards Institution|page=43}}</ref><ref>In 1901 the [http://www1.bipm.org/en/CGPM/db/3/2/ third CGPM] {{Webarchive|url=https://web.archive.org/web/20120207064142/http://www1.bipm.org/en/CGPM/db/3/2/ |date=2012-02-07 }} declared (second resolution) that:
<blockquote>The value adopted in the International Service of Weights and Measures for the standard acceleration due to Earth's gravity is {{val|980.665|u=cm/s2}}, value already stated in the laws of some countries.</blockquote>
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://arquivo.pt/wayback/20160515144655/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>

The standard values of acceleration of the standard gravitational field (''g''<sub>n</sub>) and the [[Pound (mass)#International pound|international avoirdupois pound]] (lb) result in a pound-force equal to 
{{val|32.174049|u={{sfrac|ft⋅lb|s<sup>2</sup>}}}} ({{cvt|1|lbf|N|sigfig=14|disp=out}}).{{efn|The [[International yard and pound|international avoirdupois pound is defined]] to be exactly {{val|0.45359237|u=kg}}.}}

<math display="block">\begin{align}
  1\,\text{lbf} &= 1\,\text{lb} \times g_\text{n} \\
                &= 1\,\text{lb} \times 9.80665\,\tfrac{\text{m}}{\text{s}^2} / 0.3048\,\tfrac{\text{m}}{\text{ft}}\\
                &\approx 1\,\text{lb} \times 32.174049\,\mathrm{\tfrac{ft}{s^2}}\\
                &\approx 32.174049\,\mathrm{\tfrac{ft {\cdot} lb}{s^2}} \\
  1\,\text{lbf} &= 1\,\text{lb} \times 0.45359237\,\tfrac{\text{kg}}{\text{lb}} \times g_\text{n} \\
                &= 0.45359237\,\text{kg} \times 9.80665\,\tfrac{\text{m}}{\text{s}^2}\\
                &= 4.4482216152605\,\text{N}
\end{align}</math>

This definition can be rephrased in terms of the [[Slug (unit)|slug]]. A slug has a mass of 32.174049&nbsp;lb.  A pound-force is the amount of force required to accelerate a slug at a rate of {{val|1|u=ft/s2}}, so:

<math display="block">\begin{align}
  1\,\text{lbf} &= 1\,\text{slug} \times 1\,\tfrac{\text{ft}}{\text{s}^2} \\
                &= 1\,\tfrac{\text{slug} \cdot \text{ft}}{\text{s}^2}
\end{align}</math>