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2014-08-12T17:53:44Z

AngelEidrwvalia:

The '''poundal''' (symbol: '''pdl''') is a [[Units of measurement|unit]] of [[force]] that is part of the [[foot–pound–second system]] of units, in [[Imperial units]] introduced in 1877, and is from the specialized subsystem of English absolute (a coherent system).

:<math>1\,\text{pdl} = 1\,\tfrac{\text{lb}_m \cdot \text{ft}}{\text{s}^2} </math>
The poundal is defined as the force necessary to accelerate 1 [[pound-mass]] to 1 foot per second per second.
1 pdl = {{val|0.138254954376|ul=N}} exactly.

== Background ==
English units require re-scaling of either force or mass to eliminate a numerical proportionality constant in the equation ''F = ma''. The poundal represents one choice, which is to rescale units of force. Since a pound of ''force'' ([[pound force]]) accelerates a pound of ''mass'' ([[Pound (mass)|pound mass]]) at 32.174 049 ft/s2 (9.80665 m/s2; the [[Standard gravity|acceleration of gravity]], ''g''), we can scale down the unit of force to compensate, giving us one that accelerates 1 pound mass at 1 ft/s2 rather than at 32.174 049 ft/s2; and that is the poundal, which is approximately {{Frac|1|32}} of a pound force.

For example, a force of 1200 poundals is required to accelerate a person of 150 pounds mass at 8 feet per second squared:
:<math>150\,\text{lb}_m \cdot 8\,\tfrac{\text{ft}}{\text{s}^2} = 1200\,\text{pdl}</math>

{{GravEngAbs}}
The poundal-as-force, pound-as-mass system is contrasted with an alternative system in which pounds are used as ''force'' (pounds-force), and instead, the ''mass'' unit is rescaled by a factor of roughly 32. That is, one pound-force will accelerate one pound-mass at 32 feet per second squared; we can scale ''up'' the unit of ''mass'' to compensate, which will be accelerated by 1 ft/s2 (rather than 32 ft/s2) given the application of one pound force; this gives us a unit of mass called the [[Slug (mass)|slug]], which is about 32 pounds mass. Using this system (slugs and pounds-force), the above expression could be expressed as:
:<math>4.66\,\text{slug} \cdot 8\,\tfrac{\text{ft}}{\text{s}^2} = 37.3\,\text{lb}_F</math>

Note: Slugs (32.174 049 {{lbm}}) and poundals (1/32.174 049 {{lbf}}) are never used in the same system, since they are opposite solutions of the same problem.

Rather than changing either force or mass units, one may choose to express acceleration in units of the [[standard gravity|acceleration due to Earth's gravity]] (called ''g''). In this case, we can keep both pounds-mass and pounds-force, such that applying one pound force to one pound mass accelerates it at one unit of acceleration (''g''):
:<math>150\,\text{lb}_m \cdot 0.249\,g = 37.3\,\text{lb}_F</math>

Expressions derived using poundals for force and {{lbm}} for mass (or {{lbf}} for force and slugs for mass) have the advantage of not being tied to conditions on the surface of the earth. Specifically, computing ''F'' = ''ma'' on the moon or in deep space as poundals, {{lbm}}·ft/s2 or {{lbf}} = slug·ft/s2, avoids the constant tied to acceleration of gravity on earth.

== Conversion ==
{{Units of force}}

== See also ==
[[Slug (mass)]]

==References==
* Obert, Edward F., “Thermodynamics”, McGraw-Hill Book Company Inc., New York 1948; Chapter I, Survey of Dimensions and Units, pages 1–24.
{{reflist}}


[[Category:Units of force]]
[[Category:Imperial units]]
[[Category:Customary units of measurement in the United States]]

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