Poundal


The poundal is a 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.
The poundal is defined as the force necessary to accelerate 1 pound-mass at 1 foot per second per second.
1 pdl = 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 accelerates a pound of mass at 32.174 049 ft/s2, 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 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:
The poundal-as-force, pound-as-mass system is contrasted with an alternative system in which pounds are used as 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 given the application of one pound force; this gives us a unit of mass called the slug, which is about 32 pounds mass. Using this system, the above expression could be expressed as:
Note: Slugs and poundals 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 acceleration due to Earth's gravity. 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 :
Expressions derived using poundals for force and lb 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, lb⋅ft/s2 or lbf = slug⋅ft/s2, avoids the constant tied to acceleration of gravity on earth.

Conversion