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The poundal is a non-SI unit of force. It is a part of the absolute foot-pound-second system of units, a coherent subsystem of English units introduced in 1879, and one of several specialized subsystems of mechanical units used as aids in calculations. It is defined as 1 lb·ft·s-2, or in words, as the force necessary to accelerate a pound of mass at 1 foot per second, per second.1 pdl = 0.138 254 954 376 newton (N) exactly. English units require re-scaling of either force or mass to eliminate a numerical proportionality constant in the equation . The poundal represents one choice, which is to rescale units of force. Since a pound of force accelerates a pound of mass at about 32 ft/s² (the acceleration of gravity = g), the smaller unit of force represented by the poundal is chosen as that force which accelerates a 1-pound mass at 1 foot per second^2. The poundal is only about 1/32 of a pound of force. For example, a force of 1200 poundals is required to accelerate a man with 150 pounds mass at 8 foot per second^2, that is . The poundal-force, pound-mass system is contrasted with an alternate system in which pounds are used as force, and instead, the mass unit is rescaled by a factor of 32. If force is given in pounds rather than poundals, then an acceleration of one foot per second^2 is induced in a mass of about 32 pounds mass, for each applied pound of force. The mass unit of about 32 mass-pounds (32.2 to three digits) in this alternate system, is called a slug. In this system, the above result would be expressed as . Note that slugs and poundals are never used in the same system, since each exists to solve the same problem, so that both should not be used together. Rather than changing either force or mass units, one may choose to express acceleration in units of the acceleration due to earth's gravity. With this system, the above result would be expressed as .
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