The absolute foot-pound-second system is a system of mechanical units. The foot is the base unit of length or distance, the pound is chosen to be a unit of mass, and the second is the base unit of time. All other mechanical quantities, and specifically force, are expressed in terms of combinations of these three units. In absolute systems, Newton's second law is expressed as F = ma, and since acceleration is the second derivative of a distance with respect to time, if the unit of length or distance is denoted by L, the unit of mass by M, and the unit of time by T, the unit of force becomes a derived unit of dimensions MLT−2, in this case pound·foot/second2, which is known as the poundal.

Three approaches to mass and force units[1][2]

v · d · e


force, length, time weight, length, time mass, length, time
Force (F) F = m·a = w·a/g F = m·a/gc = w·a/g F = m·a = w·a/g
Weight (w) w = m·g w = m·g/gc ≈ m w = m·g
Acceleration (a) ft/s2 m/s2 ft/s2 m/s2 ft/s2 gal m/s2 m/s2
Mass (m) slug hyl, also called “metric slug” or “TME” lbm kg lb g t kg
Force (F) lb kp lbF kp pdl dyn sn N
Pressure (p) lb/in2 at PSI atm pdl/ft2 Ba pz Pa

This system is not much used; the gravitational foot-pound-second system is far more common in those physics books that use US customary/British Imperial units at all in their treatments of mechanics, and engineers in the United States and Britain, insofatr as they have not gone to metric units, favor the unit force-mass foot-pound-second system.

See also


  1. Lindeburg, Michael, Civil Engineering Reference Manual for the PE Exam 
  2. Wurbs, Ralph A, Fort Hood Review Sessions for Professional Engineering Exam,, retrieved October 26, 2011 
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