Pound

:This article is about the unit of weight. For other uses, see Pound (disambiguation).

Systems of units

There are three practical ways of doing calculations with mass and force in the fps systems (and other systems such as inch-pound-second systems not discussed here), which the following table summarizes and compares with the SI.

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The absolute and gravitational fps systems are coherent systems of units which both share with the SI the advantage of avoiding needless complication in several of the formulas used, whereas the engineering fps system requires the introduction of the factor gc, which is a dimensionless constant, usually 32.17405 lb·ft/(lbf·s²), which is numerically equal to the value of the standard acceleration of gravity on Earth used to define a pound-force, when expressed in ft/s². This must be distinguished from the actual local value of g.

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No one of the three fps systems is more correct than the other two. None of our ordinary measurements are made in the context of any of these specialized subsets of mechanical units, used only in calculations.

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Although the U.S. National Bureau of Standards http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf has defined the pound as a unit of mass, and the pound-force as a unit of force, this distinction is not widely recognized among working physicists, because the fps system has not been used in physics, even in the United States, since the early 20th century. When giving data to be used in calculations, it is not a good idea to use the term pound without clarifying whether mass or force is intended. If force is what is meant, the symbol "lbf" or the term "pounds-force" can be used for clarification. For mass, one can specify "pounds-mass."

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The reader is cautioned that in some contexts "lb" may be defined as force or mass by convention. For example, in structural engineering applications, "lb" is almost exclusively used as a unit of force, and the slug or kip·s²/ft{{fn|3}} is preferred to the pound-mass (lbm). Slugs are used since the factor gc is not required for proper application of Newton's Second Law.

Related Topics:
Slug - Kip - Newton's Second Law

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Notes