pound force

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The unit of force in the British gravitational system of units, approximately the force which a mass of 1 pound exerts on whatever it is resting on on the Earth; that is, the weight of a mass of 1 pound. Symbol, lbf. (Some, however, have used the symbol Lb, with the L capitalized to distinguish it from the symbol lb for the pound.¹)

The weight of a mass of 1 pound varies from place to place; it weighs less at the equator than at the poles, and is lighter at high altitudes than at sea level. The pound-force, however, is unvarying; it is defined as the weight (a force!) that a body with a mass of 1 pound would exert at a location where the acceleration due to gravity was exactly 32.1740 feet per second per second, which is about the value at sea-level at a latitude of 45°. So 1 pound-force = 32.1740 poundals. Later a standard value of 9.80665 meters per second per second was adopted for the acceleration due to gravity, and 1 pound-force = 4.448 221 615 260 5 newtons.

1. Ernst Schmidt.
International system of units. MKSA system in applied thermodynamics.
in Systems of Units. National and International Aspects.
Carl F. Kayan, editor.
Publication No. 57 of the AAAS.
Washington, D. C.: American Association for the Advancement of Science, 1959.

Page 274: “the pound-force is abbreviated Lb with capital initial letter.”

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