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Please see foot pounds.

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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