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The pound or pound-force (symbol: lb, lbf, or lbf) is a unit of force used in some systems of measurement including English Engineering units and the British Gravitational System.〔(The Engineering ToolBox )〕 Pounds of force are not to be confused with foot-pounds or pounds-feet, which are a unit of torque, and may be written as "lb·ft" or "lbf·ft". == Definitions == The pound-force is equal to the gravitational force exerted on a mass of one avoirdupois pound on the surface of Earth. Since the 18th century, the unit has been used in low-precision measurements, for which small changes in Earth's gravity (which varies from place to place by up to half a percent) can safely be neglected.〔Acceleration due to gravity varies over the surface of the Earth, generally increasing from about 9.78 m/s2 (32.1 ft/s2) at the equator to about 9.83 m/s2 (32.3 ft/s2) at the poles.〕 The 20th century, however, brought the need for a more precise definition. A standardized value for acceleration due to gravity was therefore needed. Today, in accordance with the General Conference on Weights and Measures, standard gravity is usually taken to be 9.80665 m/s2 (about 32.174 049 ft/s2).〔 In 1901 the (third CGPM ) declared (second resolution) that: The value adopted in the International Service of Weights and Measures for the standard acceleration due to Earth's gravity is 980.665 cm/s2, value already stated in the laws of some countries. This value was the conventional reference for calculating the kilogram-force, a unit of force whose use has been deprecated since the introduction of SI.〕〔Barry N. Taylor, ''(Guide for the Use of the International System of Units (SI) )'', 1995, NIST Special Publication 811, Appendix B note 24〕 The acceleration of the standard gravitational field (''g''n) and the international avoirdupois pound (lbm) define the pound-force as:〔The international avoirdupois pound is defined to be exactly 0.45359237 kg.〕 : This definition can be rephrased in terms of the slug. A slug has a mass of 32.174049 lbm. A pound-force is the amount of force required to accelerate a slug at a rate of 1 ft/s2, so: : 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pound (force)」の詳細全文を読む スポンサード リンク
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