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Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force. The model for this is the ''law of the lever.'' Machine components designed to manage forces and movement in this way are called mechanisms. An ideal mechanism transmits power without adding to or subtracting from it. This means the ideal mechanism does not include a power source, and is frictionless and constructed from rigid bodies that do not deflect or wear. The performance of a real system relative to this ideal is expressed in terms of efficiency factors that take into account friction, deformation and wear. ==The law of the lever== The lever is a movable bar that pivots on a fulcrum attached to or positioned on or across a fixed point. The lever operates by applying forces at different distances from the fulcrum, or pivot. As the lever pivots on the fulcrum, points farther from this pivot move faster than points closer to the pivot. The power into and out of the lever must be the same. Power is the product of force and velocity, so forces applied to points farther from the pivot must be less than when applied to points closer in.〔J. J. Uicker, G. R. Pennock, and J. E. Shigley, 2003, Theory of Machines and Mechanisms, Oxford University Press, New York.〕 If ''a'' and ''b'' are distances from the fulcrum to points ''A'' and ''B'' and if force ''FA'' applied to ''A'' is the input force and ''FB'' exerted at ''B'' is the output, the ratio of the velocities of points ''A'' and ''B'' is given by ''a/b'', so the ratio of the output force to the input force, or mechanical advantage, is given by : This is the ''law of the lever'', which was proven by Archimedes using geometric reasoning. It shows that if the distance ''a'' from the fulcrum to where the input force is applied (point ''A'') is greater than the distance ''b'' from fulcrum to where the output force is applied (point ''B''), then the lever amplifies the input force. If the distance from the fulcrum to the input force is less than from the fulcrum to the output force, then the lever reduces the input force. Recognizing the profound implications and practicalities of the law of the lever, Archimedes has been famously attributed with the quotation "Give me a place to stand and with a lever I will move the whole world."〔John Tzetzes Book of Histories (Chiliades) 2 p 129-130, 12th century AD, translation by Francis R. Walton〕 The use of velocity in the static analysis of a lever is an application of the principle of virtual work. 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「Mechanical advantage」の詳細全文を読む スポンサード リンク
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