system by which effort or force can be multiplied.

Merriam-Webster Online Dictionary
mechanical advantage (noun)
the advantage gained by the use of a mechanism in transmitting force , specifically the ratio of the force that performs the useful work of a machine to the force that is applied to the machine

Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input 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 machine does not include a power source, is frictionless, and is 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 departures from the ideal.

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