Force-amplifying effectiveness of a simple machine (lever, wedge, wheel and axle, pulley, or screw). The theoretical mechanical advantage of a system is the ratio of the force that performs the useful work to the force applied, assuming there is no friction in the system. In practice, the actual mechanical advantage will be less than the theoretical value by an amount determined by the amount of friction.

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Economic theory first advanced by Robert Torrens and David Ricardo that analyzes international trade in terms of differences in relative opportunity costs. The theory suggests that countries should specialize in the goods they can produce most efficiently rather than trying for self-sufficiency and argues strongly in favour of free international trade.

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