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Efficiency of a machine is always less than 100%. Why?

Efficiency is the ratio of work output and work input. As work output is always less than work output for the following reasons, the efficiency is less than 1 or 100%.

1. The movable parts of a machines are not weightless.
2.  The movable parts are not frictionless.
3.  The parts of the machines are not rigid.
4. The strings or belts are not perfectly elastic in nature.

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