The percentage of the input work that is converted to output work is called efficiency
Explanation:
The efficiency of a machine is defined (in percentage) as:
where:
is the output work of the machine
is the input work in the machine
For an ideal machine that loses no energy, the efficiency is 100 %, because
which means that all the energy in input is converted into useful work in output.
However, in the real world this is never possible: in fact, there are always some frictional forces acting in the machine, and therefore part of the work in input is "wasted" and converted into non-useful forms of energy (mainly, thermal energy), and therefore
which means that the actual efficiency is less than 100%.
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