Definitions of efficiency vary depending on how energy is being converted. Compare the definitions of efficiency for the human body and heat engines. How does the definition of efficiency in each relate to the type of energy being converted into doing work?
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In an engine or a mechanical system, efficiency can be defined as the output power to the input power or the ratio of the word done to the energy supplied.
In a mechanical system such as an engine, the fuel is supplied, which gives heat energy as the input. The work done will be in terms of the process done by the engine as per its construction, such as rotating a loaded shaft or running a wheel at constant speed.