The efficiency of a machine is

A. always greater than 100 per cent
B. the ratio of useful output energy to total input energy, expressed as a percentage
C. the ratio of input to output
D. equal to the power output

Explanation

Efficiency can never exceed 100 per cent, because energy is conserved and some is always lost to friction, sound and heat. An electric motor may reach 90 per cent while a petrol engine manages only about 25 per cent. The wasted energy is not destroyed, merely converted into forms that are not useful.

The efficiency of a machine is

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