All machines do not work at hundred percent efficiency because of friction in the machines.
Efficiency of a machine is the measure of the percentage of useful work the machine can do and the amount of the work input into the machine.
No machine has an efficiency of 100%.
The efficiency of machines are never 100% because some amount of the work input into the machine is used to overcome friction.
Since friction is not useful work, the efficiency of the machine will decrease.
In conclusion, the efficiency of a machine measures the useful work the machine can do compared to the work that has been done on it.
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