What is mechanical efficiency?

• A. Output work divided by input work.
• B. Input work divided by output work.
• C. Net work done per cycle.
• D. Power delivered per time.

Answer: (A)

Mechanical efficiency is the ratio of useful output work to the input work. It shows how effectively a machine converts the energy you put in into work done on the load, with losses like friction turning some of that energy into heat. Since some energy is always lost, the output work is less than the input work, giving an efficiency value less than one (often expressed as a percentage). The correct expression uses output work divided by input work. Using input divided by output would be the inverse, not the efficiency. Net work per cycle and power per time describe other quantities (energy change over a cycle and rate of doing work, respectively) rather than efficiency.