Which statement about the efficiency of a machine is true?

• A. Efficiency is the ratio of energy input to useful energy output.
• B. Efficiency is the ratio of useful energy output to energy input, with no units.
• C. Efficiency is the loss of energy due to friction.
• D. Efficiency is the ratio of useful energy output to total energy input × 100%.

Answer: (D)

Efficiency measures how much of the energy supplied to a machine is transformed into useful work. It is found by dividing the useful energy output by the total energy input, and expressing the result as a percentage. This captures both how effectively the machine uses energy and the fact that some energy is always lost to heat, friction, or other non-useful forms. So the best statement is that efficiency equals useful energy output divided by total energy input, multiplied by 100%. The other ideas mix up the direction of the ratio, describe losses instead of the overall efficiency, or omit the percentage form commonly used to express efficiency.