The ratio of a machine's power output to power input is the machine's __________.

• A. Efficiency
• B. Torque
• C. Speed
• D. Load

Answer: (A)

Efficiency measures how effectively a machine converts input power into useful output power. It is the ratio of power output to power input, usually shown as a decimal or a percentage. This tells you how much of the energy you supply actually becomes useful work at the output, with the rest lost to heat, friction, and other losses. In an ideal, lossless machine, efficiency would be 1 (100%), but real machines are always less than that.

Torque is the turning force the machine can produce and is related to power through P = torque × angular velocity, but it is not the ratio of input to output. Speed is how fast the machine rotates, another separate performance measure. Load is the resistance the machine works against. So the ratio of power output to power input specifically identifies efficiency. For example, if a motor consumes 1000 W and delivers 900 W of shaft power, its efficiency is 0.9 (90%).