A machine uses 200 J of input energy to produce 50 J of useful work. What is its efficiency?

A machine uses 200 J of input energy to produce 50 J of useful work. What is its efficiency?

• A. 10%
• B. 15%
• C. 25%
• D. 75%

Answer: (C)

Efficiency is the ratio of useful work output to the input energy, usually written as a percentage. Here, the machine produces 50 J of useful work from 200 J of input energy. Efficiency = 50 J / 200 J = 0.25, which is 25%. So the machine converts 25% of the input energy into useful work, with the remaining 75% lost to other forms like heat or friction. The other options would require different output amounts (20 J for 10%, 30 J for 15%, 150 J for 75%), which don’t match the given 50 J.

Efficiency is the ratio of useful work output to the input energy, usually written as a percentage. Here, the machine produces 50 J of useful work from 200 J of input energy. Efficiency = 50 J / 200 J = 0.25, which is 25%. So the machine converts 25% of the input energy into useful work, with the remaining 75% lost to other forms like heat or friction. The other options would require different output amounts (20 J for 10%, 30 J for 15%, 150 J for 75%), which don’t match the given 50 J.