A machine converts 500 J of input energy into 320 J of useful output. What is its efficiency?

• A. 0.64
• B. 0.72
• C. 0.60
• D. 0.68

Answer: (A)

Efficiency is the ratio of useful output energy to the input energy. It tells us how much of what goes in actually becomes useful work, with the rest lost to inefficiencies like heat and friction.

Here, the machine takes in 500 J and delivers 320 J of useful output. So the efficiency is 320 J divided by 500 J = 0.64, or 64%. The remaining 180 J is energy wasted. The other numbers would require different useful outputs (for example, 0.72 would need 360 J; 0.60 would need 300 J; 0.68 would need 340 J), which isn’t the case here.