Efficiency Formula for Motors

Steps:

• Efficiency η = useful output power / electrical input power, so input power = output / η = 120 W / 0.6 = 200 W.
• Waste heat power = input power - useful output power = 200 W - 120 W = 80 W.
• Verify: η = 120 W / 200 W = 0.6 or 60%, and total power balances.

Why D is correct:

• Matches input power of 200 W and waste heat of 80 W from η = P_out / P_in formula.

Why the others are wrong:

• A: Input 72 W too low; yields η > 100%, impossible.
• B: Identical to A; input 72 W too low, impossible efficiency.
• C: Input 200 W correct, but waste heat 72 W wrong; should be 80 W to balance powers.

Final answer: D