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AS Daily A Level Physics question

2026-02-27 OCR A Efficiency (M3.3) OCR-A M3.3 Work, energy & power — useful mechanical power at constant speed (P = Fv); efficiency = useful output/input OCR-A M2 Mathematical skills — ratios and percentages

In a lab, a small motor draws a steady 120 W while lifting a 100 N load vertically at a constant speed of 0.80 m s−1. Which value best estimates the motor’s efficiency?

  1. A 80%
  2. B 150%
  3. C 8.0%
  4. D 67% (correct)

Answer

The correct answer is D.

Correct: D — 67%. The useful power is force × speed = 100 N × 0.80 m s−1 = 80 W, so efficiency ≈ 80/120 ≈ 0.67 = 67%. A uses 100/120 (ignoring the speed and that power out is Fv), giving about 83% and rounded here to 80%, which is not the correct power ratio. B inverts the ratio (120/80) to get 150%, which is impossible since useful output cannot exceed input. C misplaces the decimal (treating 80 W as 8 W), giving 8.0%, far too low. D correctly compares useful output power (80 W) with input (120 W) to give about 67%.

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