A2 Daily A Level Physics question
A small electric hoist in a workshop draws 600 W from the mains. In steady operation only 40% of this power becomes useful lifting of the load; the rest is lost as heat and sound. It raises an 80 kg crate vertically at constant speed. Take g = 9.8 m s^-2. What is the approximate lifting speed?
Answer
The correct answer is A.
Correct: A — 0.31 m s^-1. Useful power is 0.40 × 600 = 240 W, weight is 80 × 9.8 = 784 N, so speed ≈ 240/784 ≈ 0.31 m s^-1. A is consistent with using useful (not input) power and P = (weight) × speed. B uses the full input power (600 W) instead of the useful 40%, giving 600/784 ≈ 0.76 m s^-1. C divides by efficiency instead of multiplying, effectively assuming the motor delivers more than the input power. D treats the weight as 80 N (or omits g), giving 240/80 = 3.0 m s^-1, which underestimates the force needed.