AS Daily A Level Physics question
A workshop hoist lifts crates vertically at constant speed. With a 20 kg crate, the electrical input power is 300 W and the hoist’s efficiency is 60%. The hoist now lifts a 40 kg crate at the same speed and with the same efficiency. What is the new electrical input power?
Answer
The correct answer is A.
Correct: A — 600 W. A: With speed and efficiency unchanged, useful lifting power is proportional to mass, so doubling the mass doubles the input power: 300 W × 2 = 600 W. B: Stays the same is wrong because to lift twice the weight at the same speed, the useful power doubles, so input cannot stay unchanged. C: 360 W confuses useful output power (which would double to 360 W) with the electrical input, ignoring the 60% efficiency. D: 1200 W assumes an extra (incorrect) doubling, as if power scaled with mass squared or speed also doubled; only mass changes here.