4 · IGCSE Physics
Work done and power
Two motors lift identical loads through the same height. Change the mass, height and lifting times to see why both do the same work but deliver different power.
W = F × d = m g h
ΔEₚ = m g h
P = W ÷ t
g = 9.8 m s⁻²
ΔEₚ = m g h
P = W ÷ t
g = 9.8 m s⁻²
Try this
- With m = 50 kg and h = 10 m, set A to 10 s and B to 20 s. Compare the work done by each motor, then compare their powers.
- Double the mass while keeping both times fixed. By what factor do the work done and each power change?
- Set m = 20 kg and h = 5 m. Find the time for motor B that gives a power of 98.0 W, and check it with P = W ÷ t.
Exam tip: when two machines lift the same load through the same height they do the same work, so compare power using P = W ÷ t — and convert minutes to seconds and kJ to J before dividing.