An electric hoist A lifts a cargo crate of mass 500 kg through a vertical height of 12 m in a time of 30 s.
An electric hoist B lifts a cargo crate of mass 400 kg through a vertical height of 10 m in a time of 16 s.
Which hoist develops the greater average useful power output and why? (Take g=9.8 m s−2g = 9.8\text{ m s}^{-2}g=9.8 m s−2)
Hoist A, because it transfers a greater total amount of energy (58.8 kJ58.8\text{ kJ}58.8 kJ) over its cycle.
Hoist B, because it transfers energy at a greater rate (2.45 kW2.45\text{ kW}2.45 kW) despite transferring less total energy.
Hoist A, because it lifts a larger load to a higher vertical position.
Hoist B, because it takes less time to complete its lift.
37 exam-style questions on OCR GCSE Physics Power and efficiency. Each one has a worked solution and a mark scheme showing where the marks go.