An electric motor opens a theatre curtain. During one opening, the motor is supplied with 18 000 J18\,000\ \mathrm{J}18000 J of energy in 30 s. Its useful power output is 420 W.
The moving parts are lubricated. This reduces the wasted power by 60%, while the useful power output remains unchanged.
Calculate the efficiency of the motor after lubrication.
Give your answer as a percentage to two significant figures.
128 exam-style questions on Edexcel GCSE Physics 8.3 Power and efficiency, covering 8.3.1 Power and 8.3.2 Efficiency. Each one has a worked solution and a mark scheme showing where the marks go.