Work, power and efficiency — Physics, 14–17 years
These ideas separate the amount of energy transferred from how quickly it is transferred and how much is useful. They make machines and human effort measurable.
Three useful measures
Mechanical work is energy transferred when a force moves an object: work = force × distance in the force’s direction. Power tells how fast that work is done: power = work ÷ time. Efficiency compares useful output with total input, usually as a percentage.
Why these measures arose
Lifting a heavy box and lifting a light box can require different energy, but two machines may do the same job at very different speeds. Engineers needed language for the job, the rate and the losses. Work, power and efficiency answer those three practical questions.
Worked example
A hoist lifts a 50 kg load by 4 m in 10 s. Taking g = 10 N/kg, the lifting force is 50 × 10 = 500 N. Work is 500 × 4 = 2,000 J. Power is 2,000 ÷ 10 = 200 W. If it used 2,500 J, its efficiency was 2,000 ÷ 2,500 × 100 = 80%.
The common mistake
A common mistake is to say that a larger force always means more work. That seems natural because pushing harder feels like doing more. But if the object does not move, or the force is sideways to the movement, that force transfers no mechanical work; direction and distance matter.
Where it is used
A kettle’s power tells how quickly it can heat water, while its efficiency compares useful heating with electrical input. Car engines, lifts and solar panels are also rated by power and efficiency. Athletes use power to describe how quickly they can transfer energy during a sprint or jump.
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