Energy
Work, energy and power
Work out work done, kinetic energy, potential energy and power, say when work is positive, negative or zero, and explain what a pulley, a ramp and a lever actually change.
Lift a 5 kg bag of wheat one metre and you have done some work on it. Lift three of them the same metre and you have done three times as much; lift one bag three metres and you have done three times as much again. Both routes point at the same definition: the work a constant force does is the force multiplied by the displacement along that force.
Work, energy, power
Work done is force × displacement, measured in joules. Whatever work is done on an object turns up as a change in its energy — that is the work-energy theorem — so energy is measured in joules too. Power is how fast the work is done: work divided by time, measured in watts, where one watt is one joule each second. Walking up the stairs and running up them are the same work and very different power.
Have a play
The parts of a lever, each one named.
Tap any part of the picture.
Worked example
Lifting one 5 kg wheat bag to a height of 1 m takes some amount of work, call it W. How much work is it to lift three such bags to 1 m, and how much to lift one bag to 3 m?
Three bags, one at a time, to the same height: you repeat the same job three times, so the work is 3W.
Work adds up over repetitions. A machine doing it would burn three times the fuel.
Try it together
Now find the power an engine needs.
A car of mass 1000 kg starts from rest and reaches 72 km/h in 10 s. Answer with a bare number each time.
1.First put the final speed into SI units. What is 72 km/h in m/s?
Have a go
Have a go on your own. A weightlifter raises 75 kg through 2 m in 5 s. Taking g as 10 m/s², what power does she need, in W? Answer with a number.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.