Forces And Energy

Lift a bag of rice onto a shelf and you have just paid a bill. The bag was low down and now it is high up, and the difference between those two situations cost you something real. Let the same bag fall and it arrives at the floor moving fast enough to burst, so whatever you paid has clearly not vanished: it was stored on the way up and handed back on the way down. Physics measures both the paying and the storing in one unit, the joule, and this lesson is where you learn to put numbers into that idea.

You will meet three relationships and nothing more. One prices the energy stored when something is raised, one prices the energy carried by something that is moving, and one tells you how quickly a machine can move energy from one of those places to another. Between them they explain why a hoist motor is rated in kilowatts, why a reservoir on a hilltop is a battery made of water, and why the same lift done twice as fast needs twice the power. Every one of the three is printed in the specification's own equation sheet, so the marks here are never for reciting them: they are for knowing which one the question wants and substituting into it cleanly.