Gravitational potential energy
Ep = m g h
h is the CHANGE in height, and AQA uses g = 9.8 N/kg unless the question says otherwise.
Know the equation
| Symbol | Quantity | Unit |
|---|---|---|
| Ep | gravitational potential energy | J |
| m | mass | kg |
| g | gravitational field strength | N/kg |
| h | change in height | m |
Rearrangements
- h = Ep / (m g)
- m = Ep / (g h)
Apply it — mark your own working
Work each one out on paper first, then reveal the mark scheme and tick the marks you actually earned. That is exactly how you should mark past papers.
A crane lifts a 250 kg load to a height of 12 m. Gravitational field strength g = 9.8 N/kg. Calculate the gain in gravitational potential energy.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Using the total or final height instead of the CHANGE in height.
- Using g = 10 when the question states g = 9.8 N/kg (or vice versa) — always use the value given.
- Mixing up g (field strength, N/kg) with acceleration; they are numerically equal but the unit in this equation is N/kg.
Exam tip: In energy-transfer questions, GPE gained at the top often equals KE at the bottom (if there is no friction): m g h = 1/2 m v2, and the mass cancels.
Still losing marks on the calculations?
I'll go through your working line by line and show you exactly where the marks are — your first lesson is free.