Gravitational Potential Energy
Simple Explanation
Gravitational potential energy (GPE) is the energy an object has because of its height above a reference point: GPE = mgh. It represents the energy that would be released (usually converting to kinetic energy) if the object fell back down to that reference level.
Why Do We Need It?
GPE is the energy store behind hydroelectric dams, pendulum clocks, and roller coasters — anything that stores energy by height, ready to be released as motion.
Formula
Gravitational Potential Energy
GPE = mgh
Gravitational potential energy is the energy an object has because of its height above a reference level — it is the energy that would be released if the object fell back down to that level.
- GPE
- — gravitational potential energy, in joules (J)
- m
- — mass of the object, in kilograms (kg)
- g
- — the acceleration due to gravity, approximately 9.8 m/s²
- h
- — height above the reference level, in metres (m)
When to use it: Whenever the gravitational potential energy of an object at a known height needs to be found.
Worked Example
Find the GPE of a lifted box
A 15 kg box is lifted onto a shelf 2 m high. Using g = 9.8 m/s², find its gravitational potential energy relative to the floor.
Why Does This Work?
GPE equals the work that was done lifting the object against gravity (W = Fd, with F = weight = mg and d = h) — since raising the object stores exactly that much energy, ready to be released again as it falls.
Real-Life Example
Hydroelectric dams
A hydroelectric dam holds back a huge reservoir of water at height.
The water's gravitational potential energy converts to kinetic energy as it falls through the turbines, which convert that kinetic energy into electricity — GPE is the entire energy source behind hydroelectric power.
Practice
Find the GPE of a 4 kg object at a height of 5 m (g = 9.8 m/s²).
EasyCommon mistake
Forgetting that GPE is always relative to a chosen reference height — the same object has different GPE values depending on whether it is measured from the floor, the ground outside, or sea level; only CHANGES in GPE are usually what matter physically.
Quick Review
- GPE = mgh
- GPE is the energy stored by height, relative to some reference level.
- This is the energy source behind hydroelectric power and pendulum clocks.