Gravitational potential energy
The energy stored in an object because of its height. GPE = mass x gravitational field strength x height.
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Analogy
A book on a high shelf is a 'loaded spring' waiting to fall.
Example
m = 5 kg, g = 10 N/kg, h = 2 mGPE = 5 x 10 x 2GPE = 100 J
In practice
A roller coaster climbs slowly to store energy, then uses it all on the way down.
Practice
Flashcards
- Give the gravitational potential energy formula.GPE = mass x g x height.
Interview questions
- A 60 kg student climbs stairs 3 m high in 6 s. Take g = 10 N/kg. Calculate the work done and the power.
- Gain in GPE = 60 x 10 x 3 = 1,800 J
- Work done equals the energy transferred, 1,800 J
- Power = 1,800 / 6 = 300 W
Questions people ask
What does GPE depend on?
Three things: the mass of the object, the gravitational field strength, and how high the object is lifted. Double the height and you double the stored energy. Take the same object to the Moon, where g is smaller, and the energy stored at the same height is smaller too.
How does GPE turn into kinetic energy?
As an object falls, its height drops, so gravitational energy falls. The same amount of energy moves into the kinetic store, so it speeds up. Ignoring air resistance, the GPE lost equals the KE gained. A 1 kg ball dropped from 5 m gains 50 J of kinetic energy.