Unit 4, lesson 20. About 12 minutes.

Height energy

Why does a ball held up high seem ready to do something?

Play

Getting the experiment ready…

Explain

High things have stored energy.

When they fall, it comes out.

When you lift something, you give it energy. It stores this energy because of its height.

This is called gravitational potential energy. A heavier object or a greater height stores more energy.

When the object falls, the stored energy turns into movement.

PE = m × g × h. It equals the work done lifting the object against gravity.

Energy doubles if the mass doubles or the height doubles. There is no squaring here.

Worked example: a 5 kg box is lifted 3 m. So PE = 5 × 9.8 × 3 = 147 J.

Lift it up
Energy is stored
Let go: it moves

Want the full story?

Only changes in PE really matter, so you can choose any zero height you like. The formula assumes that g is constant, which works near Earth's surface but not for rockets going to space.

The Formula

height energy = mass × gravity × height

PE = m \times g \times h

$PE$: the gravitational potential energy, measured in joules (J)
$m$: the mass, measured in kilograms (kg)
$g$: the strength of gravity, 9.8 on Earth, measured in newtons per kilogram (N/kg)
$h$: the height lifted, measured in metres (m)

Try the numbers

Mass: 10 kgHeight: 10 m

Potential energy: 981 J

Try it at home

You need a soft ball, a tray of flour or sand.

You need: a soft ball, a tray of flour or sand.

Drop a ball into sand. See the dent.

Fill a tray with flour or sand.
Drop a ball from a low height.
Drop it from higher up and compare the dents.

What's happening: more height means more stored energy, so the ball hits harder and makes a bigger dent.

Physics is life

Your body

You climb stairs, and store energy.

Climbing stairs takes effort, because your muscles are storing energy in your raised body.

Daily life

Water in a tower can flow down.

Water towers and hydroelectric dams store energy by holding water up high.

Nature

Rocks on a cliff could fall.

A rock high on a mountain has a great deal of stored energy, and that is why rockfalls are so dangerous.

Space

Rockets store height energy as they climb.

Lifting things into space takes huge amounts of energy, because they must climb far from Earth.

Quick check

Question 1 of 3

What does height energy depend on?

Words to know

Potential energy: Energy stored, ready to be used, like a ball held up high.

Potential energy: Energy stored, ready to be used, like a ball held up high.
Energy: What makes things able to happen. It is measured in joules.
Gravity: The pull between all things that have mass.

Next up

Energy is never lost: conservation