# A gravitational store of energy

##### Demonstration

A simple analogy that could be used with small groups or with individual students finding it difficult to understand the concept of gravitational potential energy ('uphill' energy).

#### Apparatus and materials

Brick, single

Cardboard box, filled with screwed-up paper or containing polystyrene packaging

#### Health & Safety and Technical notes

Ensure that the brick catcher is in the correct place with the student's feet wide apart.

#### Procedure

Ask a student to stand with their feet apart, holding the brick in their hands. As they raise and lower the brick they can imagine the brick to be attached to a long spring, fixed to the centre of the Earth.

#### Teaching notes

1 When you discuss examples involving gravitational potential energy, students will often ask questions about where the energy resides. This demonstration can help students think of the uphill energy stored in the gravitational field that connects the Earth with the raised load, by telling them an artificial story.

2 You might introduce the story with these words:
'Imagine a spring connected between the load you are holding and the centre of the Earth. There is no real spring but the pull of that stretched spring is just an imaginary idea to help you think about the way the Earth pulls on the load.'

Ask the student holding the brick to shut their eyes and imagine the spring connecting the brick to the Earth.

'Feel how heavy the brick is. Feel its weight. Feel how the Earth pulls it down. If you don't believe the Earth pulls it down, let go and see what the Earth does to the brick.'

(Protect the floor and the student at this point!)

'Pretend to yourself that the pull of the Earth, which you can feel, is the pull of a long stretched spring that is attached to your brick and runs through a hole in the ground to the centre of the Earth. You will find it difficult to imagine that spring if you keep your eyes open and can see that there is no spring there. So now shut your eyes and think about that spring.'

'Raise the brick up, holding it with your two hands. As you haul the brick up, you can feel that spring s-t-r-e-t-c-h-i-n-g. Keep your eyes shut, lower the brick, and let the spring contract a little. Now pull the brick up and stretch the spring. Were you able to imagine the spring?'

3 If students can visualize this long spring being stretched, some may use their knowledge of springs to expect gravity to increase with height above the Earth, (because the force of the spring grows as it stretches). If so, you need to explain that this imaginary spring, all the way to the centre of the Earth, is so long that ordinary stretches would not make it change its strength. Then, to avoid the story being misleading, you should add a warning that the real gravitational 'spring pull' of the Earth gets weaker as you go farther out. This is not the time to go into any inverse-square story.

This experiment was safety-checked in November 2005