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Energy and power · level 2, lesson 2 of 6

A question to start

You carry a ball to the top of a slide and let it go. It rolls down faster and faster. Where did the energy to speed it up come from?

The idea

When you lift something up, you give it energy. That energy is stored in it as long as it stays up high. When it falls or rolls down, the stored energy turns into kinetic energy, and it speeds up.

This stored energy is called height energy, or by its proper name, gravitational potential energy. Something has more of it if it is:

  • heavier,
  • higher up,
  • on a planet with stronger gravity.
stored (height)moving

On Mars, gravity is weaker, a bit more than a third of Earth's. So lifting a rock is easier, and the rock stores less energy when it is up high. If you drop it, it lands more gently. A rock dropped from a table on Mars would hit the floor with only a bit more than a third of the energy it would have on Earth.

Water stored high up behind a dam has lots of height energy. As it rushes down through the dam, it turns machines that make electricity.

Worked examples

1. A bouncing ball. At the top of each bounce, the ball stops for a moment: all its energy is stored height energy. On the way down, the height energy turns into moving energy.

2. A climbing frame. Climbing up takes effort: you are storing energy in yourself. Sliding down gives it back as speed.

3. Dropping a spanner on Mars. From a ladder, it lands with only 0.38 of the energy it would on Earth, so it is less likely to break, or to hurt a foot.

Try it

  1. What happens to the energy you use lifting a book onto a high shelf?
  2. Which stores more energy: a ball on a high shelf or the same ball on a low one?
  3. Does a rock held 1 m up store more energy on Mars or on Earth?
  4. Why does a dam store water high up?
Answers
  1. It is stored in the book as height energy (gravitational potential energy).
  2. The ball on the high shelf.
  3. On Earth: gravity is stronger there.
  4. The height energy of the water can be turned into electricity as it falls.

Common mistake

"Lifting something on Mars gives it the same energy as on Earth, because it is the same rock." Height energy depends on gravity. Weaker gravity on Mars means less stored energy, and less work to lift it.

Where it leads