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The skateboard and the ball

Rockets and landing · level 2, lesson 2 of 6

A question to start

Sit still on a skateboard (on a smooth, flat floor, with a grown-up nearby) holding a heavy ball. Throw the ball hard forwards. What happens to you?

The idea

You roll backwards. When you threw the ball, you pushed it forwards, and the ball pushed you backwards, just as hard. Nobody touched you, but you moved.

ball goes fastyou go slowly

Try it with different balls:

  • A heavier ball, thrown at the same speed, rolls you back faster.
  • The same ball thrown faster rolls you back faster too.
  • A very light ball, like a ping-pong ball, hardly moves you at all.

You go much more slowly than the ball, because you are much heavier than it. If you threw ball after ball, each one would push you back a bit more, and you would keep speeding up.

That is a rocket. A rocket is like the skateboard rider with a huge pile of balls, throwing them out of the back thousands of times a second. The "balls" are tiny bits of hot gas, thrown out very, very fast. Each one gives the rocket a little push forwards, and all together they make a big, steady push.

Worked examples

1. A garden hose. A fire hose pushes back so hard that firefighters have to hold it tight. The water rushing out forwards pushes the hose backwards.

2. A cannon. When a cannon fires, the cannonball shoots forwards and the cannon jumps backwards. This is called recoil.

Try it

  1. You throw a ball forwards from a skateboard. Which way do you roll?
  2. Would you roll back faster with a heavy ball or a light one?
  3. Why do you roll back more slowly than the ball flies?
  4. How is a rocket like a skateboarder throwing balls?
Answers
  1. Backwards.
  2. A heavy ball (thrown at the same speed).
  3. You are much heavier than the ball.
  4. A rocket throws out bits of gas, very fast, from its back, again and again, and each one pushes it forwards.

Common mistake

"You only move if your feet push the ground." Your feet were not touching the ground at all: the skateboard rolled freely. The push came from the ball. Rockets work the same way, with nothing to push against.

Where it leads