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Push back

Rockets and landing · level 2, lesson 1 of 6

A question to start

Push your hand hard against a wall. Can you feel the wall pushing back on your hand? Push harder: does the wall push back harder too?

The idea

Whenever one thing pushes on another, the second thing pushes back on the first, just as hard, the opposite way. Isaac Newton wrote this down as his third law of motion. People often say it as:

For every action there is an equal and opposite reaction.

You feel it all the time:

  • Push a wall and the wall pushes your hand back. If you are on roller skates, you roll away from the wall.
  • When you walk, your foot pushes the ground backwards, and the ground pushes you forwards.
  • When you swim, your hands push the water backwards and the water pushes you forwards.
  • When you jump, your legs push the ground down, and the ground pushes you up.
hands push wallwall pushes backrolls away

The two pushes always come as a pair, and they act on different things: one on the wall, one on you. That is why the wall's push can move you.

A rocket pushes hot gas out of its engine, backwards and very hard. The gas pushes the rocket forwards just as hard. That pair of pushes is what flies every rocket.

Worked examples

1. A boat. When you step off a small boat onto the shore, the boat moves away from the shore. You pushed the boat backwards with your feet as you stepped forwards.

2. Hitting a ball with a bat. The bat pushes the ball forwards, and the ball pushes the bat back. You feel it in your hands.

Try it

  1. When you walk, which way do you push the ground? Which way does the ground push you?
  2. You push a wall with your hand. Which pushes harder: you or the wall?
  3. A swimmer pushes water backwards. What happens to the swimmer?
  4. What does a rocket push, and what pushes the rocket?
Answers
  1. You push it backwards; it pushes you forwards.
  2. Neither: the pushes are exactly as hard as each other.
  3. The water pushes the swimmer forwards.
  4. The rocket pushes its hot gas out backwards, and the gas pushes the rocket forwards.

Common mistake

"The push and push-back cancel, so nothing moves." They act on different things. The push on the gas moves the gas; the push on the rocket moves the rocket. Each moves the thing it acts on.

Where it leads