Every Push Pushes Back
- Physics
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- Forces always come in pairs that are equal in size and opposite in direction.
- The two forces of a pair act on different objects, so they do not cancel out.
- Rockets and swimmers move by pushing something else backwards.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Forces come in pairs
Stand on a skateboard and push your friend, who is standing on another one. Your friend rolls away, but so do you, in the other direction. Every push has a partner push of the same size, pointing the opposite way. You push your friend, and at the same moment your friend pushes you. These two forces do not cancel each other out, because each one acts on a different person. If your friend has more mass than you, the same force moves them more slowly, and you speed away faster.
Pushing something backwards to go forwards
A swimmer pushes water backwards, and the water pushes the swimmer forwards. A rocket works the same way. Burning fuel makes hot gas, the rocket pushes that gas out of its nozzle, and the gas pushes the rocket the other way. Many people think a rocket pushes against the air. It does not, which is why rockets work in space, where there is no air to push against. Let go of an untied balloon and it flies around the room for exactly the same reason.
Curriculum links: NGSS MS-PS2-1 · NGSS MS-PS2-2