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ELI5 Yahaaa

Newton's third law

Every Push Has a Push Back

You cannot push something without it pushing you exactly as hard.

Push a wallYou push thewallThe wall pushesyou, exactly ashard
Forces never happen alone.

  1. Step 1 of 5

    Forces come in pairs, always.

    Impossible A one-sided pushReality Always two, equal andopposite

    There is no such thing as a lone push. If A pushes B, then B pushes A back with the same strength in the opposite direction. No exceptions have ever been found.

  2. Step 2 of 5

    You feel it when you push off something light.

    Push a wall Nothing visibly movesPush a skateboard Both of you move apart

    Push a wall and nothing moves — the Earth is holding it. Push off a skateboard and it shoots away while you go the other way.

  3. Step 3 of 5

    Equal force does not mean equal movement.

    A pushed skateboardmoves a lotThe person pushingmoves someThe Earth being walked onmovesimmeasurably

    The forces match, but the results do not. The lighter object accelerates far more, because the same push moves less mass more easily.

  4. Step 4 of 5

    This is how you walk.

    1 Foot pushesback on theground2 Ground pushesforward on you3 You move

    Your foot pushes backwards on the ground. The ground pushes forwards on you, and that forward push is what actually moves you.

  5. Step 5 of 5

    And how rockets, boats and birds move.

    1 Throw waterbackwards2 Throw airdownwards3 Throw gas out

    A propeller throws water back, a wing throws air down, a rocket throws gas out. Each one moves forward because of the push it gets in return.

    On slippery ice walking barely works — your foot cannot push back on anything, so nothing pushes you forward.

The short version

Every push comes with an equal push the other way — that pair is what makes walking, swimming and flying possible.

Try it yourself

Sit on a wheeled chair and throw a heavy book. You roll backwards. The book pushed you.

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