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Passing motion along

What Happens When Things Crash?

Motion is not destroyed in a crash. It gets handed over.

1 Moving ball2+Hits a stillball3 Motion handedover
Total motion in equals total motion out.

  1. Step 1 of 5

    Motion has a measurable amount.

    Heavy and slow Big mass, small speedLight and fast Small mass, big speed

    Physicists call it momentum: mass times speed. A heavy slow lorry and a light fast ball can carry the same amount of it.

  2. Step 2 of 5

    A crash transfers it.

    During the crashFirst ballpushed back —slowsSecond ballpushed forward— speeds up

    When a moving ball strikes a still one, the push between them slows the first and speeds up the second. Whatever one loses, the other gains.

  3. Step 3 of 5

    The total is always conserved.

    Before Total momentum = XAfter Total momentum = stillX

    Add up the momentum before and after and you get the same number every time. This holds for snooker balls, car crashes and colliding galaxies.

  4. Step 4 of 5

    Bouncy and squishy crashes differ.

    Bouncy Energy kept — theyreboundSquishy Energy spent bendingand heating

    Snooker balls bounce apart and keep almost all their energy. Cars crumple, turning motion into bent metal, heat and noise — but the momentum still balances.

  5. Step 5 of 5

    Crumpling is a safety feature.

    Rigid car — stops in 0.05shuge force onyouCrumple zone — stops in 0.15sa third of theforcePlus airbag — stops in 0.3sgentler still

    A car is designed to crush, because stretching the crash over a longer time means a gentler force on the people inside.

    Newton's cradle works because both momentum and energy have to balance — that is why exactly one ball leaves.

The short version

Crashes hand motion between objects; the total never changes, but the damage depends on how long the stop takes.

Try it yourself

Roll one coin into a line of touching coins. Only the far one shoots off — the motion passed straight through.

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