Passing motion along
What Happens When Things Crash?
Motion is not destroyed in a crash. It gets handed over.
-
Step 1 of 5
Motion has a measurable amount.
Physicists call it momentum: mass times speed. A heavy slow lorry and a light fast ball can carry the same amount of it.
-
Step 2 of 5
A crash transfers it.
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.
-
Step 3 of 5
The total is always conserved.
Shared out differently, moving in new directions, and always the same total. 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.
-
Step 4 of 5
Bouncy and squishy crashes differ.
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.
-
Step 5 of 5
Crumpling is a safety feature.
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.