Imagine a room full of super bouncy balls that never slow down. You give one a flick, it zips across, smacks a wall, and comes right back. Then it taps another ball, and something surprising happens. The first ball can stop, and the second one races off in its place. That little trick is the whole game.
#What you do
You fling a ball with your finger or your mouse. It shoots off in a straight line. When it meets a wall, it bounces, a bit like light off a mirror. When it meets another ball, they knock into each other and change how they are moving. You can fling as many as you like and watch them clatter around. Want to try it right now? You can play it and send a few flying.
#What is really happening
Here is the secret. When two balls of the same size crash into each other, they trade the part of their speed that points straight along the line joining their centers. Think of a fast ball hitting a still one head on. The fast ball hands over its push and stops, and the still ball leaves with that push. Nothing is lost. The energy and the motion simply move from one ball to the other, like passing a note down a row of desks.
#Where it shows up
You have seen this before. On a pool table, the cue ball can strike another ball, stop dead, and send that ball rolling away. A row of hanging metal balls, called a Newton’s cradle, does the same thing. Lift one, let it swing in, and one ball pops out the far end. Even a good game of marbles works this way. Once you notice it, you spot it everywhere that balls or coins or beads knock together.
#The short version
- Fling balls and watch them bounce off walls and each other.
- Equal balls swap the push along the line between them.
- No energy is lost, so the motion just changes hands.
- That is why a cue ball can stop and send another off.