Ball in a spinning ring
Balls bounce inside a spinning ring with a gap that grows. The spin only carries the gap around.
Plays from YouTube after a click.
The math
Two balls bounce under gravity inside a circle that spins once every four bars of the music. Every bounce is perfectly elastic — no energy is lost, so the balls never calm down. One thing may surprise you: the spinning changes nothing about the bounces. The wall of a rotating circle only ever moves sideways, along itself, so it cannot push a ball in or out; a frictionless spinning ring bounces exactly like a still one. The only thing the rotation does is carry the hole around — and because gravity keeps the balls near the bottom, their chances come each time the hole sweeps past the floor, once per revolution. The hole widens at the start of each round (8°, 12°, then 18°), and a ball only gets out if the whole of it fits through the gap at the moment it arrives. The run you are watching was chosen from 500 simulated starting conditions for one reason: the first escape lands in the final seconds, and the other ball is still inside when the music ends.
From the description of Two Balls, One Growing Hole.
