Newton's Cradle

Newton's Cradle is the executive desk toy: five polished steel balls hanging in a row, each just touching the next. Lift one and let it fall, and instead of the whole row swinging, the impact travels through the middle three without visibly moving them and throws the ball at the far end out by exactly the same amount. Lift two and two come out the other side. Lift three and three do. It is one of the few physics demonstrations that stays surprising no matter how many times you have seen it.

The balls are simulated as real pendulums rather than looped animation, so the timing is emergent. Each swings under gravity with a period set by its string length, and when two touch, the pair exchange velocities — which is exactly what equal masses do in an elastic collision, and is the whole reason the trick works. Contacts are resolved several times per frame so an impulse arriving at one end travels the full row within a single step, instead of creeping along one ball per frame.

Every collision plays a short metallic click, pitched and weighted by how hard the balls actually met, so a strong swing sounds different from the small residual taps as the motion dies away. Energy loss is deliberately tiny, meaning a single lift will keep going for a very long time. Drag any ball out to whatever angle you like and release it, or use the Lift buttons to set up the clean one, two and three ball cases. Free, in the browser, nothing to install.