The Swinging SpringA ball hangs from a spring, free to bounce up and down and to swing side to side at the same time. Tune the spring just right and something strange happens: a pure up-down bounce spontaneously turns into a wide sideways swing, then back into a bounce, then back again — forever trading energy between the two motions with no outside push at all.Vitt and Gorelik found this in 1933. The trick is a single nonlinear term, (L₀+s)θ̇², that lets swinging motion feed back into the spring's stretch. When the spring's natural bounce rate sits near double the pendulum's swing rate, that feedback turns resonant. Peter Lynch revived the problem in 2002 as the simplest toy model of how fast atmospheric gravity waves trade energy with the slow, balanced weather systems riding beneath them.