Rolling ball between concentric races
= Rolling ball between concentric races
{title2=$v_{\rm cm}=R\omega/2$}
If an inner race of radius $R$ rotates at <angular velocity> $\omega$ and an outer race is stationary, a ball of radius $r$ rolling at both contacts has centre speed $R\omega/2$ and spin $-R\omega/(2r)$. Its centre follows a circle of radius $R+r$. The <angular momentum> about the common race axis includes both orbital <angular momentum> and spin.