Freely expanding rotating particle ring
= Freely expanding rotating particle ring
Particles initially on a circle of radius $R$ with common tangential speed $v$ move freely on tangent lines. They remain on a circle of radius
$$
R(t)=\sqrt{R^2+v^2t^2},
$$
rotated through $\theta(t)=\arctan(vt/R)$, while total angular momentum remains $mRv$.