Moment of inertia of a uniform annulus
= Moment of inertia of a uniform annulus
{title2=$I=\frac M2(a^2+b^2)$}
A uniform planar annulus of outer radius $a$, inner radius $b<a$ and mass $M$ has surface density $M/[\pi(a^2-b^2)]$. Integrating $r^2$ over its mass gives the axial <moment of inertia> $I=M(a^2+b^2)/2$. Subtracting the inner disc from a complete disc gives the same result, provided their surface densities match.
// Target: electromagnetism.bigb