Planar lamina
= Planar lamina
{title2=$dm=\sigma(x,y)\,dA$}
A planar lamina is an ideal <mass distribution> confined to a <plane>, with <surface mass density> $\sigma$ and infinitesimal <mass> $dm=\sigma\,dA$. Its <moment of inertia> about an axis is the <integral> of squared perpendicular distance against this mass element. Three mutually perpendicular axes through one point, with two in the lamina's plane, obey the <perpendicular axis theorem> even when the density is nonuniform.