Past exam of the mathematics course of the University of Cambridge / 2010 / ia / Paper 3 / 9C / c / Solution

For $i\ne j$, the inertia component is $I_{ij}=-\rho {\int }_V{x}_i{x}_{j}dV$. Reflecting just one of these coordinates preserves the cylinder and its uniform density but changes the integrand's sign. Odd-integrand cancellation by reflection therefore makes every off-diagonal entry zero. Rotations about the axis also give $I_{11}=I_{22}$.

Using cylindrical coordinates $x_1=r\cos \theta$, $x_2=r\sin \theta$, $x_3=z$, with $dV=rdrd\theta dz$, the required integrals areThus the inertia tensor isThe mass is $M=4\pi \rho$, so these entries are also $19M/12,19M/12,M/2$, respectively.