${{{\text{m}}^\prime } = \frac{{I4{\ell ^2}}}{\pi }}$ ${{\text{r}} = \frac{{2\ell }}{\pi }}$
${{{\text{m}}^\prime } = \frac{{I4{\ell ^2}}}{\pi }}$ ${\pi {r^2} = \frac{{\pi 4{\ell ^2}}}{{{\pi ^2}}} = \frac{{4{\ell ^2}}}{\pi }}$
${{{\text{m}}^\prime } = \frac{4}{\pi }\,{\text{m}}}$

