\( = \left( {\sigma dA} \right){r^2}\)
\( = \left( {\frac{{{\sigma _0}}}{r}2\pi dr} \right){r^2} = \left( {{\sigma _0}2\pi 0{r^2}dr} \right)\)
\(I = \int {DI = \int\limits_a^b {{\sigma _0}2\pi {r^2}dr} } \)
\( = {\sigma _0}2\pi \left( {\frac{{{b^3} - {a^3}}}{3}} \right)\)
\(m = \int {dm = \int {\sigma dA} } \)
\( = {\sigma _0}2\pi \int\limits_a^b {dr} \)