$\mathrm{dQ}=\ell \mathrm{d} \mathrm{x}$
$\mathrm{I}=(\mathrm{dQ}) \mathrm{f}$
$\mathrm{dm}=(\mathrm{I})(\mathrm{A})$
$\int \mathrm{d} \mathrm{m}=(\mathrm{d} \mathrm{Q} \mathrm{f})\left(\pi \mathrm{x}^{2}\right)$
$\int \mathrm{d} \mathrm{m}=\int_{0}^{\mathrm{Q}} \lambda \mathrm{f} \pi \mathrm{x}^{2} \mathrm{d} \mathrm{x}$
$=\frac{Q}{\ell} \times f \times \pi \times \frac{\ell^{3}}{3}$
