\(\omega_{0}=3 \mathrm{rpm} \Rightarrow \frac{2 \pi}{60}(3) \frac{\mathrm{rad}}{\mathrm{sec}}\)

\(\omega^{2}=\omega_{0}^{2}-2 \alpha \theta\)

\(0=\left(\frac{3 \times 2 \pi}{60}\right)^{2}-2 \alpha\left(4 \pi^{2}\right)\)

\(\therefore \alpha=\frac{1}{800} \mathrm{rad} / \mathrm{s}^{2}\)

\(\tau=\frac{m R^{2}}{2} \alpha=\frac{2}{2} \times\left(\frac{4}{100}\right)^{2} \times \frac{1}{800}=2 \times 10^{-6} \mathrm{Nm}\)