A solid disc of radius $20 \mathrm{~cm}$ and mass $10 \mathrm{~kg}$ is rotating with an angular velocity of $600 \mathrm{rpm}$, about an axis normal to its circular plane and passing through its centre of mass. The retarding torque required to bring the disc at rest in $10 \mathrm{~s}$ is ___________ $\pi \times 10^{-1} \mathrm{Nm}$.
$\tau=\frac{\Delta \mathrm{L}}{\Delta \mathrm{t}}=\frac{\mathrm{I}\left(\omega_{\mathrm{f}}-\omega_{\mathrm{i}}\right)}{\Delta \mathrm{t}}$
$\tau=\frac{\frac{\mathrm{mR}^{2}}{2} \times[0-\omega]}{\Delta \mathrm{t}}$
$=\frac{10 \times\left(20 \times 10^{-2}\right)^{2}}{2} \times \frac{600 \times \pi}{30 \times 10}$
$=0.4 \pi=4 \pi \times 10^{-2}$
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