Answer
$8.38 N \cdot m$.
Work Step by Step
Apply Newton’s second law for rotations.
Calculate the angular acceleration.
$$\frac{400rev}{60s}\frac{2 \pi rad}{rev}\frac{1}{8.00s}=5.24\frac{rad}{s^2}$$
Now calculate the net torque.
$$\tau=I\alpha=1.60kg\cdot m^2 (5.24\frac{rad}{s^2})=8.38 N \cdot m $$
