(a) $\alpha = \frac{2\pi(n_{2} - n_{1})}{t} = \frac{2\pi(0 - 20)}{10} = - 4\pi\ rad/s^{2}$ Negative sign means retardation Now $\tau = I\alpha = 5 \times 10^{- 3} \times 4\pi = 2\pi \times 10^{- 2}N - m$
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