Four masses each 2 kg are placed at the corners A, B, C, D of a mass less square frame. 40 kg mass is at the centre O of a square frame of side 0.2 m . It is to be rotated about an axis passing through the centre O and perpendicular to the plane of the frame. Calculate the torque in $\mathrm{N}-\mathrm{m}$ required to produce an angular acceleration of $\frac{\pi}{2} \mathrm{rads}^{-2}$.

A particle starts rotating from rest. The instantaneous angular displacement is $\theta=3 t^3-t^2$, where $\theta$ is in radian and $t$ in s; The angular velocity at $t=1 \mathrm{~s}$ is