A short bar magnet of magnetic moment  $2{\mathrm{Am}}^2$and moment of inertia  $6\times {10}^2\mathrm{kg}/{\mathrm{m}}^2$ is freely suspended such that the magnetic axial line is in the direction of magnetic meridian. If the magnet is displaced by very small angle ( $3^{\mathrm{o}}$), the angular acceleration is $\mathrm{y}\times {10}^{-6}\mathrm{rad}/{\mathrm{s}}^2$ . Find y. (Magnetic induction of earth’s horizontal field = $4\times {10}^{-4}\mathrm{T}$)

• A

  $\frac{\mathrm{\pi }}{20}$

• B

  $\frac{\mathrm{\pi }}{45}$

• C

  $\frac{\mathrm{\pi }}{60}$

• D

  $\frac{\mathrm{\pi }}{75}$