# A bar magnet of moment of inertia I is vibrated in a magnetic field of induction $0.4 \times 10^{-4}T$ . The time period of vibration is 12 sec. The magnetic moment of the magnet is 120$Am^2$ . The moment of inertia of the magnet is ( in $kgm^2$) approximately

$\begin {array} {1 1} (1)\;172.8 \times 10^{-4} & \quad (2)\;2.1\: \pi^2\: 10^{-2} \\ (3)\;1.57 \times 10^{-2} & \quad (4)\;1728 \times 10^{-2} \end {array}$

(1) $172.8 \times 10^{-4}$