Rotational Inertia of compass needle

[olyviab]

Homework Statement

A small compass needle is situated in a uniform magnetic field. When displaced from its stable equilibrium position through a small angle and then released, it oscillates with a frequency of 5 Hz. A millijoule of work is needed to twist the compass needle round through 180º, so that its magnetic moment is in the opposite direction to the magnetic field. What is the rotational inertia of the needle?


2. The attempt at a solution

I don't understand how to start this problem any help would be great.

 

[zhermes]

What equations have you learned that involve the frequency of oscillation? What about the work (or energy) of a dipole in a uniform magnetic field?