일반물리학 2 — Maxwell's Equations; Magnetism of Matter: Problem 40

40 You place a magnetic compass on a horizontal surface, allow the needle to settle, and then give the compass a gentle wiggle to cause the needle to oscillate about its equilibrium position. The oscillation frequency is 0.312 Hz. Earth’s magnetic field at the location of the compass has a horizontal component of 18.0 \mu T. The needle has a magnetic moment of 0.760 mJ/T. What is the needle’s rotational inertia about its (vertical) axis of rotation?