The moment of inertia of the rod around the horizontal axis of O is (1/3) m (2r) 2, where 2r is the length of the rod.

Next, find the moment of inertia of the circle around the O-axis, and the moment of inertia of the axis perpendicular to the plane of the circle is mr^2；; ; According to the parallel axis theorem, the moment of inertia of a circle around the O-axis is Mr 2+m (r+2r) 2, where r+2r=3r is the distance from the center of the circle to the O-point. Here, the center of the ring is the center of mass of the ring.