Consider a solid, uniform circular disk of mass and radius , with its center at . The disk resides within the plane.
Each point on the disk can be represented in polar coordinates as , where is the distance from the origin (not from the center of the disk) and is the angle with respect to the positive axis.
Let us move each infinitesimal bit of mass on the disk from to , where for and for . The resulting mass arrangement is as shown in the diagram.
What is the moment of inertia of this split disk with respect to the axis?
Note: The mass of each infinitesimal portion is preserved as it is moved.
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Let's denote the right half of the "nutmeg" by D and proceed in polar coordinates. The moment of inertia we seek is 2 ∫ ∫ D x 2 d m = π 2 ∫ ∫ D x 2 d A = π 2 ∫ − 4 π 4 π ∫ 0 2 sin ( θ + 4 π ) r 3 cos 2 θ d r d θ = 4 3 + 3 π 4 ≈ 1 . 1 7 4 4