MR² is the moment of inertia of the disk ring system.
The term "moment of inertia" refers to a physical quantity that quantifies a body's resistance to having its speed of rotation along an axis changed by the application of a torque (turning force). The axis might be internal or exterior, fixed or not. But the moment of inertia (I), which is always expressed in relation to that axis, is calculated by adding the results of multiplying each particle's mass by the square of its distance from the axis in a particular body. The moment of inertia is comparable to mass in linear momentum when calculating the angular momentum of a rigid body.
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