Routh's rule of inertia

Routh's rule of inertia

[′rau̇ths ′rül əv i′nər·shə]
(mechanics)
The moment of inertia of a body about an axis of symmetry equals M (a 2+ b 2)/ n, where M is the body's mass, a and b are the lengths of the body's two other perpendicular semiaxes, and n equals 3, 4, or 5 depending on whether the body is a rectangular parallelepiped, elliptic cylinder, or ellipsoid, respectively.