Applied Mechanics and Graphic Statics
M.I. of a thin ring (external diameter D, internal diameter d) about an axis perpendicular to the plane of the ring, is

π/32 (D⁴ + d⁴)
π/32 (D⁴ × d⁴)
π/64 (D⁴ + d⁴)
π/32 (D⁴ - d⁴)

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Applied Mechanics and Graphic Statics
The angle of friction is:

The force of friction when the body is in motion
The ratio of the friction and the normal reaction
The force of friction at which the body is just about to move
The angle between the normal reaction and the resultant of normal reaction and limiting friction

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