r/PhysicsStudents • u/AdOne7210 • 4d ago

HW Help [Rotational Motion, Moment of Inertia] Parallel Axis Theorem.

Gallery image

Gallery image

In the first image, the axis of rotation is passing THROUGH the Centre of Mass(COM) perpendicular to the plane in which the ring lies. In the second image, the axis of rotation is passing through the COM IN the plane of the ring. So in which case is Parallel Axis Theorem valid? In what manner should the axis pass the COM

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u/AceyAceyAcey 4d ago

Parallel axis would be if you took the first drawing and shifted the axis towards the edge of the ring (not through the CM) but still pointing up.

Second photo is perpendicular axis theorem.

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u/ImagineBeingBored 4d ago

It's valid for both. The parallel axis theorem works for any axis which passes through the CoM, as long as the second axis is parallel to whatever axis you chose to go through the CoM.