Angular Momentum weird scenario

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SUMMARY

The discussion centers on the calculation of angular momentum for a wooden ring with a mass M and radius R, which is in pure rotation with a small particle of mass M placed on its circumference. The confusion arises from the fact that the rotation occurs about the ring's axis of symmetry, which does not pass through the center of mass (COM). The established formulas for angular momentum, Icm*w for rotation about the COM and Iaxis*w for rotation about any other axis, are applicable, but the scenario presents a unique challenge in defining the axis of rotation. The conclusion emphasizes that every axis has a well-defined angular momentum, and contributions from individual components can be calculated and summed.

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  • Understanding of angular momentum and its definitions.
  • Familiarity with moment of inertia (I) and angular velocity (w).
  • Knowledge of rigid body dynamics and rotation about different axes.
  • Basic calculus for integrating to find angular momentum.
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PhysicsKid703
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Homework Statement


Before anything, please address this first question:
Angular momentum, at least for pure rotation, is defined about the axis of rotation only, correct?
[/B]
We are given a wooden ring of mass M, radius R.
I place a small particle of mass M on its circumference.
Now the wooden ring is put into pure rotation.

This is a self made conceptual question so if you're wondering why it's so weird, that's why.
my questions;
a) How do we calculate Angular Momentum of the body? What do we define it about?

Because we know that
--->for a normal rigid body in pure rotation about it's axis of symmetry(which passes through COM), angular momentum is Icm*w , where Icm is moment of inertia about COM and w is angular velocity.
--->for a normal rigid body in pure rotation about any other axis of rotation other than its axis of symmetry(which still passes through COM) , angular momentum is Iaxis*w , where laxis is moment of inertia about that axis and w is again angular velocity.
(All this is obtained through ∫ dm r2w , where r in the first case is position vector of each particle from axis of rotation, and the same in the second case, right?)

The main reason I'm getting confused is that the situation in question lies somewhere in the nether:
The body is in pure rotation about its axis of symmetry, but the axis of symmetry is not passing through the com.

The Attempt at a Solution


My thoughts are this, so correct me if I'm wrong:
I have no thoughts I'm blank.
Please explain elaborately for I have lost almost all patience with angular momentum today.
 
Last edited:
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PhysicsKid703 said:
Angular momentum, at least for pure rotation, is defined about the axis of rotation only, correct?
Every axis has a well-defined angular momentum. It does not matter which object rotates around which axis.
The two cases you describe are just special cases - you can still use them to find the contribution of the individual components, and add them.
 

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