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Anonymous asked:

Why

is it mr^2 omega and not some other weird formula that is conserved? Why not mr^3 omega or mr^2 omega^2 ?

This is a great question. And to be honest, there is no intuitive answer as to why it is defined this way or that.

Conservation laws can be understood better through the Lagrangian formulation of classical mechanics.

That’s the conservation of momentum for a free particle. It means that this quantity **mv **remains constant with time *(not m ^{2}v, not m^{2}v^{2 },just mv*).

And similarly for a rotating body, one can find that the quantity that remains constant wrt time is the angular momentum.

And that’s the best rationale using modern physics that can be provided for why Angular momentum takes the form that it does.

Any other form would just not be conserved. Sure, you can construct a Lagrangian that would give you the form that you need but that would not represent anything physical !

Hope that answers your question. Thanks for asking !

** If you have not heard about Lagrangian formulation of classical mechanics, the wiki article on Principle of Least action is a really good place to start..

The principle of Least/Stationary action remains central in modern physics and mathematics, being applied in thermodynamics, fluid mechanics, the theory of relativity, quantum mechanics, particle physics, and string theory.

This leads to **Quantization of θ **because there are only specific values that become possible for * θ *when we impose that after n rotations it has to return back to its same starting point. **

And the following are those values:

The problem of finding the values of *θ* for given values of *n* is more generally known as the N roots of unity.

We will leave it as an exercise, but the following animation plots all the possible the values of *θ* for integer values of *n=2,3,4,.. *from the above equation that we found:

And those are the roots of the equation z^n = z i.e if you start at these points on the unit circle and make n rotations you will get back to the same point that you started with.

Have a good one!

** A more physical way to think about this is matching boundary conditions.

For more insight on how boundary conditions lead to quantization, take a

look at this post.