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-   -   Angular momentum of rod (http://www.physicsforums.com/showthread.php?t=660936)

 kuyt Dec26-12 11:02 AM

angular momentum of rod

What is the angular momentum of a rod rotating about one end (mass M and angular velocity ш),about its center of mass?

 Doc Al Dec26-12 11:13 AM

Re: angular momentum of rod

Quote:
 Quote by kuyt (Post 4209051) What is the angular momentum of a rod rotating about one end (mass M and angular velocity ш),about its center of mass?
Is the rod rotating about one end or its center of mass?

 kuyt Dec26-12 12:08 PM

Re: angular momentum of rod

Re: angular momentum of rod

A little research is in order.
http://scienceworld.wolfram.com/physics/
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
"There is no need to know all the answers when you know how to find all the answers that have been found before."

 kuyt Dec26-12 01:13 PM

Re: angular momentum of rod

But this is something fundamental,calculating angular momentum of a system about arbitary points in space.:confused:

Re: angular momentum of rod

Navigate through "Mechanics", then "Rotation", then "Moment of Inertia".
Click on "Common Forms" for Enlightenment!

 schaefera Dec26-12 01:27 PM

Re: angular momentum of rod

Find the angular momentum of a differential length (dm*v*r) and integrate from r=0 to L.

 HallsofIvy Dec26-12 01:30 PM

Re: angular momentum of rod

Quote:
 Quote by kuyt (Post 4209128) But this is something fundamental,calculating angular momentum of a system about arbitary points in space.:confused:
Yes, it is, and you can answer it by calculating the angular momentum of individual point about the end of the rod, then integrating them along the length of the rod. The result of doing that would be the formula at http://scienceworld.wolfram.com/phys...nertiaRod.html
that you could get to following the links you were given in tadchem's response.

 kuyt Dec26-12 01:43 PM

Re: angular momentum of rod

Quote:
 Quote by HallsofIvy (Post 4209151) Yes, it is, and you can answer it by calculating the angular momentum of individual point about the end of the rod, then integrating them along the length of the rod. The result of doing that would be the formula at http://scienceworld.wolfram.com/phys...nertiaRod.html that you could get to following the links you were given in tadchem's response.
it gives moment of inertia not angular momentum! Can we use the general formula(if its correct):angular moment abt any point=angular moment of a fictitious particle (of mass m at the position of COM)abt that point + angular moment of the body abt com ?

 dev70 Dec26-12 01:52 PM

Re: angular momentum of rod

hello kuyt, i think you should have a look at the angular momentum equation ie., L= r X P and v=rw. So, use r=l/2 where l=length of rod. hope that makes sense..

 kuyt Dec26-12 02:04 PM

Re: angular momentum of rod

No,it not that easy I guess.
P.S can anyone giveme the final answer(in terms of angular velocity,mass and length)and ofcourse the explanation,instead of links

 schaefera Dec26-12 02:06 PM

Re: angular momentum of rod

It's (1/3)wmL^2 for angular speed w.

 kuyt Dec26-12 02:09 PM

Re: angular momentum of rod

^but that about one of the ends ,not com

 BruceW Dec26-12 02:57 PM

Re: angular momentum of rod

Ah, so you're trying to find the angular momentum about the com, in a system where the rod is rotating about one end.

 kuyt Dec26-12 03:02 PM

Re: angular momentum of rod

yes :approve:

 BruceW Dec26-12 03:54 PM

Re: angular momentum of rod

hmm. Tricky one. Well, I'm pretty sure that the whole point of saying the angular momentum "about a point" is equivalent to calculating the angular momentum, given that the origin is the point about which we want to find the angular momentum.

So I think the angular momentum about the COM is simply angular momentum, given that our origin is the COM. And in our original reference frame, the rod was rotating around the end. So in a reference frame where the COM stays at the origin, the angular momentum will simply be
$$\omega \frac{mL^2}{12}$$
(in other words, same as what the angular momentum would be for a system where the rod is rotating around it's COM.)

 Astronuc Dec26-12 06:02 PM

Re: angular momentum of rod