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Angular momentum question... I have the answer

Posted: Thu Feb 20, 2014 2:47 pm
by jueves
I just don't understand why the answer is like that. The prob is:

A uniform cylindrical puck of radius r and mass M is attached to the end of a cord that passes through a
hole in a fixed horizontal frictionless table. The center of the puck moves in a circle of
radius R with angular speed ω

8. The magnitude of the angular momentum of the puck about the hole is


So first I just calculate the Inertial moment of the pluck around the center of the table ,
I= Icylinder + M.R^2 = M(r^2/2 + R^2)
I would expect the answer: L to be simply that I times ω

but the correct answer is this WEIRD thing:
|L|= M (r^2/2 + R^2)^1/2 ω^2

I don't get it , where does that rootsquare term comes from and also why the ω^2????
Please help me out folks, I don't know what the heck am I doing wrong :cry:

Re: Angular momentum question... I have the answer

Posted: Thu Feb 20, 2014 5:49 pm
by TakeruK
Are you sure you have the correct answer? The units of the "weird thing" you gave are not the units of angular momentum.

Re: Angular momentum question... I have the answer

Posted: Thu Feb 20, 2014 10:07 pm
by jueves
TakeruK wrote:Are you sure you have the correct answer? The units of the "weird thing" you gave are not the units of angular momentum.

I know, I'm thinking that maybe comes from the "module" part of the question, so I have to square them and so, but still that omega square is so out of place ... Is from a chinesse booklet that I got. Maybe it's wrong? .. but maybe it's me cause asians are never wrong :P

Re: Angular momentum question... I have the answer

Posted: Tue Aug 30, 2016 9:48 pm
by Penemonie
I have the same question here...