Angular momentum question... I have the answer

jueves
Posts: 3
Joined: Tue Jan 28, 2014 6:06 pm

Angular momentum question... I have the answer

Postby jueves » Thu Feb 20, 2014 2:47 pm

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:

TakeruK
Posts: 812
Joined: Mon Jan 02, 2012 3:05 pm

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

Postby TakeruK » Thu Feb 20, 2014 5:49 pm

Are you sure you have the correct answer? The units of the "weird thing" you gave are not the units of angular momentum.

jueves
Posts: 3
Joined: Tue Jan 28, 2014 6:06 pm

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

Postby jueves » Thu Feb 20, 2014 10:07 pm

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

Penemonie
Posts: 2
Joined: Wed Jul 06, 2016 4:28 pm

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

Postby Penemonie » Tue Aug 30, 2016 9:48 pm

I have the same question here...




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