Moment of inertia - disk - finding dm

AI Thread Summary
The discussion centers on calculating the moment of inertia for a disk, specifically how to determine the mass element dm. The user initially struggles with the expression dm = (rho)*t*dA, particularly with the t*dA component. They attempted to use t*dA = (pi)(r^2)/2 but found it ineffective. After a period of no responses, the user later confirms they resolved their confusion independently. The conversation highlights the importance of clarity in defining variables and the integration process when calculating moment of inertia.
O011235813
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hey, I am trying to figure out the moment of inertia with respect to various axis. I am, having trouble figuring out how to find out dm.
I know that dm = (rho)*t*dA, but i am having trouble figuring out the t*dA part. i tried using

t*dA = (pi)(r^2)/2
That didn't work. If some1 could help me figure this out i'd be much grateful.

Here is the picture of the disk

[URL=http://img165.imageshack.us/my.php?image=semidiskad3.png][PLAIN]http://img165.imageshack.us/img165/6294/semidiskad3.png[/URL] http://g.imageshack.us/img165/semidiskad3.png/1/ [/PLAIN]
 
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Hi O011235813! :smile:
O011235813 said:
hey, I am trying to figure out the moment of inertia with respect to various axis. I am, having trouble figuring out how to find out dm.
I know that dm = (rho)*t*dA, but i am having trouble figuring out the t*dA part. i tried using

t*dA = (pi)(r^2)/2
That didn't work. If some1 could help me figure this out i'd be much grateful.

hmm … no replies in almost 24 hours …

so I'm guessing everyone's as confused as I am :confused:

dm is an element of mass, and rho is density, but what are t and dA supposed to be, what shape volume are you considering (the diagram doesn't make it clear), about which axis, and how are you trying to "slice up" this volume for integration?
 
hey,

yeah sorry for the confusing. I figured it out so I'm good now. Thanks for replying back though :D
 

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