Moment of Inertia

  • Thread starter revres75
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  • #1
revres75
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Homework Statement



This was an exam question that I got wrong, my teacher tried to explain it but it only left me more confused. I found some websites that also had explanations but they were also confusing.

"Derive the moment of inertia of a uniform disk which rotates along a central axis , radius R , disk thickness d, mass M , density p
I= R^2 dm"


Homework Equations


I= 1/2*M*R^2 ?


The Attempt at a Solution



mass M = Pi*R^2*d*p

My teacher mentioned something about a third integral.

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
Doc Al
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Presumeably you are to derive the moment of inertia by evaluating [itex]\int r^2 dm[/itex]. So what did you do?
 
  • #3
revres75
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If I derive [itex]\int r^2 dm[/itex] I get 2r but I not sure what to do with the dm
 
  • #4
Doc Al
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If I derive [itex]\int r^2 dm[/itex] I get 2r but I not sure what to do with the dm
By "derive" I didn't mean "take the derivative". You need to evaluate that integral, which is the definition of moment of inertia. Start by expressing "dm" in terms of density and distance from the axis. (Hint: Think in terms of concentric rings.)
 

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