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Torque and angular momentum

  1. Nov 8, 2009 #1
    Hey all,

    I have a physics problem here and I have come across an odd scenario, I wonder if you could tell me if this means that I have the wrong answer.

    I have an ideal bar rotating about an arbitrary axis. The angular velocity vector [tex]\omega[/tex] is NOT colinear with the angular momentum vector. However, [tex]\omega[/tex] is constant, so when I try to find the torque on the bar by using hte time dervitive of the angular momentum vector, I get 0.

    My question is this: is it possible for there to be 0 torque on the bar if the angular momentum and angular velocity vectors are not parallel? Because I was under the impression that the only time when they were not parallel was when there was some applied torque, but I have done this problem using three different methods now, all with the same result.
     
  2. jcsd
  3. Nov 8, 2009 #2

    bcrowell

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    Hi, KBriggs,

    In general, the relationship between L and w for a rigidly rotating body is not L=Iw, where I is a scalar; I has to be a 3x3 matrix. Even for an isolated body with no torques acting on it, it is not necessary for L and w to be collinear. Here is a discussion of this kind of thing that might help: http://www.lightandmatter.com/html_books/0sn/ch04/ch04.html#Section4.3 [Broken]

    Ben
     
    Last edited by a moderator: May 4, 2017
  4. Nov 8, 2009 #3
    Thanks

    So getting 0 torque doesn't mean I went wrong somewhere.

    I know about the matrix notation, but I am new to this stuff so I am getting a little lost in the definitions.
     
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