1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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]

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

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook