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Having problem in crossproduct, dotproduct.

  1. Oct 2, 2007 #1
    [SOLVED] Having problem in crossproduct, dotproduct.

    Sorry, i have a lil problem in solving for some math in physics equation.
    I'm doing simulation of rigid body collision reaction,
    and i applied the angular momentum,
    which is L = p x r = I ( w2 - w1);
    where p = mv is the impact of the collision,
    w1,w2 is the angular velocity.
    r be the vector from the center of body to the collision point.

    I'm wondering if the eq is p x r,
    in a 2D system, the third coordinate system is ignored.
    and in some circumstances,
    i let the equation to be reformed into something like p * n x r,
    where n is the reflect/normal vector at the collision point.
    since p x r is actually the cross-product,
    but some books are telling me to take dotproduct of it,

    i'm wondering and hoping to seek for help.

    Thanks in advance.
  2. jcsd
  3. Oct 2, 2007 #2
    L=p x r= pr sin (a), where a is the smaller angle between p and r. The resultant vector L requires and resides in a third dimension by definition. The angular velocity omega (w), parallel to L, also lies in this third dimension.

    Given p (momentum) and r (momentum arm), you do not find the angular momentum by taking their "dot product."

    What two-dimensional system did you have in mind?
  4. Oct 2, 2007 #3
    bleessh i just found that i got it figured, just it skipped few steps n i did proved it, not actually dotproduct of them, but cross of them ^^
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