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Classical mechanics, angular momentum and velocity not parallel, why?

  1. Nov 12, 2007 #1
    1. The problem statement, all variables and given/known data
    A single particle of mass m is revolving at steady speed in a circle of radius a about the z axis at a height h above the origin. In this situation the particle has angular velocity w = wz

    1 Show that the angular momentum L is not parallel to w

    2 If a second particle revolves in the same circle at the opposite end of a diameter, show that the combined angular momentumof the two particles rotating rigiidly is parallel to w.
    2. Relevant equations

    3. The attempt at a solution
    For the first question, i have found a webpage that talks about this situation and describes the problem of w and L not being parallel. But im finding it hard to extract what information to show that this is true. Here is the link: http://sites.isel.ipl.pt/fisica/pedagogia/ajp75(2007)53.pdf

    I am concentrating on the first question first so any help would be greatly appreciated.
  2. jcsd
  3. Nov 12, 2007 #2
    To answer part (1), consider this:

    In rotational motion, angular momentum [tex]\stackrel{\rightarrow}{L} = \stackrel{\rightarrow}{r} \times\stackrel{\rightarrow}{P}[/tex], where P is linear momentum.
  4. Nov 12, 2007 #3
    Does this show that the components of L are in the x and y axes aswell as z. Therefore it isnt parallel because of this?
  5. Nov 12, 2007 #4
    I think i have the answer to the first part by showing that the moment of inertia in this case has components that are perpendicular to the z axis therefore L and w arent parallel. Any help on the second part would be much appreciated.
  6. Nov 12, 2007 #5
    Have you drawn a picture? It is incredibly helpful when considering multiple different vectors. Consider: what would the cross product be of z and P if z were parallel to P?

    As for part (2), remember that "combined" is just the sum of the two angular momenta, and that each angular momentum is a separate vector. Again, draw a picture.
    Last edited: Nov 12, 2007
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