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Homework Help: Angular momentum of a particle system

  1. Mar 26, 2009 #1
    1. The problem statement, all variables and given/known data

    Calculate the total angular momentum of the system of particles pictured in Figure P.10 about the origin at 0. Figure is at www.webassign.net/reese1/p10-10.gif.

    2. Relevant equations

    L = r X p

    p = mv

    3. The attempt at a solution

    Since you are taking a vector product, there is no i or j component to the angular momentum, meaning there will just be k perpendicular to the plane. I read somewhere that when doing a system of particles, you should take the angular momentum of each separate particle and then add it up. However, when I do this I don't get the right answer... Using L = r X p:

    For 1st 3m: -4i + 2j X -3i = 6k
    For 1st 2m: 1i + 2j X 2i = -4k
    For 2nd 2m: 1i -2j X 2j = 2k

    And 6 - 4 + 2 = 4k, which is NOT the right answer. I don't know what I've done wrong...
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 26, 2009 #2


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    Homework Helper

    Welcome to PF.

    Your 3rd r vector should be <1.2i, -2j> shouldn't it?
  4. Mar 26, 2009 #3
    Indeed it should be. That 1.20 is so close to the 1.00! I got the correct answer now. Thanks a lot!
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