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Homework Help: Angular momentum uniform rod problem

  1. Nov 7, 2004 #1
    well, ive tried, and ive tried, but i cant seem to get this one right, my TA did it in class, but left something out i think, he did it in a big hurry, so i cant get an answer thats right.... ill show you what ive tried, and maybe you can spot my error? any help is much appreciated

    Figure 11-49 is an overhead view of a thin uniform rod of length 0.600 m and mass M that is rotating horizontally at 80.5 rad/s counterclockwise about an axis through its center. A particle of mass M/3.00 and speed 41.5 m/s hits the rod and sticks. The particle's path is perpendicular to the rod at the instant of the hit, at a distance d from the rod's center.
    (a) At what value of d are rod and particle stationary after the hit?

    ok, so our ta worked out the cross produt of r and the particle, and used that to get the equation (1/12)mL^2 omega^2-d(m/3)v=0.... so you sove that out, and get d=(L^2omega^2)/4v but thats just not right... what am i doing wrong? any help? thanks in advance
    Last edited: Nov 7, 2004
  2. jcsd
  3. Nov 7, 2004 #2
    somebody has to have SOME ideas.... come on guys...
  4. Nov 7, 2004 #3
    i guess nobody knows how to do it, at least im not the only one
  5. Nov 7, 2004 #4
    well i don't think u wrote down what ur TA wrote correctly

    [tex]L = d * p = d(M/3)v [/tex] cause its perp to the rod
    [tex]L = Iw = \frac{1}{12}ML^2w[/tex]

    [tex]\frac{1}{12}ML^2w - d(M/3)v = 0[/tex]
    [tex]d = \frac{L^2w}{4v} = 0.175m[/tex]

    u don't square the angluar velocity im not sure why u had that..
    Last edited: Nov 7, 2004
  6. Nov 7, 2004 #5
    maybe i did, but i think it was cause he was in a hurry, he did it in less than 2 minutes, cuase we had to take a quiz before we left, but either way, thanks for the help man, stupid mistakes always hurt : (
  7. Nov 7, 2004 #6
    glad to help!
  8. Nov 7, 2004 #7
    thnk you could help on that other one i posted down lower, about variables? cause that one ive racked my brain on for a while... still no result though.... you seem to know this stuff pretty well too
  9. Nov 7, 2004 #8
    post a link
  10. Nov 7, 2004 #9
    hahah, nevermind, gingerbread helped me out.... i was overlooking the obvious, but thanks for the offer!
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