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Conservation of angular and linear momentum

  1. Nov 5, 2013 #1
    1. The problem statement, all variables and given/known data

    Look at the picture in "oppgave 1".

    Suppose you have an incoming mass which hits the very thin rod straight on in a completely inelastic collision. the incoming mass is ##m##, the rod has a mass of ##M##, and the little mass hits the rod at a length ##l## from the top.

    According to the text, the linear momentum right before and right after are NOT preserved, while the angular momentum is.

    3. The attempt at a solution

    I calculated the total linear momentum before and after, and indeed I got:

    [tex] p_0 = m v[/tex]
    [tex] p_1 = \frac{m v + M L/2l}{MvL^2 /3ml^2 +1}[/tex]

    so the two momentums are seemingly unpreserved. Why is this so? I realize it's a translational motion going over to a rotational one, but what does this have to do with linear momentum not being preserved??
    Last edited: Nov 5, 2013
  2. jcsd
  3. Nov 5, 2013 #2
    You forgot to mention that the rod is hinged at its top. What is the implication of that?
  4. Nov 5, 2013 #3
    That the motion after the collision is rotational?
  5. Nov 5, 2013 #4
    That too. But on a more fundamental level, conservation of momentum works only when no external forces act on the system. Is that the case with the hinged rod?
  6. Nov 5, 2013 #5
    ahh, of course. How stupid of me. thanks for the help :)
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