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Homework Help: Bar coming to a rest (vertical position)

  1. Jun 8, 2012 #1
    Bar collision with ground

    1. The problem statement, all variables and given/known data
    A uniform bar of mass m and length l has no angular velocity when the extremity A hits the ground without bouncing. If the angle α is 30º, what is the magnitude of v1, so that the bar rotates, around A, until it immobilizes on the vertical position.

    http://img15.imageshack.us/img15/4899/pictrt.png [Broken]

    2. Relevant equations

    [tex]\frac{dL}{dt} = \tau[/tex]
    [tex]\frac{dp}{dt} = F[/tex]

    3. The attempt at a solution

    After the collision one can solve applying conservation of energy. From now I'm only considering the collision.
    Now, my teacher says that the impulse by gravity is negligible and so, using conservation of angular momentum:

    [tex]m v_1 \sin {30º} \dfrac{l}{2} + 0 = mv_{CG}\dfrac{l}{2} + I_{CG} \omega_2 \Rightarrow v_1 = \frac{4}{3}l\omega_2 = \frac{8}{3}v_{CG}[/tex]

    If we apply conservation of linear momentum in the direction perpendicular to the bar, the only force acting on this direction is gravity and if we neglect it (as above), we have:

    [tex]\Delta p = F \Delta t = 0[/tex]

    And so, I would conclude that the velocity perpendicular (vCG above) remains constant and so

    [tex]v_{CG} = v_1 \sin \alpha = \dfrac{v_1}{2}[/tex]

    Am I missing something? If yes, what?

    EDIT: I was thinking and maybe, my error was thinking that the ground reaction had to in the same direction of the bar. Now I don't believe that this is necessarily true. Is this the error?

    Thank You
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jun 8, 2012 #2


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    Science Advisor
    Homework Helper

    Hi Krappy! :smile:

    I don't understand why you didn't stop here …
    … you now have initial ω as a function of v1, what else is there to do? :confused:

    (and yes, you can't apply conservation of momentum separately, since you have no idea which direction the reaction force is, unless you deduce it from the ω equation which you already have)

    now apply conservation of energy :smile:
  4. Jun 8, 2012 #3
    Hi tiny-tim! Thanks for answering.

    I was confused, because if I applied angular momentum conservation I would get a different result of applying linear momentum conservation. What I was missing and later found (which you confirmed) is that I couldn't apply the latter because I don't know the reaction force (initially I thought that it was directed along the bar, but I now I know it isn't.)

    Thank you
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