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Homework Help: Analytic mechanic, disk and rod

  1. Apr 23, 2015 #1
    < Mentor Note -- thread moved to HH from the technical engineering forums, so no HH Template is shown >

    A rod rolls without creep. And the disk rolls without creep on Q. The rod can just moves on y. Which is the relation among Va and Vohm?
    Va= velocity in A
    My resolution:
    in Q we know that velocity is zero.Q is also the instant rotation center ( disk).so the P point ( disk) is the fastest. How to bound speed in
    ##\Omega## and Va?
    I mean, just seeing the picture, without using the fondant formula of rigid cinematic. I could think it's like the half of Va.
    Ps.it's my first article and my english isn't so good, I hope you'll understand anyway.

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    Last edited by a moderator: Apr 23, 2015
  2. jcsd
  3. Apr 23, 2015 #2


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    Gold Member

    Your picture does not define point P very well. Is it the top of the disk, or is it the tangent point? If it is the tangent point (as I suspect), it is "not the fastest point." The "fastest point" is directly on the top.

    This is a kinematics problem, and as such, you need to define some more variables and write the suitable relations among them. Define a variable name for the height of point A, a variable for the distance from A to the tangent point (P?), and the angle of the bar with respect to the horizontal. Then write the equations defining the closed loop from the origin to A to P to the center of the disk to Q and back to the origin; there will be two such equations. These can be solved for what you need.
  4. Apr 24, 2015 #3
    Thanks for reply,
    Point P is tangent, the question that teacher asked to me was "give me a qualitative consideration of velocity in ##\Omega##"
    For example the direction, and approximately the entity like "it's 1/3...2/3 of Va". The problem gives me just Va and nothing else. I tried to set problem analytically but professor said no.
  5. Apr 24, 2015 #4


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    Afraid I can't help you if a proper mathematical formulation is not allowed.
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