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Homework Help: Dynamics Help Needed Relative velocities problem

  1. Sep 30, 2011 #1
    Dynamics Help Needed!! Relative velocities problem

    1. The problem statement, all variables and given/known data

    see link:


    2. Relevant equations

    Can you please help me solve this problem?
    I have 3 unknowns: Va, Vb, and Thetha; but only two equations

    Vb(cos(theta)) - Va(cos30) = 5.6cos70
    Vb(sin(theta)) - Va(sin30) = 5.6sin70

    I need a third equation???!?!

    3. The attempt at a solution

    To get the two above equations, I took into account:

    Vb/a = Vb - Va

    Then I broke each vector into components. I end up with the two above equations. But I don't know where to go from here. I need a third equation either relating theta to Va or Vb, or a cable constraint equation, etc. I need some help.
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Oct 1, 2011 #2
    Re: Dynamics Help Needed!! Relative velocities problem

    Is it right to use the cable constraint equation of: 2Xa+3Xb=constant.
    then taking the derivative to get: 2Va+3Vb=0? Or is that not correct?
  4. Oct 1, 2011 #3

    rude man

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    Homework Helper
    Gold Member

    Re: Dynamics Help Needed!! Relative velocities problem

    How about energy conservation?

    (I haven't done this problem, so this is truly just a suggestion, but I think it would lead to your answer).
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