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FBD of Flexible and Rigid Body Coupling

  1. Nov 18, 2014 #1
    Problem:

    I am having a bit of problem with the signs on the force balance with the constraining force. The system is:

    1.jpg


    Attempt:

    I drew the FBD with the constraining force for the lumped mass:

    2.jpg
    Then I tried to solve for the constraint force in terms of the spring and damper:

    3.jpg

    For the constraint force, Q(t), I end up getting:

    Q(t) = k[u(L,t) - y] + c[(du/dt) - (dy/dt)]

    The answer that the professor gives has the signs changed on the k and c terms. Where am I going wrong?

    Thank you!
     

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  2. jcsd
  3. Nov 18, 2014 #2
    The expression you wrote for Q(t) is positive when u(L,t)>y And the same for derivatives. Now look at your FBD; it shows positive Q to the let. There is your sign error.
     
  4. Nov 22, 2014 #3
    Thank you Dr.D, that makes sense now!
     
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