FBD of Flexible and Rigid Body Coupling

  • Thread starter ThLiOp
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  • #1
ThLiOp
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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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Answers and Replies

  • #2
Dr.D
2,411
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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.
 
  • #3
ThLiOp
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Thank you Dr.D, that makes sense now!
 

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