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Homework Help: Dependent motion problem with ropes

  1. Mar 1, 2012 #1
    1. The problem statement, all variables and given/known data
    I attached the problem with my attempt.

    The problem I have is that I have two unknowns. I think my set up is wrong. Does anyone know where to put the datum/set up this question?

    Thanks!
     

    Attached Files:

  2. jcsd
  3. Mar 1, 2012 #2

    gneill

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    Staff: Mentor

    What are your two unknowns? Remember, you're looking for dependencies between certain length changes.
     
  4. Mar 1, 2012 #3
    I know it's A and C but I don't know how to get rid of B :S
     
  5. Mar 1, 2012 #4

    gneill

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    Staff: Mentor

    Here's your drawing with the distances labeled.

    attachment.php?attachmentid=44622&stc=1&d=1330662443.gif

    Your equation is:

    L = SA - SB + 2SC

    What you're looking for is the rate of change of SA with respect to a rate of change of SC. So rearrange your formula to have SA on the left and the rest on the right. Now, are you familiar with calculus (derivatives)?
     

    Attached Files:

  6. Mar 1, 2012 #5
    Oh... I see, so if you take the derivative Sb and L are eliminated since they are constant?
     
  7. Mar 1, 2012 #6

    gneill

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    Staff: Mentor

    Yup.
     
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