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Homework Help: Acceleration of a system with three blocks

  1. Jul 13, 2013 #1
    1. The problem statement, all variables and given/known data

    What is the acceleration of this system?

    2. Relevant equations

    3. The attempt at a solution
    I tried the following:

    Let blocks with mass 9kg, 4kg and 6kg be m9, m4, m6
    This is the free body diagrams I drawn:


    Then I applied Newton's 2nd Law:
    F = ma

    (For m9) mg-TR=9a
    (For m4) TR-TL=4a
    (For m6) TL-w*sin(30)=6a

    I solved for a and it didn't gave the right answer.
    What I am doing wrong?
  2. jcsd
  3. Jul 13, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor


    What about gravity?


    Gravity applies to m4 just like it does to m6.
  4. Jul 13, 2013 #3
    I didn't included that because as m4 is "in the middle" I thought it wasn't necessary but it seems I was wrong. Now it gives the right acceleration.
    Thank you! :biggrin:
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