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Homework Help: Double Atwood Machine Question

  1. Oct 4, 2011 #1
    1. The problem statement, all variables and given/known data

    I've tried and tried and can't solve this to look like this equation. Please Help!

    http://i233.photobucket.com/albums/ee237/biggyjoe210/IMAG0175.jpg

    1. Show that the Acceleration of Mass A is given by : (look at picture)

    IMAG0175.jpg

    2. Relevant equations

    F = ma

    3. The attempt at a solution

    So far all I have is a = g(mA-mB)/(mA+mB)

    Im lost
     
  2. jcsd
  3. Oct 5, 2011 #2
    Did you draw any free-body diagrams?
     
  4. Oct 5, 2011 #3
    Yeah all have tension going up and mg going down except the pulley which has tension up and tension down as well *** mg. The tension of string attaching a and b are the same and c and d are the same.
     
  5. Oct 5, 2011 #4
    Does the following look like I'm on the right track? Six equations in six unknowns.
     

    Attached Files:

  6. Oct 5, 2011 #5
    Yeah, I have that down. I solved for the smaller system(mA, mB, mD) for acceleration and tension. I'm guessing x is acceleration. If so thats exactly what I have. I can't get the algebra right or something when I solve for a1 and a2(the two separate accelerations of the strings). I keep getting close to the given formula but I'm messing up somewhere.
     
  7. Oct 7, 2011 #6
    hey do you go to tech??
    m having problem with the same question...did you solved it??
     
  8. Oct 7, 2011 #7
    the trick is displace one body and see the motion of other bodies eg.if u displace the "Mc" body then the other two bodies(Ma and Mb) will as a whole go down but one of them will go up and other down relative to each other.then draw the free body diagram on each.NOTE: tension in the Mc body will be 2times that of ropes of other two.then write the equations involving T and g.gud luck
     
  9. Oct 7, 2011 #8
    I'm also at Tech. I get some of it, but there are like 3 parts to the final equation that I'm missing. I'm still not exactly sure on how to do this.
     
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