Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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!


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


    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


    User Avatar
    Gold Member

    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


    User Avatar
    Gold Member

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook