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Tension and acceleration problem

  1. Mar 18, 2013 #1
    1. Problem statement/ given data
    The weights of the objects are 200N and 300N. The pulleys are essentially frictionless and massless. Pulley P1 has a stationary axle, but pulley P2 is free to move up and down. Find the tensions FT1 and FT2 and the acceleration of each body.

    2. Relevant equations.
    F=ma

    3. Attempt.
    I drew a FBD for each mass with m1 going down with down as the positive and m2 going up with up as the positive direction. I know that in this system the tension for mass 2 is twice the tension of mass 1 and that the acceleration for mass 2 is half of mass 1. I have the answer and the explained solution but I don't know how I'm getting a different answer than they are. Can someone check what im not seeing here? I know if I find the right acceleration I just have to plug that in to find the tensions.

    29mq1c1.jpg

    The answer in the book is a= 1.78m/s^2
     
  2. jcsd
  3. Mar 18, 2013 #2

    TSny

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    It looks to me that your second equation has a sign error.
     
  4. Mar 18, 2013 #3
    For mass 2? Since that mass is going up shouldnt the upwards be the positive direction? So it would be T - mg ?
     
  5. Mar 18, 2013 #4

    TSny

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    Yes, that's right. I should have been clearer. I was referring to the second line of your equations where you wrote for m1: FT1 = m1ay + m1g. Check the signs in this equation.
     
  6. Mar 18, 2013 #5
    Oh I see it now, got it thanks!
     
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