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Homework Help: Three masses suspended on pulley

  1. Sep 24, 2010 #1
    1. The problem statement, all variables and given/known data
    A pulley is massless and frictionless. 3 kg, 2 kg, and 6 kg masses are suspended on a pulley.

    (a)What is the tension T1 in the string between the two blocks on the left-hand side of the pulley?

    (b)What is the magnitude of the acceleration of the lower left-hand block?



    2. Relevant equations
    I solved for (a) using the equations:
    T2-m1g=m1a
    m2g+T1-T2=m2a
    m3g-T1=m3a


    3. The attempt at a solution

    the combining those equations:
    -m1g+m2g+m3g=m1a+m2a+m3a

    then I solved for a
    -29.4+19.6+58.8=11a
    a=4.45455 m/s^2

    then plugged into T1=m3g-m3a and got T1=32.0727N (right

    for part (b) I thought that I already had a since I needed it to slove for T1, but it is wrong...

    So does the lower left-hand block have a different acceleration?


    Also, I tried putting in -4.45455 m/s^2 since m3 is 6kg and ways more than m1 and m2 combined... also didn't work
     
  2. jcsd
  3. Sep 24, 2010 #2

    PhanthomJay

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    You have a signage error in your 2nd relevant equation....the weight acts down, so it should have a minus sign in front of it. Redo your calc for the acceleration and value of T1.
     
  4. Sep 24, 2010 #3
    In front of the whole equation or just infront of all m2?
     
  5. Sep 24, 2010 #4

    PhanthomJay

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    Well, just in front of m2....on the left side of the equation, the weight acts down, and T2 acts down , so both those terms are minus; T1 acts up, so that's plus; on the right side of the equation, the acceleration is up, so that's a plus sign.
     
  6. Sep 24, 2010 #5
    What about m2 in front of acceleration?
     
  7. Sep 24, 2010 #6

    PhanthomJay

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    Repeat...positive.
     
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