1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

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