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Homework Help: Torque/acceleration with masses on pulley

  1. Oct 28, 2004 #1
    Wer are given two objects of mass m1 = .20 kg and m2 = .67 kg, which are connected by a massless cord that is wrapped around a uniform disk of mass M = 0.50 kg and radius R = 0.12 m. The disk can rotate without friction about a fixed horizontal axis through its center; the cord cannot slip on the disk. The system is released from rest.

    (a) Find the magnitude of the acceleration of the blocks____m/s2
    (b) Find the tension T1 in the cord at the left____ N
    (c) Find the tension T2 in the cord at the right____N

    well so far i have three equations - clockwise motion
    1) T2*R - T1*R = I*alpha = net torque where alpha = a/R for pulley
    2) T2 - m2*g = m2*a for mass on pulley
    3) -T1 + m1*g = m1*a for mass on pulley

    Then I solve 2) and 3) for respective tensions T2, T1 and substitute into 1) and solve for "a" I mean, am I close here or no? Cuz I don't wanna write all of my solving out if I'm at the wrong starting point anyway...kinda messy.... but I'm getting an answer of

    a = - 7.429m/s2 but I get it as wrong. Don't have actual answer. Negative? So I'm guessing goes in opposite direction...???

    Without correct a can't solve for T1 & T2 ... for answer a) since magnitude is being asked, even if acceleration is negative answer is +ve value.

    I'd really appreciate some input on this questions! Thanks in advance!

    Attached Files:

  2. jcsd
  3. Oct 28, 2004 #2
    can anyone show me how to include a figure/graph in my question area instead of putting in as an attachment??
  4. Oct 29, 2004 #3
    can anyone help on this one?

    surely there is someone that can help out with this problem? i understand the concept of the torque acting with the forces, but i am unsure as to how i can relate this with the tensions! help anyone??? i would really appreciate it!!!
  5. Oct 29, 2004 #4

    Doc Al

    User Avatar

    Staff: Mentor

    sign problems

    You are mixing up the signs for the acceleration. You need to use a consistent sign convention. If you take up as positive, then the acceleration of m1 is up (+a), but the acceleration of m2 is down (-a). Rewrite equations 2 and 3.
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