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: System of pulleys

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data

    Mass M1 and M2 are connected to a system of strings and pulleys as shown. The strings are massless and inextensible and the pulleys are massless and frictionless. Find the acceleration of M1.

    (image attached.
    just to clarify the image, the upper left pulley is fixed to the ceiling and the bottom center black box is a fixed point on the floor. Imagine everything is perfectly centered as my skills on Paint are not so great)

    2. Relevant equations


    3. The attempt at a solution

    So, I do my FBD's for the 2 masses. Straightforward: gravity pointing down, tension pointing up.

    It starts to get tricky on the pulley I labeled P.

    Clearly, it will start to accelerate at some point. So, my FBD on it was T1 pointing up where T1 is the tension of the string due to M1. For the force pointing down, I said there would have to be at least 2*T2 because the first one comes from the mass M2 and the other comes from the "normal" force of the stationary hook (my black box).

    The problem is the pulley is massless and I'm not sure how to treat it.

    So I said treated the second pulley as a system. So it would have a mass of m2, with forces T1 pointing up, and 2*T2 pointing down.

    Now, I have 3 equations, 3 unkowns. So I solve.

    The problem was taken from Kleppner and Kolenkow, Q2.12 and the hint is : if M1=M2, then a=g/5.

    And I get a=-g/2. So there must be something wrong with the way I treated the pulley.

    Attached Files:

  2. jcsd
  3. Oct 13, 2008 #2


    User Avatar
    Homework Helper

    Hi thejinx0r,

    What did you get for these two equations? I'm thinking you probably got these incorrect, but if you don't show your work I can't be sure.

    I don't believe this is correct. It's massless, so it has a mass of zero. This shows that

    T1 - 2 T2 = 0

    g/5 looks correct to me for identical masses.
  4. Oct 31, 2008 #3
    I just figured this problem out, literally 5 minutes before it was due.

    You can a: Set up a system of a equations of the Height of the Pulley (attached to M2), and compare it to the height of M2 itself.

    Or you can just use your intuition and realize that for every 1 meter that M1 moves, M2 moves 2 meters, meaning that A1 = A2/2 or 2 A1 = A2
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook