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Homework Help: Two pulley problem

  1. Oct 30, 2008 #1
    1. The problem statement, all variables and given/known data

    A pulley hangs from the ceiling. A string is run over the pulley and one end is attached to the ceiling, while a 10 lb weight hangs from the other end. In between the first pulley and the end of the string that is attacked to the ceiling rest another pulley on the string with a 5 lb weight hanging from it, as in the diagram below. Find the tensions of the various segments of string, in terms of applied force.


    2. Relevant equations

    T1= ?
    T2= ?
    T3= ?
    T4= ?
    W1= 10lbs.
    W2= 5lbs.

    This is the section I'm having the most trouble with. Which tensions are part of which system? Does T1+T2+T3=15pounds or 5 pounds, does T4 = the weight of the whole system, or only part of it?

    3. The attempt at a solution

    I can't begin to solve the problem without being able to identify all the systems. Also, I'm not sure how to draw the force diagrams without the systems. Here are some things I think I know about the problem:

    There is no normal force counteracting the weights from underneath. All normal forces will counteract the forces operating against them in the system that is suspended from them at the points attached to the ceiling.

    so, we have Fn1 at the leftmost ceiling-mounted point, and Fn2 at the rightmost ceiling-mounted point. So, we have a force diagram for each of these with an N axis. The magnitude of the vector for normal force is given as the equal and opposite force of the system hanging from the string.

    Other than this, the only thing I'm fairly sure of is that the 10lb weight exerts 10lbs of force downward on the right side of the ceiling mounted pulley, and 10lbs upward on the left side of the pulley.

    Someone please help me out, this is the first problem I've seen like this. Thanks.
  2. jcsd
  3. Oct 30, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Well... not exactly. The 10lb weight would exert 10lbs of force on the rope if there were no acceleration, but that's not the case here. You have to solve for the tension in the rope.

    Identify the forces acting on each mass and apply Newton's 2nd law. Combining the two equations (one for each mass) will allow you to solve for the acceleration and rope tension. Hint: The tension is the same throughout the rope.
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