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Homework Help: Moments and couples - Very stuck

  1. Apr 18, 2004 #1
    I am having trouble getting to grips with moments and couples and i have got stuck on the following question:

    "A uniform horizontal rod AB is 120cm long and weighs 250N. It is supported by two veritcal wires, one 10cm from A and the other 20cm from B. Loads of 100N and 350N hang from the rod at distances of 50cm and 100cm respectively from A. Calculate the tension in each wire"

    I'm not quite sure what 'tension' means exactly but I am guessing that I have to use the formula (force x distance) to somehow work the problem out. Initially, can one can say that the tension from the weight of the rod in each wire is the (distance from centre of mass that wire is attached x 250N) which would mean that the tension in the first wire is equal to (0.5m x 250N) as the wire is attached 50cm from the centre of mass. Therefore the tension from the weight of the rod in the other wire is equal to (0.4m x 250N). However it is at this point that i get stuck!! If any of the above is wrong please tell me and if anyone could give me an idea where to go, that would be great.

  2. jcsd
  3. Apr 18, 2004 #2

    Doc Al

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    Staff: Mentor


    Tension just means the force that each wire must exert to support the rod plus loads.

    Start by identifying all the forces acting on the rod. I count 5 forces: the tensions in each wire, the two loads, the weight of the rod itself (which acts through the center of mass). Draw a diagram.

    For equilbrium to hold:
    (1) The sum of the forces must equal zero. (What does that tell you about T1 + T2?)
    (2) The sum of the torques (about any axis) must equal zero. I recommend using the point where a wire attaches to the rod as an axis--find the sum of torques about that point.
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