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Homework Help: Yank and Pull - String Tension Problem

  1. Feb 2, 2008 #1
    [SOLVED] Yank and Pull - String Tension Problem

    1. The problem statement, all variables and given/known data

    A heavy object (of mass ~10kg) hangs from a hook by a light string. There is another light string of equal configuration (mass, length, etc) hanging down from the object. Pulling on the string slowly will cause the top string to rupture and the object will fall. However, yanking on the string will cause the bottom string to break and the object will remain suspended by the top string.

    2. Relevant equations

    a = F/m (all I have so far)

    3. The attempt at a solution

    My professor mentioned that the following could be involved in the explanation of the solution: kinematics, newton's second law, the fact that macroscopic objects undergo deformation to create a force, and breaking tension. Here's what I got so far:


    (Sorry for the poor quality, my camera is not that good)

    I drew separate graphs for the yanking and the pulling. In the yanking graph, the tension of the lower string reaches breaking tension first. In the pulling graph, the tension in both strings increase at an equal rate, until the top string reaches the breaking tension first. The top string has more initial tension than the bottom string because the object is hanging on it, and it's weight is causing the tension.

    Thanks for the help!
  2. jcsd
  3. Feb 2, 2008 #2


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    Gold Member

    Your graphs look very good. For the 'slow pull' case, you're on the right track, but you should talk a bit more how Newton 1 comes into play here (it's more than the weight that causes the upper string tension as the lower string is pulled). For the second case, it is indeed Newton 2 in effect, but you've got to explain more about the deformations of the non-rigid strings. And the magnitude of the acceleration under the impulsive force which acts over a very short time period.
    Last edited: Feb 3, 2008
  4. Feb 7, 2008 #3
    Alright, I think I figured it out, thanks!
  5. Feb 8, 2008 #4
    how does kinematics play into this magical phenomenon?
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