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Homework Help: Maximum load that a 2mm steel cable can hold.

  1. Nov 23, 2015 #1
    1. The problem statement, all variables and given/known data

    A platform (m = 5,0kg) is suspended from 4 steel cables whose length is 3,0m and diameter 2mm.

    a) How much further will the platform sink if a mass of 50kg is placed on the platform?

    b) What is the maximum load that the wires can hold before breaking?

    2. Relevant equations

    [tex]\dfrac{\Delta F}{A} = E\dfrac{\Delta L}{L}[/tex]

    3. The attempt at a solution

    Part A was easy enough; simply assume that the wires remain in the elastic zone of deformation and that A doesn't change much and plug in the values. However part B baffles me, and I'm not sure where to start.

    Surely the cross sectional area A doesn't go to zero as the cable breaks(division by zero)? Am I supposed to use calculus to figure out when E changes(doesn't really make sense since E is a constant for elastic situations)? How would I even do that? Does steel have documented values for when a wire of certain L and A starts deforming plastically? I couldn't find any.

    I don't have the right answers for this problem, so I can't just try one of the above methods to find out if it was the correct one, not that any of them make any sense to me. If someone could point me in the right direction, that would be great.
  2. jcsd
  3. Nov 23, 2015 #2


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    Just as you must be given E to solve part a, you must be given the ultimate breaking stress of the cable to solve part b. The ultimate breaking stress usually assumes no change in area.
  4. Nov 23, 2015 #3
    Alright, so I used Wolfram alpha to find out the ultimate breaking stress of steel and calculated the maximum force the 4 wires could take using [tex]F = pA[/tex] This gave me a value of 10 327.04N and a maximum mass of [tex]m = \dfrac{F}{g} = 1.053\cdot 10^3 kg[/tex] Seems realistic enough, but I cant be sure until Thursday when I have to turn these problems in.
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