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When will a string break?

  1. Oct 3, 2012 #1
    I don't know much about physics but I know some math.
    So say I want to set up an experiment (hopefully easily reproducible) where I have a string (I can find out what I like about the string) tied to a spot above the ground. It is hanging straight down. Say I tie a zip lock back to it and I want to figure out what the maximum weight is that I can put into the bag without the string breaking.
    I guess I could repeat this experiment over and over with varying string lengths and see what the function of the weight is with respect to the weight (hopefully there's some meaningful function). However I do not have the resources currently to reproduce this experiment over and over again.
    Does anyone know of an equation that models what I'm trying to do?
    I need an approximate weight not an exact weight.
     
  2. jcsd
  3. Oct 3, 2012 #2
    Yeah tie weights to the string until it breaks, and that's the maximum force the string will hold before breaking. Done. The length of the string doesn't matter.
     
  4. Oct 3, 2012 #3
    I suppose it would depend on the tensile strength of the string, which would probably have several factors - what the string is made of, how the string is made (the ply of the string etc), the thickness of each ply etc. I am not aware of any equation that would work it out for you, but if you search for 'tensile strength' you may have more luck
     
  5. Oct 3, 2012 #4

    cjl

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    In pretty much every case, it should be completely independent of string length.
     
  6. Oct 3, 2012 #5
    Yes you can search for "yield strength" and "ultimate tensile strength".
    See for example:
    http://www.instron.us/wa/applications/test_types/tension/default.aspx
    http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Tensile.htm

    If you double the area (for example if you take a double string) then you double the max. weight.

    And if you know the material then you may be able to find a theoretical value from which you can work it out for yourself, for example:
    http://www.ides.com/generics/polyester/polyester_typical_properties.htm

    1 Pa = 1N/m2
    F=mg with F is force in Newton [N], m is mass in kg and g is gravitational acceleration = ca.9.8 m/s2
     
    Last edited: Oct 3, 2012
  7. Oct 3, 2012 #6
    The only thing about strings is that you have to add the weights gently.

    If you drop a large weight, tied to a string or drop one into a pan supported by a string you are applying impulsive forces that can be up to double the weight.

    Crane drivers have to be very careful to avoid this situation. They call them snatch forces.
     
  8. Oct 3, 2012 #7
    I've tested much cordage over the years. Often the package will list a tensile strength, which often is the smallest number you might expect, 2 or 3 sigma below the mean. You will also find that your knot is the weak point, and that the strength of the knot is also highly variable. All this uncertainty is the reason that most codes require a safety factor of 5, but for some applications 8 or 10.
     
  9. Oct 3, 2012 #8
    I think it has something to do with Youngs modulus although I could be wrong. Basically the string (or any material) will stretch to a point and snap. Youngs modulus is a linear equation relating the tensile stress to the strain. Use wikipedia for more info, I'm rusty for explaining it detail.
     
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