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Breaking strength of a piece of material

  1. Dec 14, 2005 #1

    Pengwuino

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    Ok so I was wondering something... in which situation, if any, is a piece of material more likely to break in the real world...

    1) A piece of rope or wire that is being pulled on by two people in opposite directions.

    2) A piece of rope or wire being pulled but the other side is a looped around a stationary poll

    Now if both of the pieces of rope are just closed loops... I assume that they'll both break under the same conditions since (if the two people use equal force) the normal force by the poll will simply be equivalent to the 2nd person 's pulling strenght from 1).

    Am i right :)
     
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  3. Dec 15, 2005 #2

    FredGarvin

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    Yes. You are correct. In the two person scenario, the one is the reaction force in the opposite direction. If the two people are pulling with diifferent forces. the lesser force is going to be what the rope/wire sees. The excess force will go towards accelerating the system (both people moving).
     
  4. Dec 15, 2005 #3

    vanesch

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    Although, when the wire is considered a 1-dim object, you are correct of course that the two situations are equivalent, if you take into account the finite thickness of the rope, then I'd imagine you get a much uglier distribution of internal stress where the rope is tied up to the pole. In that case, you are more likely to (locally, somewhere within the cross section of the wire) to go beyond the maximally allowed stress and break the rope, then when two people nicely hold the rope in their hands and you get a nice, uniform distribution of stress across the section of the rope.
     
  5. Dec 15, 2005 #4

    Danger

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    To extrapolate Vanesch's answer a tad...
    If you decrease the diameter of the pole, you'll increase the local stress on the rope. You can see that if you get it down to a few molecules of thickness, it'll slice right through it like a knife.
     
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