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I read a few times that a knot can reduce the strenght of a rope, but I can't understand why this happens. Can someone explain me what happens to a rope tied with a generic knot and stretched? Is there a way to calculate the reduction of resistance from the form of the knot?

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A knot always requires the rope in the knot to be curved. This increases the stress on the outside of the curved bit of rope, and decreases the stress in the inside. This increase in the stress in a knot means the rope breaks at a lower overall stress than a straight rope would.

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Interesting answer. Since you have a feeling for everyday problems, how would you respond to this question: physics.stackexchange.com/questions/23485. No answer received satisfactory support there. –  Pygmalion Apr 24 '12 at 8:40
    
I think it's always dangerous to give an answer that seems plausible rather than one you are fairly sure is correct. Apart from anything else you risk looking a twit :-) A plausible answer to 23485 is simply that the corners experience more deformation as pages are turned. The fact they splay seems an obvious consequence of the when you open a book you bend the corners in opposite directions. However I don't know if anyone has ever actually tested this idea. My own experience is that I'm careful not to open pages at the corners, and my books don't have dog ears. –  John Rennie Apr 24 '12 at 8:46
    
OK, thanks, this is an interesting piece of advice. –  Pygmalion Apr 24 '12 at 8:55
    
Seems a good answer, thanks! –  MFH Apr 24 '12 at 9:56

This subject gives some nice research papers! For a comparison of the effect of the different knots on the weekening of the rope see http://iopscience.iop.org/1367-2630/3/1/310/fulltext/ (and have a look at the video showing the knot breaking the spagetti!)

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Simply amazing! Thanks. –  MFH Apr 24 '12 at 16:24

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