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I don't understand why a rope doesn't untwist when an attempt is made to increase its length. It seems counter-intuitive to me, because untwisting would allow the rope to increase its length, but in practice it does not untwist.

Do the braids spiral-like shape and the friction between the braids prevents this from happening ?

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up vote 6 down vote accepted

Have a look at http://usscouts.org/scoutcraft/BoyScoutRopemaking.pdf

When you make a rope the individual fibres are twisted before you combine them into the rope. It's the "untwisting" force of the individual fibres that locks them together in a rope. You're quite correct that if you try to make a rope without twisting the fibres your rope will just fall to bits.

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+1 for the "Pioneering Merit Badge" :-) –  Alexander Jun 27 '12 at 23:00
    
+1 for cool link –  ja72 Jun 28 '12 at 17:58
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