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Tension in cables help please!

  1. Nov 14, 2007 #1
    1. The problem statement, all variables and given/known data

    Hi, I'm actually teaching myself some basic mechanics/physics at the moment and I'm having difficulty getting my head around all the ideas.

    Basically I've been reading up on a cables problem where a flexible, uniform cable of length L has its two ends held at a distance 'd' apart at the same height/level.

    There is a corresponding graph which shows the relation between the tension and the length. The tension is large when L is small, and starts decreasing and attaining a minimum before increasing and becoming large again as L gets bigger.

    Can anyone explain to me why this is so? Many thanks!!!

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 14, 2007 #2
    Was this in a book or online? If online, could you please give me the link?
  4. Nov 14, 2007 #3
    Hi, this was in a book, sorry!
  5. Nov 14, 2007 #4
    Is 'd' constant?
  6. Nov 14, 2007 #5
    Yes, the distance is constant
  7. Nov 14, 2007 #6
    Well, if you're treating the cable like it has mass, I think the tension would be coming from the increasing force of gravity.

    EDIT: Also, are we speaking merely of magnitude here?
    Last edited: Nov 14, 2007
  8. Nov 14, 2007 #7
    hmm i think so, im not too sure actually.

    But why is tension large when the length is L = 1? and then it decreases? I can understand that if the cable gets longer, then there is more force needed in holding it up.
  9. Nov 14, 2007 #8
    Because initially the cable is taught, that is, when L = d. When L increases just a little, T shoots down very fast because the length can't really "feel" the force. It's like if you're holding a length of string tight between two hands and then you move them in just a little bit, you feel the tension in the drop off incredibly.
  10. Nov 14, 2007 #9
    Oh I see! that makes sense now :smile:

    that's brilliant thank you! Thanks for your help :biggrin:
  11. Nov 14, 2007 #10
    Of course.
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