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The Origin of Tension

  1. Apr 4, 2014 #1
    Does the tension force in elastic springs and strings arise inevitably as a consequence of Newton's third law? Or is it due to the electrostatic forces within the material itself? Or are both of these explanations basically the same thing?
  2. jcsd
  3. Apr 4, 2014 #2


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    You've got it - they are the same thing.

    Newton's 3rd law holds "regardless" of how the rope is made, or held together. But in fact almost everything you will ever encounter directly is held together by electromagnetic forces. The exceptions are atomic nuclei, hadrons, and massive objects held together by gravitation - like the earth or moon.
  4. Apr 4, 2014 #3
    But doesn't tension appear only when the spring or string is pulled from both sides?
  5. Apr 4, 2014 #4


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    Yes, but it still works that way.

    You can treat each individual atom in the spring or string as a tiny spring fastened to its neighbors, and subject to Newton's third law. Tension is when these springs are stretched and compression is when those springs are compressed, and you can't do either unless you're pushing/pulling both ends of the object (if you only push/pull at one end, the forces are imbalanced and the object moves away/towards you instead of compressing/stretching). But either way, each individual atom/spring is being pushed/pulled by its neighbors, and exerting equal and opposit forces on them.

    The attachments and forces within and between these 'tiny springs" are the electromagnetic forces you asked about in your original post.
  6. Apr 5, 2014 #5


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    Yes. If the spring / string has no mass you cannot get any tension in it unless something is attached to the other end. You have to be either distorting something, keeping equilibrium with another external force or accelerating it.
    In the same way, you can't 'push' against 'nothing'. Press-ups work as an exercise but Press-downs are useless.
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