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What exactly is tension?

  1. Mar 4, 2012 #1
    I am asked by my student this seemingly easy question.

    I searched on the net on got two answers.
    1) tension is the pulling force exerted by a string, cable, chain, or similar solid object on another object
    2) tension is the pulling force in a material under stretching

    Picture a rope suspending a weight....

    My personal opinion is that,
    Tension is defined as the 'Force on rope by weight'
    and because 'Force on weight by rope' equals numerically to 'Force on rope by weight', we loosely call both 'tension'.

    Is that right?
  2. jcsd
  3. Mar 4, 2012 #2
    The language on this is confusing; but I think '2' is the better definition. Tension is not the force on the rope by the weight---thats just gravitational force. Tension is the resulting internal force in the rope, and from the rope onto the weight.

    It sounds like you're not a native english speaker, but people often use the phrase 'under tension' (e.g. the rope/beam/table is 'under tension') --- which means that it is in a situation in which there is an internal tension force. In this way, you could say that 'the rope is under tension from the weight'---but this means something slightly different, it means that 'the rope is under tension' because of the weight. The tension force itself would be the reactionary force acting on the weight.
  4. Mar 5, 2012 #3
    Tension is caused by intermolecular forces which are fundamentally electrodynamic in nature. When you pull on something that is bolted down, it pulls back (as opposed to shattering to a million pieces) because the of the material's bonds. This pulling back is what we call tension. Whether the original pull is caused by the object's own weight or the object's own weight plus something else is irrelevant. All the object sees is some total external force pulling it apart and it resists. For instance, guitar strings are pulled apart by the bridge and tightened screws at top, and they pull pack (instead of snapping or stretching indefinitely) on the bridge and screws. But the weight of the guitar strings does not have much role in creating the initial pulling because they are so light. So the word "tension" actually means three things going on all at once in a given system:
    1) The original external force pulling an object apart (You provide tension to the guitar strings)
    2) The internal state of an object when pulled (The guitar string are under tension)
    3) The response force the object supplies externally against the original force (The guitar string exerts tension on the guitar).

    These multiple meanings can lead to confusion. As usual, its best to use more math and diagrams, and less words to make clear what forces exist in a given problem.
  5. Mar 5, 2012 #4
    I also didn't know how the forces will look like last time. Imagine a box connected to another box by a rope. There will be a backwards on the box from the rope (in front) and a forward force on the box at the back. These two forces are equal and opposite.
  6. Mar 7, 2012 #5

    Andrew Mason

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    Tension, of course, results from a whole lot of competing electromagnetic and gravitational forces. It is a macroscopic concept. It does not really exist at the atomic level: if you are looking at those forces at the atomic level, then you are not going to need to bother about "tension".

    I would say that tension is what is left over you have accounted for the net force ie. the mass x net acceleration. In the interaction of macroscopic bodies you have the net force on each body + a lot of tensions within the bodies (including bodies that connect other bodies such as ropes).

    Rope tension can be confusing because it is necessarily the result of (at least) two bodies applying force to each other, and very often the other body is ignored. If you are looking only at one body and the tension force of the rope, you are only looking at part of the picture.

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