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I have been learning about tension and was doing some reading on trying to explain these forces at the intermolecular level and later also trying to see how it fits in view of Newton's third law- I did come to certain conclusions but I am not sure if I am understanding them correctly.

As much as I have understood, I gather that on applying a force on either side of a string the molecules in the string move away from their mean positions and that tension develops as an internal force to try to bring the molecules back to their mean position. Is that a correct explanation?

WRT Newton's third law of motion, consider a block attached to a pulley. When we draw free body diagrams for such systems, we usually indicate tension in two directions on the string. Am I right if I understood that because the block pulls on the string, tension develops in the string as a reaction force - acting on the block(but then how will tension restore the molecules in the rope back to their mean positions ?) and in the same diagram since the pulley exerts a force on the string and a reaction force develops which is the tension in the opposite direction. But as action and reaction forces should be equal, according to my explanation the same string (let's say massless) will be experiencing different tensions?

What have I understood wrong?

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  • $\begingroup$ physics.stackexchange.com/a/583319/265226 $\endgroup$
    – dnaik
    Oct 29, 2020 at 5:42
  • $\begingroup$ Thanks for sharing this. It was helpful. But again, I did read somewhere that tension is the restorative force. Is there anything wrong with that understanding? $\endgroup$
    – Sevfeynn
    Oct 29, 2020 at 11:13

2 Answers 2

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Assuming a massless frictionless pulley, the tension will be the same throughout the string. Suppose a 1 Newton force is applied to the string, its tension will increase to 1 Newton so that it is applying the 1 Newton of force to the block. The string will have 1 Newton of tension throughout the entire length. The force will pull one direction and the block's pull from resistance to moving will be in the opposite direction. So you have equal and opposite forces per Newton's third law of motion, applied by the string which must have equal tension.

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  • $\begingroup$ The block's resistance will be due to inertia, right? Also is the opposite force you mentioned that the string applies- is it tension or just a reaction force in itself? $\endgroup$
    – Sevfeynn
    Oct 29, 2020 at 11:20
  • $\begingroup$ Inertia and friction cause the block's resistance. Once tight the string can be considered to be reaction to the force at one end and action force on the block at the other end. $\endgroup$ Oct 29, 2020 at 12:06
  • $\begingroup$ I see. Thanks for the clarification. Could you also tell me if my understanding of tension as a restorative force is correct fundamentally? I did read a detailed answer from the previous comment on my question but it didn't mention any view of tension as a restorative force. $\endgroup$
    – Sevfeynn
    Oct 29, 2020 at 16:22
  • $\begingroup$ When the tension stretches the string past its relaxed length it can have a restorative force. This means it may continue pulling, until it reaches its relaxed length, after the force stops. For a better explanation you should read the second paragraph here; en.wikipedia.org/wiki/Tension_(physics) $\endgroup$ Oct 29, 2020 at 16:51
  • $\begingroup$ That helped a lot in clearing up the concept. Thanks for the response! $\endgroup$
    – Sevfeynn
    Oct 30, 2020 at 14:19
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The action/reaction pair occurs at each place inside the string where one layer of molecules pulls or pushes on an adjacent layer of molecules. Consider two adjacent layers $A$ and $B$:

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If the string is in tension in the vertical direction then layer $A$ exerts up upwards force $F$ on layer $B$, and layer $B$ exerts a downwards force of the same size on layer $A$.

At the bottom of the string, therefore (for a vertical string in tension), the final layer is being pulled upwards by the layer above. And at the top of the string, the uppermost layer is being pulled downwards by the layer below it. If the bottom layer of the string isn't accelerating then it must be being pulled downwards by something else. e.g. it is a stuck to a heavy weight being pulled down by gravity. If the top layer of the string isn't accelerating then it must be being pulled upwards by something else. e.g. it is stuck to the ceiling.

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