An idealized model for a rope is a single long chain of molecules held together by intramolecular forces. Suppose we fix the left end of the rope and apply a force F on the right end. The rightmost molecule will accelerate due to this force and the distance between this molecule and its adjacent neighbor will increase that will increase the intramolecular force acting upon it and it will be pulled leftwards. At the same time, this rightmost molecule will pull this neighbor towards the right ( due to increased intramolecular distance or action-reaction law ). Similarly, we can extend this analogy up to the leftmost molecule. This reasoning implies that every molecule in the rope is moving. But in reality, we see no apparent movement ( unless the rope breaks ). What is wrong with the above logic and what is actually happening at the molecular level when we fix one end of the rope and pull it on the other? I am very confused about how the Tension develops in ropes, strings etc.

  • $\begingroup$ You do see every molecule in the rope moving. In particular, the whole rope is moving. Since the rope is made up of its molecules, this is the same as every molecule moving. You can also see it stretch - this is the macroscopic result of all its molecules moving a bit further apart from each other. $\endgroup$ Commented Jan 9, 2018 at 7:17

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The logic is fine. But there is a range of how elastic various strings are. For a mostly inextensible string, the forces that arise from the increased distance rise very dramatically with distance. In this way, microscopic motion quickly adds to sufficient force to resist the pull.


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