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B How do forces propagate through the chain?

  1. Jun 15, 2016 #1
    If I have a chain suspended from a hook, such that gravity is pulling it downward, how does that gravitational force propagate through the chain?

    What would happen if the gravitational source suddenly disappeared?

    What would happen if the hook suddenly disappeared?

    What I'm interested in, is how the reaction propagates through the chain.
  2. jcsd
  3. Jun 15, 2016 #2


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    It doesn't. It acts on the entire chain all the time.

    This might give you an idea:

  4. Jun 15, 2016 #3
    But even if the gravity acts upon the entire chain all the time, it's still a wave propagating through the chain, such that if the gravitational source should suddenly disappear, the bottom of the chain would be the first to be aware of the disappearance, correct? I would assume therefore that there would be no discernible change in the chain other than a loss of tension, as the wave propagated through it. Neither end of the chain should move because there's no force acting upon it. It's simply a matter of the gravitational wave propagating upward through the chain, and tension disappearing. Or would there be two waves propagating upward through the chain, one for gravity and one for tension?

    On the other hand, if the hook should suddenly disappear, it's the information about the loss of the hook that has to propagate downward through the chain, this time only in the form of a loss of tension. But in this case the information should propagate at something significantly less than the speed of light. But I'm not sure when the information reaches the bottom of the chain, and when it begins to fall. There should be a delay between when the top of the chain begins to fall and when the bottom of the chain begins to fall. But is this only due to tension, or will the information reach the bottom of the chain before that?
  5. Jun 15, 2016 #4


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    Only if the chain is not stretched at all under gravity.. A real chain would have some deformation, and the relaxation would accelerate the bottom slightly.

    That isn't a gravitational wave


    That is basically what the slinky videos show in an exaggerated way (slinky is much more stretchable than a chain).
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