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Foucault pendulum string medium

  1. Feb 15, 2013 #1
    This question is literally driving me crazy. How does the String or rope or whatever is used in a Foucault pendulum not twist with the rotation of the earth (because the rope is attached to bases that are attached to the ground).

    Actually if the bases are attached to the ground, that should be the ENTIRE thing is rotating so how are we able to see the effect?

    please if you're going to explain it dumb it down to the most reduced form of an explanation visually because this problem is on my nerves. Thank you in advance!
  2. jcsd
  3. Feb 15, 2013 #2


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    Science Advisor

    What should prevent it from untwisting? With rigid Foucault pendulums you obviously need a bearing in the attachment, that gives the vertical axis free.
  4. Feb 15, 2013 #3
    I'm sorry i'm not quite clear on the things you described. If it twists it should mess up the motion of the swing, we know that it DOESNt twist..Why doesnt the rope twist?
  5. Feb 15, 2013 #4


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    Gold Member

    To think about that case start with a simplified setup, then add features back to it.

    In this case start with a pendulum at one of the Earth's poles, say, the South pole. Also consider first a non-swinging pendulum.

    So you have that pendulum bob, suspended above the south pole. That pendulum bob is co-rotating with the Earth. The wire of the non-swinging pendulum will not twist/untwist because the bob is rotating around its own axis, co-rotating with the Earth.

    Next step: such a pendulum on any latitude. Let's call that a 'latitudinal pendulum'. Such a pendulum is circumnavigating the Earth. The thing is: just as the polar pendulum bob is rotating on it's own axis, the latitudinal pendulum is rotating on it's own axis. So just as the polar pendulum bob will not twist the wire the latitudinal pendulum will not twist the wire.

    Adding swing makes no difference either, not to the polar pendulum and not to the latitudinal pendulum.

    (As many have pointed out in earlier threads: on a scientific station located right at the south pole, in the winter of 2001, three guys have actually rigged a foucault setup, in a high staircase. Allan Baker recounts it was tough going; that staircase was unheated.)
    Last edited: Feb 15, 2013
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