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Thought Experiment on perfect strings

  1. Jul 29, 2009 #1
    I've recently had this thought experiment, and wanted to share it.
    My HS physics teacher had talked about "perfect" strings that had no mass, does not stretch, and is completely flexible when describing physics experiments. Using such a hypothetical string means it does not interfere with the experiment. It has no mass to alter momentum, does not stretch under stress and smoothly wraps around pulleys. In this thought experiment I adopted this.

    Take a length of material that, much like the perfect string, has no mass and does not bend under stress. Balance or suspend it across a fulcrum, so that it appears like a see saw or teeter-totter. Like so.

    Code (Text):

    If its initial position is perfectly horizontal, and someone pushes down on one end , the other end rises up, correct? The answer, of course, is yes.

    However the question I have been asking myself is, how much time passes before the other end rises up? As the material does not bend and has no mass for momentum be an issue, as near as I can determine myself, it rises up simultaneously as the opposite end is pushed down, or t=0.

    Now, extend the length of the material to a light second (299,792,458 m). Again, push down on one of the ends and simultaneously send a photon/s down the length. The time for the photon/s to reach the opposite end (assuming vaccuum) is t=1 sec, while the time for the opposite end to rise remains the same, t=0. I cannot see how the time for the opposite end to move is anything but t=0 regardless of its length, or by how much force is applied. t=0.

    If an observer was stationed at each end, they each would be aware when the opposite end was moved prior to the photon/s arriving. Simple, yet seems profound and yet almost not true.

  2. jcsd
  3. Jul 29, 2009 #2


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    Congratulations, you've neatly outlined the argument for why there are no perfectly rigid bodies in the real world :biggrin: Your ideal material could not really exist.

    When you push down on one end of the bar, the force is transmitted along its length by means of electromagnetic forces, which are carried by photons. So there's little difference between the "messenger photon" you'd send along next to the bar, and the ones that are carrying the "message" that one end has been pushed. Both signals travel at (or in the case of the bar's motion, below) the speed of light. As an observer stationed some distance to the side of the bar, you'd see it bend as the disturbance caused by the initial push propagates along its length. And consequently, the other end of a light-second-long bar would only begin to rise one second (or more) after the initial push.
  4. Jul 29, 2009 #3
    Assuming one could see the other end, and the gravitational field is on a flat surface for the seesaw, it would be an incredible experience. You would hit the ground and still see your friend rising. Good thought experiment
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