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I Musings on waves - possible or already realised?

  1. May 16, 2018 #1
    A couple of idle musings but I hope interesting ones:

    1. If a drop of water lands in a still pond, circular concentric waves spread out. Is it possible to reverse this process with a large circular wave generator such that the waves converge and meet in the middle, and a drop of liquid jumps out of the pool? Has anyone ever demonstrated this?

    2. A guitar string makes a definite pitch because it's constrained by the nut and fret, allowing only waves that divide the string length a whole number of times. Could you make a fixed-pitch musical instrument from circles of metal (similar to the triangle but closed and, obviously, circular)? Would the oscillations of the metal be of a too-high frequency for this to work properly?

    The inspiration for the latter is the electron in a hydrogen atom.
  2. jcsd
  3. May 16, 2018 #2


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    Waves on a water surface are a bit like waves on a circular membrane and there is a lot of info about them, This link shows some good animations. The waves could be excited from the periphery which is what you are referring to, I think. Real waves on a pond would be a combination of several of the modes shown, probably.
    The analogy of vibrations on a circled of wire to the De Broglie wave of a bound electron is something that A level students are often presented with. This link shows a demonstration of the normal modes of vibration of a wire loop. The demo would have been good if you were actually there but the video is pretty poor. But you can see that happens - with a bit of imagination. Note, the vibrations are transverse in the demo (low speed waves). You could have longitudinal waves too but compression waves are a lot faster and the wavelength would be much longer so you would need a very long wire for a standing wave to be established.
  4. May 16, 2018 #3


    Staff: Mentor

  5. May 16, 2018 #4
    Skip to about 00:30.
  6. May 17, 2018 #5


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    You can try submerging partially filled circular vessels, such that the water overflows the rim simultaneously on all sides. Happens sometimes, when submerging the ladle back into the soup pot.

    It can also potentially happen, when you drop something in the center of a circular pool, and then the waves are reflected off the walls, to meet back at the center. In a sense, also right after the object fell in, and the gap it created quickly fills with water from all sides.
    Last edited: May 17, 2018
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