arfa's musings on Elastic surfaces

Discussion in 'Free Thoughts' started by arfa brane, Feb 3, 2022.

  1. arfa brane call me arf Valued Senior Member

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    Understanding the what, of what happens in the electron diffraction experiments, means I think understanding the difference between an elastic and an inelastic collision; right James?

    In the first case momentum is conserved, in the second it isn't. James can explain to us what happens to it. Right James?

    Although, it means James will have to also explain that any material surface, in physics, is an elastic surface. It's why guitar strings vibrate, for instance.
     
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  3. origin Heading towards oblivion Valued Senior Member

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    No, it has nothing to do with collisions, this is purely a wave phenomena.
    The lattice of the material is deformed, producing heat in an inelastic collision.
    Why would you think all materials have an elastic surface, that is clearly wrong. The only elastic collision I can think of would be a neutron - neutron collision, and I am not sure they are actually elastic.

    I am sure James will have a more detailed and accurate answer than mine, but your questions were pretty simple so I just jumped in.
     
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  5. arfa brane call me arf Valued Senior Member

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    Can you give an example of a surface that is not elastic.

    Just because I'm bored.
     
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  7. exchemist Valued Senior Member

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    The surface of a pond?

    YAWN............
     
  8. arfa brane call me arf Valued Senior Member

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    How is the surface of a pond of water not elastic?

    First hint--your explanation will need to explain why it responds to a sudden impulse the way it does; I assume you can recall high speed photography.

    You will also need to explain why leaves and insects can float on the surface.
     
  9. arfa brane call me arf Valued Senior Member

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    There it is again; I feel like I'm back in high school. Those were the days, huh?
     
  10. exchemist Valued Senior Member

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    No, you will need to explain when you last threw a pebble into an unfrozen pond and it rebounded off the surface elastically. When you've done that, we can talk about surface tension. Maybe.

    If I don't get even more bored in the meantime..................
     
  11. arfa brane call me arf Valued Senior Member

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    You are a science dick, exchemist. The elasticity in the surface is the surface tension.

    Because coins don't usually fall through solid wood doesn't mean you can't find something that will.

    Oh hey, you skipped over the other thing I said, about explaining why a fluid rebounds (responds), like water does when you drop that pebble in . . .

    That's usually called an impulse function, you know, in the time domain.
     
    Last edited: Feb 3, 2022
  12. exchemist Valued Senior Member

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    So, according to you, the pebble rebounds from the water surface, right?
     
  13. arfa brane call me arf Valued Senior Member

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    No, according to physics, there is an elastic response from the surfaces involved. You can ignore the pebble and focus on what the water does. So, what does it do, does it rebound elastically?

    Wait, I know this one.
    But, I'd be absolutely fascinated with your explanation of why the pebble doesn't just slip beneath the surface.
     
  14. arfa brane call me arf Valued Senior Member

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    You might have seen that old film of the Tacoma Narrows bridge collapsing, because of insufficient damping.

    The elastic response in the steel beams approached a critical resonance point--the bridge began to output too much power in its frequency spectrum. It couldn't radiate it away, and eventually a section gave and took the rest with it.

    The wind played it like a violin till it broke all the strings.
     
  15. origin Heading towards oblivion Valued Senior Member

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    Name a material and that will be an example of a surface that is not elastic. An elastic collision would mean that there would be no change in the kinetic energy of the objects before and after a collision.

    Are you saying you didn't do very well in high school?
     
  16. arfa brane call me arf Valued Senior Member

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    So . . . what material would you like to name, then? It has to be material, and not stretch or bend in a way that stores tension which then "unstretches" and "unbends"; there has to be no response to deformation.

    Please tell us what it is.
     
  17. arfa brane call me arf Valued Senior Member

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    And you went to high school?
    If you did, you should easily spot the difference between a material surface, and a collision ('nother name for interaction, which you of course also know).
     
  18. arfa brane call me arf Valued Senior Member

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    Because all crystalline solids, Greek marble included, are made of atoms that are connected together by 'springy' bonds--all materials and their surfaces, are elastic.

    That means all materials, including gases, respond elastically to a sudden impulse. The impulse is an elastic function, so an inelastic collision means something was exceeded, a critical point was reached and energy gets lost as a new equilibrium is reached in the system.

    It's like this; suppose you have a pristine length of stainless steel pipe. not a mark on it.

    If you tap it with a little force using a metal hammer or striker, it rings (the elastic response in the metal is radiated away as sound waves).
    If you really whack it and deform the surface, leave a dent, that's inelastic because, although mass has been conserved, heat was generated by the friction.

    Notice, in there is the idea that thermodynamic "lossy" interactions are irreversible. Something about time goes in there too, about aging or annealing a metal pipe with a hammer.

    Take the idea of thermodynamic annealing back to dots appearing over time, on a screen, and there we have it.
     
    Last edited: Feb 4, 2022
  19. arfa brane call me arf Valued Senior Member

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    Here's another interesting factoid about springs.

    Metal springs are annealed so they are like standing waves of elastic material. When you extend a spring (within a linear range), it pulls back with that elastic force thing, -kx. It also rotates with one end fixed. The rotation is a response to the linear deformation.

    Waves and rotations are closely connected ideas, oh yea.

    Anyhoo I thought I'd may as well announce how this idea of quantum annealing means you get a dot's worth of interference pattern so you 'age the screen' . . . then what would it cost to erase a dot? Each dot is there because that part of the screen absorbed the bulk of an electron's forward momentum.

    If you look again at the example of aging a metal pipe, if you make a mark what kind of information is in it and what would it cost to erase the mark? I'd say it would take more work to hammer a mark out, than to make one; is that really a general principle, as Landauer holds?

    It does seem to hinge somewhat on whether you accept that scratching or denting a metal surface is storing information. I guess I could manage.
     
    Last edited: Feb 4, 2022
  20. James R Just this guy, you know? Staff Member

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    arfa brane:

    Wrong. By why not address your question to all members of the forum? Why single me out?
    Wrong again.
    Define "elastic surface" for me, please. And how is a guitar string a "surface"?
    What does it mean for a surface to be elastic?
    Please don't call names. It is against out site posting guidelines.
    So when you say "elastic surface", you just mean any surface with surface tension? So, liquids only, then?
    Can you find something that will?
    What thing was that?
    You've lost me. What's called an impulse function in time domain?
    What do you mean by "elastically"?
    The main problem there was the resonance. I suppose you can talk about insufficient damping of the specific resonant mode, if you like.
    Radiate it away?
    Define "elastic".
    Define "elastically".
    Define "impulse" and "elastic function".
    Hint: what, exactly, is not conserved in an inelastic collision?
    Define "elastic response".
    So something was lost? What was lost?
    There we have what?
    In what way are they "like" standing waves?
    And so...?
    This idea that you've just mentioned for the first time?
    What kind of screen are you talking about? What kind of dot?
    What kind of mark are you talking about?
    So you're talking about a scratch or a dent? How are you measuring the "information"?
     
  21. arfa brane call me arf Valued Senior Member

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    James, you're back.

    To explain your experience, I see.
     
  22. arfa brane call me arf Valued Senior Member

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    James, can you please explain why momentum is conserved in an inelastic collision. if we get past that, I'll see how I'm feeling.
     
  23. arfa brane call me arf Valued Senior Member

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    This place, this forum, is now one where the sarge comes along and demands to know what the hell you're talking about. What do you mean by an elastic surface? You 'orrible little man.

    For fuck's sake, James, I know you know what an elastic material is, also that it has a surface, made of the same material. Don't be such a goddam dick.
     
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