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I Two sheets of glass stick together. Why?

  1. Dec 19, 2016 #1
    When the sheets are very smooth and are pressed together strongly, I guess they could be viewed somewhat as a unit, so the force to pull them apart would be approximately equal to or less than the atmospheric pressure times the area of the sheets, but definitely not more than, is this correct?

    Now if you add a few drops of water between the sheets of glass, it becomes significantly harder to pull apart. I can see two reason for this, please tell me if I'm missing anything or if something I say is wrong.
    1. Eliminates any air gaps, and so decreases the pressure between the plates of glass
    2. Adhesion forces, for smaller distances, the glass might actually pull together because of the adhesion forces.
     
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  3. Dec 19, 2016 #2

    phinds

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    I'm not sure the upper limit is that high with just dry glass. For very precisely ground Jo Blocks on the other hand it might be that high.

    1 is definitely true and I THINK 2 is also true but don't know that for a fact.
     
  4. Dec 19, 2016 #3

    photo_guy

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    Just a thought for what it's worth. I expect it would be gravity... That is the gravity produced by every atom in each of the two thin sheets. With each sheet being pressed to the other they would tend to draw themselves even closer... Obviously not the same as if they were all in one sheet but they might not know to what extent they are not in one sheet.. :) ?
     
  5. Dec 19, 2016 #4

    ZapperZ

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    Er... try estimating the gravitational force between the two plates. Do you really think it can be as strong as what has been described? And do you think this gravitational force that you are subscribing to changes significantly when you moisten the surface that it causes the force to separate it to change by THAT much?

    Zz.
     
  6. Dec 19, 2016 #5

    phinds

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    The force of gravity between two sheets of glass is approximately zero so no, I don't think so.

    EDIT: I see ZZ beat me to it.
     
  7. Dec 19, 2016 #6

    photo_guy

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    Ok... I understand water eliminating air gaps... But what exactly are adhesion forces..? Glass is not my idea of an adhesive...
     
  8. Dec 19, 2016 #7

    phinds

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    Google "molecular attraction".
     
  9. Dec 19, 2016 #8

    ZapperZ

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    Air pressure!

    Zz.
     
  10. Dec 19, 2016 #9

    phinds

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    I disagree. Air pressure is specifically a DIFFERENT force than adhesion in this discussion.
     
  11. Dec 19, 2016 #10

    photo_guy

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    Re molecular adhesion...

    Is that not gravity..? What other force is there..? Atoms produce gravity. Molecules are made of atoms...
    I just think we underestimate gravity... It's pretty strong, balancing and holding the cosmos together as it does...
    Not to disagree if there is another force of course...
     
  12. Dec 19, 2016 #11

    berkeman

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    I w
    I was thinking air pressure as well, but will look at the search that you suggested. Could you compare the two contributions in magnitude? (Yes, I'm being lazy here...) :smile:
     
  13. Dec 19, 2016 #12

    phinds

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    No, in the context of this discussion, gravity is utterly trivial and you vastly OVERestimate it. And you have not answered ZZ's request that you do the math.
     
  14. Dec 19, 2016 #13

    ZapperZ

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    This is where you need to adhere to the principle that physics is MORE than just saying "what goes up, must come down". It must also say "when and where it comes down", meaning that there must be a QUANTITATIVE ASPECT of what you are saying.

    I asked you to make an estimate of what the gravitational force is between two glass plates. Did you attempted doing that before continuing to insist that it is gravity? This is something elementary that one can do, and it will save you from having to keep on insisting this line of thought.

    And, just in case you are not aware of it, the electromagnetic forces are the most dominant, as in occurrences, force in our everyday lives. The atoms in your body are held together into a solid NOT by gravity!

    Zz.
     
  15. Dec 19, 2016 #14

    phinds

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    As far as I'm aware air pressure is WAY more of a contributing factor but I have read (sorry, no citation) that molecular attraction is measurable for something like extremely precisely milled jo blocks.

    EDIT: ZZ beat me to it again !
     
  16. Dec 19, 2016 #15

    photo_guy

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    Well I'm never able to do math to disprove myself... It never seems to come out right... :)
    Others can do the math to disprove me and I may see where I went wrong...
    I'm not insisting it's gravity... It was a thought... And I didn't argue air pressure or the addition of water.., or even molecular adhesion...
    Although air pressure is only effective when there is less on the other side... Water takes away that possibility... And molecular adhesion seems strikingly similar to gravity... And I did look it up in Google... It didn't say much more than I already knew...

    I offered a thought... Who's is wrong on the strength of gravity remains to be seen afai'm concerned... Cause we're in our own particular inertial frame, at rest, and basically balanced out with the cosmos... We may well be underestimating it's cosmic wide strength... Just another thought...

    I see two glass surfaces with a flat plane of differentiation between sheets of atoms but basically in the same mass.. Holding themselves together pretty much as they do in each single sheet... Singly or constituting molecules... ?

    P.S. I understand EMF... I wasn't referring to how atoms hold themselves together...
     
  17. Dec 19, 2016 #16

    berkeman

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    Thread closed for Moderation...
     
  18. Dec 19, 2016 #17

    berkeman

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    Then you should ask for our help in showing you the right equations to understand the magnitude of the forces involved. We are happy to show you the relevant equations to use and the values of the constants involved. We can then check your math to be sure you got the answers correct.
    No, we do not have the obligation to disprove anything. A a poster, you have the obligation to prove any claims that you are making. And you must prove them using mainstream science.
    No, this is incorrect. Without water between the sheets, it is easier for air to "leak" into the space between them. With water between the sheets, it is harder for air (and the resulting link to the surrounding air pressure) to make it between the sheets.
    This is complete nonsense, and is not allowed here.

    Please re-read the PF rules (under INFO at the top of the page). We do not allow nonsense and non-mainstream posts here. The PF is a very valuable place to learn about physics and the real world. And that is true precisely because we do not allow nonsense posts here.
     
    Last edited: Dec 19, 2016
  19. Dec 19, 2016 #18

    berkeman

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    Thread re-opened for mainstream science discussion and questions.
     
  20. Dec 23, 2016 #19
    I think i recall approximately that the force of an infinite charged plane on a charge L away from the plane is the same as the force of a sphere of radius L/2 on the point, if the point is right on the surface of the sphere. It should be trivial to extend this to mass, since the gravity and electrical forces are pretty much analogous. So I'll be using this to approximate the force of gravity one sheet on the other...

    Note that the gravitational attraction by a sphere is equivalent to that of an equivalent mass concentrated at the center of the sphere.

    very very rough diagram
    glass layer 1 --------------------------------------------------------------
    center of equivalent sphere -----------x
    glass layer 2 ----------------------------(dm)----------------------------

    m = mass of a glass layer
    d= distance between layer one and two
    where I'll find the force on a mass element dm and then just sum over entire sheet of glass.

    the force on the mass dm is just
    dF= (G*dm*m)/(d/2)^2 - from the equivalent sphere
    which if you integrated over the area of the glass slab, you would get approximately
    F= (G*m*m)/(d/2)^2,
    "approximately", because at the edge, a mass element is really only attracted to half of the apparently "infinite plane", so this is an overestimate. Honestly, I'm fairly sure that if you multiplied the answer I'm about to get by several orders of magnitude, the force would still be really small, but anyways I'll plug in some numbers.

    d=1mm = 0.001 m (thickness of a thin thin sheet of glass = 1 mm)
    m = (density of glass) (volume) = 2500 (N/m^3) * (0.1m)^2 (0.001m) = 0.025 kg (for 10cm x 10cm glass)
    G = 10^(-11) (roughly)

    F= 10^(-11) * (0.025)^2 / (0.0005)^2 = a whopping 2.5*10^(-9) Newtons of force to pull apart two 10cm x 10cm sheets of glass.

    Even if I used some dubious estimations here, mind that all of them were purposefully made to be favorable to your theory, just to prove my point further. It's still off by at least 9 orders of magnitude. So no, not gravitational forces for sure.

    I apologize for any sloppy math and estimations I did, this is the best I can do with my current meager high school education :p
     
  21. Dec 23, 2016 #20
    but going back to the original question, the two most relevant forces are then just adhesion (much smaller), and air pressure, right?
     
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