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Normal Force: A summation of electromagnetic forces?

  1. Nov 25, 2013 #1
    Obviously it makes sense when considering the force of weight and the fact that the object is not moving up or down, but what is it composed of?

    What I mean is, the ground you are standing on is made of molecules bonded through the electromagnetic force, right? But those bonds are parallel to the ground, and presumably they exactly offset each other, which seems to me to mean that there is no possible way for them to exert a force vertically when you stand on those molecules.

    Unless it's the molecules of your feet that are getting electrically repelled by the ground?


    But, aren't these forces from protons and electrons essentially cancelled so that both you and the ground are electrically neutral? And doesn't the electrical force decrease by the square of distance? But how close are you really to the molecules on the floor when you stand on it? Is that close enough for the electrons from your feet to be repelled by the electrons from the floor in just the right amount to overcome the downward force of your weight?


    And then how is it that the weight + the protons in your foot being attracted to the electrons on the ground not stronger than the force of the electrons on the ground repelling the electrons on your feet?




    I know this is kind of all over the place, but if anyone could explain in some detail exactly how normal forces work (in particular, why you don't fall through the floor) in terms of a summation of component forces from the matter in question itself on a molecule-detail-level I would be very thank full.
     
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  3. Nov 25, 2013 #2

    mathman

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    In general the ground and your feet are electrically neutral, so the is no net electromagnetic force between them.
     
  4. Nov 26, 2013 #3
    Also, the bonds are not parallel to the ground, they are in all directions, and they certainly don't cancel each other.
     
  5. Nov 26, 2013 #4
    How come when you step on water you fall through it but when you step on a floor you don't?

    The reason is because a solid floor is more rigidly connected between the molecules. You can imagine the molecules in the floor as having a really strong electromagnetic connection that prevents them from moving around etc etc.

    I'll have to disagree with mathman on this one, I believe that there IS an electromagnetic force between your feet an the floor. I.E. when you step on the floor, the negative electron shells on the outer edges of the atoms repel each other. You can imagine two atoms connected very rigidly and then a third atom attempting to pass in between them. Obviously, it would require a huge amount of force to push those two atoms out of the way and then pass right between. This is why you can't stick your finger through a steel plate but you can easily stick your finger through some sort of gaseous or liquid solution.

    edit-
    And no, the force from the protons and the force from the electrons don't cancel each other out. Imagine two hydrogen atoms moving toward each other. There is a single electron shell on the outer edge of each and a single proton in the middle. A proton and an electron have the same charge, so the two electron shells will repel each other MORE than the electrons will be attracted to the other atom's nucleus. I.E, they will feel a net repulsion. I believe that if they have enough speed to resist this repulsion and the electron shells actually pass over each other, then a bond is formed.
     
    Last edited: Nov 26, 2013
  6. Nov 26, 2013 #5
    In my opinion, the main force is that reaction between nuclues. Because they are the main mass. You can ignore the force from electron.
     
  7. Nov 26, 2013 #6

    Nugatory

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    No. To get a rough sense of the scales involved, imagine that a typical atom were the size of a very large sports stadium, spectator seating and all. If so, then the nucleus would be like an apple in the middle of the playing field, and all the rest of the space would be taken up by the electrons. Push two of these together, and the electrons are interacting before the nuclei are anywhere near one another.
     
  8. Nov 28, 2013 #7
    But electrons are too light to make difference.
     
  9. Nov 28, 2013 #8

    sophiecentaur

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    The bonds are not just in tension. They are in equilibrium (or the material would compress). Each molecule is in a potential well and energy is needed to displace it in any direction.

    This model applies more standing on a thin diaphragm than on solid ground. One stage further and it's like a horizontal stretched rope. When you hang any load on the rope, the forces are no longer horizontal because the rope will sag a finite amount and there will be a vertical component, which adds up to the weight of the load. Tension will increase accordingly.
     
  10. Nov 28, 2013 #9

    Nugatory

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    It's not their mass that matters, it's their electric charge. Two negative charges close to one another will repel much more strongly than two positive charges at a greater distance.
     
  11. Dec 1, 2013 #10
    That makes a lot of sense. In fact that pretty much seems to be a perfect explanation.

    But it makes me wonder about the stepping through water example brought up by Hertz. Are you then sort of breaking some sort of bond when you step through water? (the one analogous to tension?)


    In any case, the tension analogy seems pretty legit. But just to throw this out there, someone recently told me that the normal force was mainly due to the Pauli Exclusion Principle (of which I know pretty much nothing). Something about more than two electrons not being able to be in the same state. Would this have anything to do with it? If so, how, and why does it not seem to apply to water (density maybe?)?


    Thanks again! If anyone has anything please contribute.
     
  12. Dec 1, 2013 #11

    mathman

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    Water situation involves intermolecular forces, while Pauli principle (as applied to electrons) is mainly concerned with what is happening within atoms.
     
  13. Dec 1, 2013 #12
    When you stand on the ground (and don't move) there is a force balance. Your weight is being opposed by the vertical reaction force of the atoms. This vertical force arises from the electric repulsion between your feet and the floor, and this force is transferred through the solid ground because solids have the capacity to maintain their molecular shape under shear loading.

    In other words, the ground deforms a little bit downward which acts in the same way as a guitar string when you push down in the middle. The deformation creates an angle which allows the tension to push back up. When this matches your weight, you find equilibrium
     
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