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Going back to basics: The Normal Force

  1. May 13, 2014 #1
    So, usually I like to give a long description of how/why I am asking such a question, but I think this time, I'll be quick and to the point. Truth is when I first came to this site, I think I started asking questions that were well beyond my current knowledge, so I would like to take the time now to go back to some of the more basic questions. So without further ado, here they are:

    1. The Normal Force- the force that keeps things from falling into other things, the force that opposes gravity, bla bla bla; we've all heard those "high-school wives' tales." Question is: What truly explains the normal force? I mean from a quantum mechanics point of view, what is the main dominant or rather more important phenomena that makes the normal force possible? On the stockexchange-physics site (which I know for a fact, is extremely untrustworthy, which is why I'm here), I've been told that the normal force is mostly due to the pauli principle and also due to the electrostatic repulsion forces, but their explanations seemed somewhat-- lacking, not to mention, in some places, downright, incorrect.


    2. The normal force is usually "applied" to things on top of surfaces/ solid objects, but could the phenomena responsible for the normal force be used, in theory, to make non-solid objects repel other objects? Ie: Could we use the phenomena that manifests the normal force in such a way to have, say, water repel a person that is on top of it? (walking on water, anyone? :wink:)


    3. Kind of a last, fun theoretical question, but imagine this scenario:

    You're getting close to a blackhole, and you think "@##$ I've really done it this time. How am I going to get out of this one?". You realize that not even light can escape it. But then, you remember what your high-school professor told you, that the normal force opposes gravity (kind of bull---, but just go with it for now). And let's just say that you have access to some kind of a black-box or a star trek replicator that can make a huge amount of tables. Granted that the normal force is only nearly equal to the opposing force of gravity, but would it be theoretically possible to just stack those tables on top of each other, climbing on top of the newly stacked table, and keep stacking them until you get so far away from the black hole, that you get out of its gravitation pull and climb your way to freedom? (yes, I know that's a lot of tables, but who knows, maybe it's a small black hole :smile:)


    As always, awaiting your replies, but please no flaming/spamming/ spitting on the sidewalk, and of course, Thanks for reading,
    - Matt-er
     
  2. jcsd
  3. May 13, 2014 #2

    Simon Bridge

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    1. the normal force of classical mechanics comes from the electromagnetic interaction.
    What do you find lacking about this?

    2. sort of - it works best for rigid bodies though... non-rigid bodies tend to deform significantly under a force so the distribution of interactions becomes important.

    3. No - all the tables are falling at the same rate as you.
    The force from the table is thus zero which is how hard you are pressing the table.
    Tables do not magically produce forces.

    There is nothing magical about normal forces - it is a name given to an effect.
    On the Earth your seat pushes against you as hard as you push against it, but the chair is also pushing on the ground and the ground pushing back ... internal pressures in the Earth stop the ground from collapsing into a black hole so that pressure is what results in the normal force from the Earth on the chair.
     
  4. May 13, 2014 #3

    adjacent

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    Simon Bridge
    What about if we stack large tables having masses of 1000kg? Won't it resist motion(Due to inertia) and give us acceleration by the normal force?
     
  5. May 13, 2014 #4

    Simon Bridge

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    Even classically, the acceleration of gravity is the same for all masses - so the mass does not matter.
    The tables will have their own gravity, attracting you, but it makes no difference to you falling into the black hole.

    What education level are you thinking about this at folks?
     
  6. May 14, 2014 #5

    adjacent

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    Oh. I forgot about this part!

    Gr.10 :wink:
     
  7. May 14, 2014 #6
    Even though one often talks about the normal force being a result of electrostatic repulsion, it is very difficult (or downright impossible) to quantify this with classical mechanics.
    You will find that physics is not as powerful as many claim, we can qualitatively explain many things however often a full quantitative analysis is very problematic.
    In principle one can model most of the natural world with current theories, however the actual computation gets complicated very quickly as one looks at more complex systems.
     
    Last edited: May 14, 2014
  8. May 14, 2014 #7

    Simon Bridge

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    You can demonstrate the link between electrostatic repulsion on an atom-by-atom basis and the macroscopic mechanics involving surfaces though.
    That's all we need for the connection.

    You need to be specific here - not as powerful as who claimes?
    Otherwise this statement is pretty meaningless.

    ... again...

    But who is claiming otherwise?

    And how does this help OP?
     
  9. May 14, 2014 #8

    Simon Bridge

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    So we have adjacent at grade 10 ... and Matt-er?

    At grade 10 USA you've done Newton's laws and maybe free-body diagrams.
    Normal forces are a handy way to model contact effects.
     
  10. May 14, 2014 #9
    @ #1: Well considering that you wrote one sentence, that is kind of lacking of a true explanation, lol. But, I think I understand the concept behind it from other sources. Thanks.

    EDIT: I posted a counter- question that was completely incorrect, referring to the repulsion of antimatter in matter to antimatter interactions (they attract each other, so that was a stupid quesiton, lol). This was completely wrong, and have no idea how I could have posted it. All things seem correct. No further questions regarding this.


    @ #2: Well, there goes my idea of walking on water; unless there is a way to make non-rigid bodies become temporarily rigid? Any thoughts on this? :smile:

    @ #3: That makes sense. And that's what I initially thought the answer to be as well, but truth is, I never got a truly good understanding of the normal force, so I wanted to clear up some holes/ambiguities in my knowledge. And that's mostly because it wasn't taught well in highschool, and I guess I never thought about reviewing it until now. It's a shame how horribly wrong they teach things in highschool; clearly my teacher(s) had no idea what they were talking about, otherwise, I wouldn't have had these questions on what is relatively a basic topic.


    Bare in mind that this could have been asked at any education level (I mean just look how dumb my highschool teachers were, and they finished college, lol), but as for me: I am currently in college, and to be honest, I am not even a physics major/minor. I am getting a degree in computer science, and have taken only two courses in physics (I also took two courses in highschool, but neither of the two college courses went over stuff related to classical mechanics-- so things like the normal force were not covered, and hence why I am here).
     
    Last edited: May 14, 2014
  11. May 14, 2014 #10

    Simon Bridge

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    ... so the answer was unsatisfactory because it only had one sentence? How many sentences do you need? Or is it something to do with the content of the sentences?

    Please be aware that there is no such thing as a complete answer to any physics question: that would require a Theory of Everything ... so all answers are going to fall down somewhere.

    If you were to tell me what was not good enough about the answers you had, I would have been able to fill in the gaps. Otherwise you have left me the task of guessing your needs.

    Freezing? Add lots of corn starch and stamp your feet? ;)

    That is exactly what I was bearing in mind. (I try not to bare in mind too much though - leads to dribbling...)

    There are infinite possible replies - what's important is to give you an answer you can use right? I have to figure that out from clues you leave me when you ask. That is why some statement about how you are thinking about a problem, at the risk of looking stupid, is very valuable here.

    The origin of the normal force, or "contact forces" in general, is not usually covered in any detail until year 3 in college. I didn't see a mathematical treatment until post-grad.

    I cannot comment on your high school experience - I'm sure there must exist some high schools who teach normal forces well - was this in the USA: US secondary education has something of a reputation in the Rest of the World. Even so, I would not expect anything like a complete description of such advanced subjects at the level where you've only just been introduced to Newton's Laws.

    You keep saying that such-and-such was not good enough etc but you don't seem willing to say what it was about the explanations or whatever that were not up to your standard. Without that information, how can you expect anyone to meet that standard?

    Have a look at:
    http://home.comcast.net/~szemengtan/ [Broken]
    classical mechanics, systems of particles and rigid bodies has foundation treatment ... you can build a model of contact forces within the classical framework from there.
     
    Last edited by a moderator: May 6, 2017
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