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B How did Newton come up w/ 1st law? Why does friction exist?

  1. May 29, 2016 #1
    So I'm trying to understand Newton's laws, and I know that friction is supposed to be the force that stops an object in movement. But I don't really understand why this is the case, as the force of friction is not something that can be perceived intuitively (unlike, for example, the force I could apply to a box in order to move it around).

    In fact, if I remember correctly, it used to be believed (before Newton's time) that objects needed a constant force to move. There was no such thing as "friction", things were supposed to naturally come to a halt after being applied a force. Why did this notion change?

    Basically, what I'm trying to ask is, how did Newton come up with this idea? Because it's not an empirical finding: after all, he was the one to define what a force was in the first place. Or is it considered just to make inertia and the first law valid? And if so, how do we even know inertia is truly a real property of objects?

    My apologies if I haven't expressed myself well or posted in the wrong section of the forums (wasn't sure where I should write this, because it isn't a homework exercise or anything of the sort).
     
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  3. May 29, 2016 #2

    Andrew Mason

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    The first law was probably first stated by Galileo. It was based on observations done in experiments of rolling a ball down the side of a bowl and observing it roll up the other side. He observed that as the angle of the other side bowl declined the object tended to rise always to the same height. He suggested that if you could eliminate any resistance to rolling it would always roll to the same height and, therefore, if you made the angle 0 it would keep going for ever. So he made a postulate: the law of inertia which Newton made into the first law of motion. No one has ever been able to find any evidence that contradicts that first law.

    AM
     
  4. May 29, 2016 #3
    I see. So is the law of inertia an axiom then? In the sense that you start building the laws from the hunch that there's such a thing as "inertia"?

    And when you say there's no evidence that contradicts the law, are you also saying there IS specific evidence that makes the first law valid? The only example I can think of is launching spacecraft into outer space, where an initial force is enough to give the object a constant velocity through time (as there's no "friction" there, according to my book) and never making it stop. Examples that concern bodies on earth don't seem to indicate anything though, at least to me: the body will eventually stop because of this "force of friction", which can only be empirically estimated (thus not being very refutable in an experiment I suppose).

    Thank you for the help so far :)
     
  5. May 29, 2016 #4

    Andrew Mason

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    It is a generalization made from observation. Whether it is true in every sense is not known. Science does not provide absolute truth. Science provides models which seek to explain observations and allow us to make predictions. If they do that, we use them.

    We have found that the law of inertia and the other laws of motion provide the least complicated and most useful explanation for our observations (eg. why the planets keep going around the sun in elliptical paths) and enables us to make predictions about the physical world. Those predictions are usually very, very accurate. So we use the theory. If a theory leads to a result that can be disproven by experiment, we look for a better theory. General relativity was found to explain what Newton could not and changed how we view inertia. Someone, someday, may find evidence that the theories of both Newton and Einstein are not quite correct.

    AM
     
  6. May 29, 2016 #5

    phinds

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    Not completely sure what you mean here, but keep in mind things are only "proven" in math, never in physics. The best we can ever do is have a theory or law or whatever that so far has never been invalidated by empirical evidence, but that is not a "proof" of "truth", it's just the best we know at the moment. Newton's Law of Gravity was long widely believed to be "true" in the sense that you probably mean but was shown by Einstein to be a subset (a limiting case) of a wider truth and we already know that his Theory of General Relativity has to be a subset of some still wider truth that will encompass the area where it doesn't work, and this is a good example of why we don't believe in absolute truth or proof.
     
  7. May 29, 2016 #6

    David Lewis

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    Galilei came up with the idea, and Newton enunciated it as his first law. Aristotle said force must be continuously applied to keep a body in motion, and indeed that appears to agree with everyday experience. However, Galilei hypothesized air resistance and friction obscured what was really going on, and devised experimental apparati that reduced friction to negligible levels.
     
    Last edited: May 29, 2016
  8. May 29, 2016 #7
    Friction is not a fundamental force but it definitely is measurable.
    So in the standard model it either arises from electromagnetism or from gravity, (or interactions of both ??)
    (I think we can exclude nuclear forces).
    Is there a known relationship between EM and gravity which explains friction?
     
  9. May 30, 2016 #8
    How is gravity involved in friction?
    You will have friction even in the absence of gravity.
     
  10. May 30, 2016 #9

    phinds

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    Indeed. The issue is the force pressing two object together as they move against on another. Gravity is certainly one way that force can be applied, but not the only one.
     
  11. May 30, 2016 #10
    Yes, this Is what I expected regarding gravity - it can contribute to friction and often does, but it isn't required,
    So does that mean that friction is an emergent property of electromagnetism?
    Are electrons behaving like a kind of velcro in a situation where two rigid bodies are sliding against one another, (or are attempting to do so)?
     
  12. May 30, 2016 #11

    Drakkith

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    Yep.
     
  13. May 30, 2016 #12
    I thought the origin of frictions and the normal force was due to Pauli's principle, but a quick search on internet just gave me contradictory answers.
     
  14. May 30, 2016 #13

    Drakkith

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    The interaction between the atoms of object and the surface is from the electromagnetic force, and friction is a direct result of that interaction. I have no wish to get into a discussion on the end-all-be-all origin of either friction or the normal force.
     
  15. Jun 2, 2016 #14
    Thank you guys for all the help! Forgot to get on PF after Sunday, so apologies for being so late.

    The only thing that still bugs my mind has to do with what u/rootone said. How is friction to be measured? Thought the best we could do were friction coefficients. And what does it have to do with EM?
     
  16. Jun 2, 2016 #15

    phinds

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    It has everything to do with EM. The reason you don't fall through the chair you are sitting in is because of EM force between the electrons in your pants and those in the chair.

    Now rotate you and the chair 90 degrees and have someone push you really hard against the chair. Same thing. Now make 10,000 little bitty you's and 10,000 little bitty chairs and glue all the chairs onto one surface and all the little you's one another surface above it and push the two across each other. Same thing. As you smooth out your butt and angle the chair, it gets easier but to the extent that there is still "friction" it is the same EM force.
     
  17. Jun 2, 2016 #16
    So electrons stop you from falling through the chair you are sitting in,
    but equally they stop you from floating away from it.
    I'm getting there slowly.
     
  18. Jun 2, 2016 #17

    jbriggs444

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    Gravity is what keeps you from floating away from it -- unless you are holding hard on the arm rests or have velcro on your butt.
     
  19. Jun 2, 2016 #18
    Damn. I was nearly covinced by earlier posts that gravity has nothing to do with it (friction).
     
  20. Jun 2, 2016 #19

    jbriggs444

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    Gravity has nothing to do with friction. But it does have something to do with you not rising out of your chair.
     
  21. Jun 3, 2016 #20

    Andrew Mason

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    Friction between two surfaces depends on the characteristics of surfaces and the forces normal to the surfaces. If the normal forces are due to gravity, then gravity does have something to do with friction in that sense.

    AM
     
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