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What is a force?

  1. Dec 28, 2009 #1
    Hi

    I read that some people see forces as only a fictional idea, they do not exist.

    I don't get it, If I move the table, then the table moves because I moved it (extracted force) on it, no ?


    Every animal is a force , it can makes changes, move things.


    please explain.

    Thanks
     
  2. jcsd
  3. Dec 28, 2009 #2

    HallsofIvy

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    I don't know where you saw that "some people see forces as only a fictional idea". I've never heard of such a thing, at least not about all forces. It is true that General Relativity treats gravity as a "warp" in space rather than a force.

    And, of course, "centrifugal force" is a "ficticious" force.
     
  4. Dec 28, 2009 #3
  5. Dec 29, 2009 #4

    Vanadium 50

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    And the word "force" doesn't appear in that article.
     
  6. Dec 29, 2009 #5
    it is his idea ....

    others too, they see everything like mass force as fictional ideas only.
     
  7. Dec 29, 2009 #6

    Vanadium 50

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    You still haven't provided a single example of what you are talking about.
     
  8. Dec 29, 2009 #7
    You can read about Mach's incomplete theory in Brian Greene's FABRIC OF THE COSMOS including pages 23-38 summarized very briefly here: (This relates in a general way to Newton's spinning bucket of water example.)
    "Without other material...without any benchmarks for comparsion...Mach claimed there would be no way to experience acceleration....Mach's suggestion was not a complete theory since he never specified how the matter content of the universe would exert the proposed influence..."

    Mach believed a spinning bucket of water in an empty uinverse would be flat on top.....or that with two spinning rocks tied with a rope, the rope would be slack....spinning and not spinning are the same....no forces present.....

    Greene's next chapter explains how these issues were resolved: Einsteins theory of relativity.

    More generally, what is "real" and what is "fictional" depends on one's interpretations and perspectives..."reality" is NOT obvious.....and has been discussed on these forums in many guises....For example, If you free fall into a blackhole the horizon is not observed (it's "fictional") yet if you remain stationary at such an horizon you will be burned alive by radiation....

    And you may think for example that metals are solid and hard yet they are 99.99% empty space!! (I likely left out a lot more "9's")

    For yet another view of force, read the now locked thread on degeneracy pressure....
    https://www.physicsforums.com/showthread.php?t=364464
    What "kind" of a force is it that keeps,say, a neutron star from collapsing??
     
    Last edited: Dec 29, 2009
  9. Dec 29, 2009 #8

    Vanadium 50

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    I don't think even Mach would have argued that when, say, a cannon fires a shell, that there is no force on the shell - or on what is impacted. But in any event, eranb2 clearly has some idea in mind - he or she needs to express it before we can discuss it.
     
  10. Dec 29, 2009 #9

    A.T.

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    Yes, all forces are just an idea. The same is true for all physical quantities. They exist in the same sense, as numbers: as an abstract concept in our brains.

    Don't confuse this with fictitious forces vs. interaction forces debate. That is just a finer classification within the abstract concept of forces.
     
  11. Dec 29, 2009 #10
    I don't get that standpoint. If that is true how can experimental physics yield any results?

    Best regards, Henrik
     
  12. Dec 29, 2009 #11
    I read that a force is =ma, but that if you substitute ma for each force it leads to computational or other difficulties.
     
  13. Dec 29, 2009 #12

    A.T.

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    In math you count apples: The apples are real, but the number that quantifies their amount is just an abstract idea.

    In physics you describe the fall of an apple: The movement of the apple is real, but the numbers quantifying it (velocity, acceleration, force) are just abstract ideas.
     
  14. Dec 29, 2009 #13
    Maybe because in Lagrangian and Hamiltonian formulations the concept of a force isn't really necessary. You would still need the potential though.

    And to whome - you need to invoke a known force to get anything out of F = ma. ie for a mass on a spring (Hooke's law) F =-kx, so -kx = ma is the equation of interest.
     
  15. Dec 30, 2009 #14
    Yes. The concept of "force" is just that. Just a word - or a number if you please. And so is the concept of an "rollercoaster". Still you can ride it. And have fun doing it.
     
  16. Dec 30, 2009 #15

    A.T.

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    1) You don't ride the "concept of an rollercoaster", but a particular object which fits that general concept.

    2) For me, numbers and physical quantities are a different level of abstraction than general terms for a group of similar objects (like "rollercoaster" or "fruit").
     
  17. Dec 30, 2009 #16

    russ_watters

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    I think you are misusing the word "abstract". Just because the word "apple" isn't an actual apple that does not mean the word is abstract. Abstract doesn't refer to the representation (otherwise anything described with words would be abstract), it refers to CONCEPTS which are, themselves, not well defined.
     
  18. Dec 30, 2009 #17

    russ_watters

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    ....in any case the OP was clearly not asking a question about linguistic philosophy, but a question about physics. The apple is physically real as is the force when it hits you on the head.
     
  19. Dec 30, 2009 #18

    A.T.

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    Well, I prefer to distinguish between what we can observe and what we made up to quantify the observation. I don't think that confusing reality with the mathematical models that we use to describe it is helpful.
     
  20. Dec 31, 2009 #19
    I agree. It is important to make that distinction.

    But in a discussion on whether forces are fictious or not I find it equally important to underline the fact, that this distinction doesn't make the forces fictious.

    I do recognize that we have no good explanation of why there are forces and what they actually are. But whatever they are they are certainly real. Otherwise the whole of physics is reduced to a mere philosophy imo.
     
  21. Jan 1, 2010 #20

    A.T.

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    Actually making statements like "they are certainly real" is mere philosophy. There is no physical quantity called "realness".

    And no, physics is not reduced to mere philosophy because forces are an idea. Just like math is not reduced to mere philosophy because numbers are an idea. Physics and math make reliable quantitative predictions using these ideas, while philosophy debates what is 'real'. I'm trying to avoid that philosophical discussion about "realness", and just separate "observation" and "mathematical models".
     
    Last edited: Jan 1, 2010
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