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I Is the normal force just kinetic energy?

  1. Dec 20, 2016 #1
    I'm confused; forgive me if this dumb.
    I'm trying to reason what "force" is, on a molecular level. I'm only concerned about normal forces here (pushes and pulls), not field forces. Forces in Newtons are vector quantities and only represent relationships between two things right? It isn't anything fundamental - just a way of deciding whether a mass accelerates?

    Let me give an example to illustrate my confusion. You pick up a twig and press it in the middle so that it snaps, and you can measure the force that surpassed the twigs shear strength that your forearms generated. Now I understand that such forces are a measurable result of molecular orbitals refusing to give way to each other, but I don't understand where the movement leading to the confrontation is coming from in the first place.

    In this example, you only need to go as far back as the chemical energy inside your muscle cells. Myofibers contain ATP and signalling molecules which can initiate the release of chemical energy at will. A reaction is activated where myosin heads pull along a titin molecule, and chemical energy is released from ATP afterwards. This is explained as "chemical energy being converted into movement", but I can't find any explanation more descriptive.

    So what's happening? Hydrolysis of ATP releases mostly heat. So is heat being absorbed by the myosin heads which then have kinetic energy, and that's the "pulling" which is then being carried down through the muscle cell so that it contracts, and the cell has tight junctions with other cells and proteins, which eventually join to tendons and bone, creating the movement of your fingers against the twig which is the "force"? So basically it's a molecular collision, where the energy comes from heat but is converted into kinetic energy, and when that kinetic energy is compromised by an obstacle, there's some conversion into force, based on mass and stiffness etc?

    Or does nothing on the macro scale really have kinetic energy here, and it's all more to do with angular movement/configurational change, that creates the tension between the molecular orbitals of your hands and the twig, and twisting and pivoting is ultimately being created by heat release from ATP?
    Many thanks.
     
    Last edited by a moderator: Dec 20, 2016
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  3. Dec 20, 2016 #2

    Dale

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    I am not sure what you are asking. Are you interested specifically in the biochemistry of muscles, or are you asking a general question on forces?
     
  4. Dec 20, 2016 #3
    Muscles is just an example, I'm asking a question about normal forces. An equally good example would be a rock rolling down a hill - is the force it enacts against the trees it crushes on the way down just a function of the gravitational energy it has? Where's the force coming from? Is it gravitational energy -> kinetic energy -> force, and likewise chemical energy -> kinetic energy -> shear stress/force?
     
  5. Dec 20, 2016 #4

    Dale

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    In classical mechanics forces arise from a change in the Lagrangian with respect to the generalized coordinates. Things like the normal force are usually treated a little different. They are the result of a constraint, and they can be found through the Lagrange multiplier approach.
     
  6. Dec 20, 2016 #5

    russ_watters

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    I'm sorry, but it looks to me like you are headed down a rabbit-hole of reasoning that doesn't go anywhere useful: forces are not only resistance to motion, so heading on a chase for motion (kinetic energy) as a source of force will lead to tortuous paths to dead-ends.
     
  7. Dec 20, 2016 #6

    anorlunda

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    Do you need anything more than Newton's second law to answer that question. F=ma. The forces come from acceleration of the objects.
     
  8. Dec 20, 2016 #7

    PhanthomJay

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    I am not sure if this is what you are asking, but normal and all 'contact' forces are a result of the electromagnetic force causing repulsion between electrons as the objects 'touch' each other. Thus, the contact force comes from electromagnetic interaction.
     
  9. Dec 20, 2016 #8
    Thanks Dale,
    Do you think you could elaborate on how constraints create forces, also how this fits into the picture of minimizing potential energy and the lagrangian?
     
  10. Dec 20, 2016 #9
    But in the context of moving things with your body, where are the forces coming from. One second you are rest and the next you are enacting a force on something. Forces must be coming from quantities that are non-force, and I'm asking if this source is chemical/thermal/gravitational energy.
     
  11. Dec 20, 2016 #10

    A.T.

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    Chemical.
     
  12. Dec 20, 2016 #11
    Okay, what I meant was, the rock and the tree have to first come into contact, and as PhantomJay points out, the two objects then repel due to the electrostatic repulsion between molecular orbitals. What I am confused about is, how is the acceleration and mass then being communicated through these electromagnetic repulsions into a force upon impact? When the molecular orbitals of the tree and rock come into contact, how does the rocks mass and acceleration come into the picture in order to get out a force large enough to snap the tree?
    I still don't understand how chemical energy, being released as heat, is allowing the myosin heads of the myosin molecule to "pull" the titin molecule together, causing muscle contraction. I'm not understanding how we're getting from chemical energy -> heat/configurational change -> force.
    I assume it has something to do with the electromagnetic repulsion of the molecules. How are force quantities coming from energetic quantities?
     
  13. Dec 20, 2016 #12

    A.T.

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    Force fields have potential energy associated with them. Chemical energy is fundamentally such a potential energy.
     
  14. Dec 20, 2016 #13
    I suppose a boiled down version of my question is: if one long stick of molecules is pushing against another, and that pushing is coming from the release of potential energy, then does it go like: potential energy goes down, kinetic energy and therefore acceleration goes up, and those things, through F = ma, gives you the resultant pushing force?
    And it doesn't matter that the force is ultimately coming from electromagnetic repulsion between their molecular orbitals, since they can just be treated as solid Newtonian objects - only their masses and shapes matter?
     
  15. Dec 20, 2016 #14

    jbriggs444

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    A force does not come from the release of potential energy. A weight can hang motionless at the bottom of a cord for years without any expenditure of energy at all.
     
  16. Dec 20, 2016 #15

    Stephen Tashi

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    It isn't yet clear whether the use of phrase "normal force" in this thread refers to a force perpendicular to a surface - i.e. "normal" to the surface or whether "normal force" refers to any "ordinary, everyday force" - a force that can be measured with a spring scale, a macroscopic phenomenon.
     
  17. Dec 20, 2016 #16

    russ_watters

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    I think this is the dead-end rabbit hole I referred to in post #5. Though there are specific cases where energy is expended in applying a force it is not generally (always) true.
     
  18. Dec 20, 2016 #17

    Stephen Tashi

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    It may clarify things if we consider a more general issues.

    Notice that "force" is technically not "a thing" or "a system" it is a physical property of a thing or system. Likewise "potential energy", "kinetic energy" . "acceleration", and even "mass" are not things that have , by themselves, a physical existence. Even though it is common jargon in physics textbooks to pose problems like "A mass of 1 kg is sitting on a table..." , there cannot literally be a "mass" of 1 kg sitting on a table. The thing sitting on the table must be a coffee pot or a circular saw or some thing that has the property of having a 1 kg mass.

    So when you speak of "force" or "acceleration", you have to explain what thing or physical system those properties are associated with. You mentioned "molecules" and they qualifiy as a thing or physical system. Can you phrase your question about "kinetic energy", "acceleration" and "force" so that those properties are clearly associated with specific things or physical systems?
     
  19. Dec 20, 2016 #18

    Dale

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    Well, first it isn't the energy or even the Lagrangian that is minimized, it is the action. The action is the integral of the Lagrangian. So, for example, a parabola is the path that minimizes the action for a projectile.

    Regarding constraint forces, if you choose your coordinates well then you don't even need them. Usually you just use them because it is easier to write an unconstrained Lagrangian and the constraints than it is to figure out the Lagrangian in better coordinates.
     
  20. Dec 20, 2016 #19

    Dale

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    For something like this, there is certainly no need to go to a molecular level. A simple continuum obeying Hookes law is fine.

    A solid object, by definition, requires a force to deform, i.e. There is a relationship between stress and strain. Analyzing a boulder rolling down a hill and breaking a tree does not require quantum mechanics. Newtons laws and Hookes law are sufficient.
     
  21. Dec 23, 2016 #20
    :(
    But I'm not asking what is needed or sufficient, I am asking where the force is coming from when the boulder molecules touch the tree molecules. I'm not trying to do a calculation or exercise, I just want to know how the energy is turning into a force on the molecular level (or quantum level if necessary).
     
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