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What is the cause of friction?

  1. Jun 28, 2007 #1
    Hi

    In our textbooks, they say that friction originates due to electrostatic charges. But electrostatic charges themselves originate due to friction. Then what is the real reason behind friction? I have searched extensively on the internet, but nobody seems to know exactly how friction is caused. I know that friction is caused due to charges, but what is the actual process involved?
    Another question: why is heat produced due to friction?


    According to me, when atoms/molecules of one object come extremely close to atoms/molecules of another object, the atoms/molecules having greater number of valence electrons repel the valence electrons of the atoms/molecules on the surface of the other object (due to their larger value of negative charge), causing the surface atoms/molecules to polarize slightly. This develops a slight positive charge on the surface of the object (in other words, one surface causes slight polarization of the other). This attraction between the surfaces of the two objects results in formation of these weak bonds which we call "gluing up of surfaces" at the points of contact. The strength of the bonds, naturally, depends on the nature of atoms/molecules of the two objects.
    Regarding emission of heat due to friction:
    When bonds are formed, heat is released. When bonds break, heat is consumed. Now if we just place one object on top of the other, heat is produced due to formation of bonds, though it is quite small. When we slide one surface on top of another, there is a continuous formation and breaking of these bonds. The amount of heat produced is proportional to the weight of the body being moved: more the weight, stronger is the bond strength and more is the heat released.
    But this is where I get stuck up. If one body is moving on top of another, then bonds are continuously forming and breaking. This means, emission and absorption of heat must be almost equal. Then why is so much heat evolved in movement?

    Waiting for your answers and comments.

    Mr V
     
  2. jcsd
  3. Jun 28, 2007 #2

    daniel_i_l

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    Cause of friction: I was taught that the main cause of friction is attraction between molecules near the surface of the objects. What you describe is electrostatic induction. This could be the source of the attractive force in some cases.
    Heat: The temperature (different than the heat - heat is the transfer of energy which causes a change in temperature) of the surfaces goes up because when the the objects are forced against each other the atoms close to the point of contact get knocked around and gain kinetic energy.
     
  4. Jun 28, 2007 #3
    How exactly does this attraction build up? Please explain.

    Can you then please explain why increase in kinetic energy of atoms leads to rise in temperature? I never really thought about this question until I read your answer.
     
  5. Jun 28, 2007 #4
    I mean I know that vibration of charged particles produces heat. But how does this work out in case of atoms?
     
  6. Jun 28, 2007 #5

    Danger

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    Quite simply, it's because heat is the measurement of molecular motion. The faster they move, the more energy (which we define as 'heat') is conducted or radiated to another body.
     
  7. Jun 28, 2007 #6
    Can you give a link to any site explaining this. I mean I want to understand this in a bit more detail. Or you yourself could give an explanation (if you have time, of course).
    Why I am asking details is because I had quite a different notion that heat is produced by oscillating/accelerating charges. I mean how does movement of an atom/molecule produces a photon of heat?


    Mr V
     
  8. Jun 28, 2007 #7

    Danger

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    I'm perfectly willing, and have the time, but I'm really not qualified. As for links, most folks here know that I'm not a 'net guy'. I can't direct you to one, because I don't know which ones are legitimate and which aren't. Your best bet is really to just hang in here and wait for someone else to give you the proper explanation. The experts are monitoring this thread, even if they haven't yet responded to it. One of them will show up shortly.
     
  9. Jun 29, 2007 #8

    rcgldr

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    There's also the larger macroscopic effect due to the fact that surfaces aren't perfectly smooth. In effect, you have very small hills and valleys in surfaces that "snag" each other, and require small amounts of deflection at the surface in order to slide.
     
  10. Jun 29, 2007 #9

    ranger

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    Could you possibly be confusing heat and temperature? Temperature the measure of average kinetic energy of particles. Heat [which is a type of energy] is transferred by a difference in temperature. So if we have two objects of dissimilar temperature, heat is transferred from the system of higher temp. to the one of lower temperature, thereby increasing the temperature of the latter.
     
  11. Jun 29, 2007 #10
    Yeah, you are quite right. But let us imagine a surface which is smooth at the microscopic scale also. What will be the cause of friction in such a case?

    Does my post sound like I am confusing the two things? In the part of my post that you have quoted, I am merely asking how vibration/movement of atoms/molecules produces heat. Maybe I mixed *temperature* and *heat* while asking my question.
     
  12. Jun 30, 2007 #11

    rcgldr

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    Unless there are molecules shaped like cubes, a surface won't be smooth even at the microscopic level.
     
  13. Jun 30, 2007 #12
    Please try to understand that what I want to get at, is - whether there are electrostatic forces involved in friction, or is irregularity of surface the sole ( and major) reason why we have friction around us?
    I mean, are the points of contact those points where
    1. the irregularities of the two surfaces get "hooked" into each other (or entangled)?
    OR
    2. the irregularities of the two surfaces "stick" to each other due to electrostatic attraction between the atoms/molecules of the two surfaces?

    Which one is correct?
     
  14. Jun 30, 2007 #13

    ranger

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    If it wasn't already apparent in my previous post. Heat is brought about by a temperature difference. An isolated system has energy, but not heat.
     
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