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Kinetic vs Static Friction Question

  1. Oct 9, 2005 #1
    Everywhere I've read that static friction is usually greater than kinetic friction, but I haven't been able to find the exception to that.
    So I have two questions:

    Is there a real life example where the coefficient of static friction is less than the coefficient of kinetic friction between two materials, and how does it work?
    Secondly, if that were to be true it leads to a semi-paradoxical situation. Imagine the following, there is an incline and a box rests on it. Let's assume the coefficient of kinetic friction is 0.6 and of static friction is 0.4. Let's say the incline is at a degree such that the parallel component of the force to the incline is greater than the maximum static friction but less than the kinetic friction. So if the box was initially moving it would slow down and stop, but once it stops the kinetic friction force becomes static, which is less than the force pulling it down... so what would happen?
     
  2. jcsd
  3. Oct 9, 2005 #2
  4. Oct 9, 2005 #3
    Thanks for the link. It shows a little of how friction works, from what I understood from it...
    So I guess there is no answer really?
     
  5. Oct 12, 2005 #4

    Andrew Mason

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    If the net force on the box in a direction parallel to the surface is less than the kinetic friction force, the box would not be able to move. So we would view it as a static friction force. In that sense, kinetic friction cannot, by definition, be greater than static friction.
    Your question really is whether there are substances whose coefficients of static friction and kinetic friction are the same. I think there are. Think of a sticky surface where molecular bonds have to continually break in order for the box to begin to move and to keep moving.
    AM
     
  6. Oct 12, 2005 #5
    I can imagine a substance with equal coefficients of static and kinetic friction, although I don't know of any real one, but I can't see how kinetic can be more than static. That was why I brought up that paradox to point out the fact that it logically does not make sense that kinetic friction can be greater than static friction, but is that a law of physics? And if it's not, how come?
     
  7. Oct 13, 2005 #6

    Andrew Mason

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    It is really a principle of logic that says kinetic friction cannot exceed static friction.

    Laws of physics are fundamental principles that are common to all physical systems. Friction is a composite macroscopic phenomenon lacking that fundamental quality.

    AM
     
  8. Oct 13, 2005 #7

    Pyrrhus

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    Well friction is a complex phenomenon. You know the law of friction in dry surfaces (or very lightly lubricated) was introduced by Coulomb (a very simplified explanation). The static friction and kinetic friction coefficients are usually found to approximated values on engineering handbooks. Also, it's good to notice that Coulolmb's law doesn't care for the contact surface, which indeed can affect on the movement of a body in some cases.
     
  9. Oct 9, 2008 #8
    perhaps they say "usually" to account for when surfaces aren't uniform. for example, you push a block from an icy surface into a rubber surface then it would be easier to get it going then to keep it going
     
  10. Oct 9, 2008 #9

    atyy

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  11. Oct 9, 2008 #10

    rcgldr

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    Is equal close enough? Teflon on teflon, coefficient of static and kinetic friction is about .04.

    "For a given pair of surfaces, the coefficient of static friction is usually larger than that of kinetic friction; in some sets the two coefficients are equal, such as teflon-on-teflon."

    http://en.wikipedia.org/wiki/Coefficient_of_friction

    By combining special surfaces with liquid lubricants on "slideways", you can end up with more kinetic than static "friction", but what this really means is that kinetic friction increases with speed, and is at it's minimum when there's no motion on the slideway. As I understand it, fluid is force injected into the slideway, so there's always a thin film present. The fluid creates a drag that increases with speed, so the result is a very controllable motion.
     
    Last edited: Oct 9, 2008
  12. Oct 13, 2008 #11
    A purely mathematical explanation of why static friction is greater than kinetic friction would be extremely complicated. On the other hand, this is a question involving physics, and it should not be left up to “common sense” to determine. There is, of course, the empirical approach, and both static and kinetic friction are measured and determined by experiment and under normal conditions, static friction always comes out higher. One explanation is that organic matter, in the form of dust, simply gathers around the stationary object, making it more difficult to get it moving. Sort of “a rolling stone gathers no moss” explanation. However, this should not be the case in a carefully controlled laboratory experiment. Another explanation is that elastic particles tend to deform and lock themselves together forming a kind of ratcheting effect which is most prevalent when there is no relative movement. All of this makes a great deal of sense, and coupled with the experimental data, should be sufficient for most applications, although it will never satisfy the mathematical physicists!:smile:
     
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