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Maximum coefficient of friction

  1. Oct 18, 2007 #1
    Can someone explain why the maximum coefficient of friction is 1??

    Why μ <= 1 ???

    Thank You.
     
  2. jcsd
  3. Oct 18, 2007 #2

    malty

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    I dunno I thought it could be greater than one . . . :confused:
     
  4. Oct 18, 2007 #3
    It could be greater then 1. For example if you have glue, or a really viscose m aterial between the two surface.
     
  5. Oct 18, 2007 #4
    But if you are talking about glues and something like this, you are not talking about friction, it's a different force.

    For definition, when the coefficient of friction is m:

    0<=m<=1

    And I wonder, why m<=1?
     
  6. Oct 18, 2007 #5

    malty

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    Are you sure this isn't the elastic coefficient you're mixing it up with???
     
  7. Oct 18, 2007 #6

    cristo

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    No, the coefficient of friction does not need to be less than one. For two surfaces glued together, for example, the coefficient of friction would be infinite.

    You may be confused by the fact that most of the surfaces you will be doing calculations for will have coefficients of friction less than one. This is simply because, in those cases, we are more interested in surfaces that slide relatively well against each other.
     
  8. Oct 18, 2007 #7
    You don't need glue. I recently attended a horse pulling contest where the winning team weighed about 3600 lb and pulled about 3900 lb. That's a coefficient of kinetic (yes, their hooves were slipping a little) friction of 1.08. Some other candidates for high coefficients might be a cheese grater, no 40 sandpaper, a file, stainless steel on aluminum, etc.
     
  9. Oct 22, 2007 #8
    Thank you everybody.

    Now I understand the coefficient friction can be higher than 1.

    Another explication is that in a ramp, the coefficient of friction is equal to the tangent of ramp's angle.

    The angle can be higher than 45º, so the tangent would be also higher than 1, as a consequence we can get a coefficient as high as we want (considering that a body is holded in the ramp).
     
  10. Oct 22, 2007 #9

    rcgldr

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    Top Fuel drag car tires are getting a coefficient of friction well over 4.5. Table Tennis rubber is probably the stickiest, take a look at this video:

    ttstick.wmv
     
  11. Oct 22, 2007 #10
    When talking about friction, never refer to glues or something like that. Glues adhere by different force,not friction.
    I agree that most of the friction coefficients are lower than 1. There is reason for that. If you magnify the two surfaces of the table and let say an iron bar, these surfaces are not smooth but made of many micro hills (which you can see in an SEM picture of a metal). The steeper the hills (triangles), the higher the friction coefficient. If the steepness is 45 degrees, probably the coe. is 1. For many materials, the hill should not be steeper than 45 degrees, that the reason why coeficients are mainly lower than 1. Anyway, there are exceptional cases.
     
  12. Oct 22, 2007 #11
    That's only going to be the case if the Net force is 0 of an object on a ramp.
     
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