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Kinetic < Static (Friction)?

  1. Dec 5, 2005 #1
    Does anyone know what is the reason behind that static friction is more than
    kinetic friction?
    Many people give reasons but they forget that friction is area independent force.
    Please help me...:confused:
    Last edited: Dec 5, 2005
  2. jcsd
  3. Dec 5, 2005 #2
    Because the amount of force required to move an object starting from rest is usually greater than the force required to keep it moving at constant velocity once it is started.

    marlon brando
  4. Dec 5, 2005 #3
    In reality it's probably some complicated solid state physics boundary problem, but I'll tell you how I think of it ;)

    Friction basically arises because the edges of the two surfaces in contact are rough. If the surfaces aren't in motion relative to eachother and they've had a bit of time to settle the protruding bits of the one surface have nestled into the valleys of the other, they're stuck together pretty well. This gives you your coefficient of static friction. If the surfaces are moving relative to eachother all the edges are hopping along, skipping along the surface. They don't have enough time to settle down and really get a good grip, this gives rise to the coefficient of kinetic friction.
  5. Dec 9, 2005 #4
    Let's suppose two bodies with different contact surface areas of same mass.

    Now, the friction acting on both the bodies will be same if the surface is same.

    According to your assumption, friction must be more in the case in which contact surface area will be more, because each part of the body's surface will possess its own frictional repose. And each part's repose will be added to give the total friction.

    If I've misunderstood your explaination, please explain more briefly, with diagrams if possible...

    and, I've also listened that FRICTION HAS ELECTROMAGNETIC ORIGIN, can this be the reason behind that, if yes please, explain...:rolleyes:
  6. Dec 9, 2005 #5

    Doc Al

    User Avatar

    Staff: Mentor

    While it's true that each part of the contact area adds to the total friction, what also matters is the pressure between the contact surfaces: It turns out that it's just the total normal force between the two surfaces that counts (in a simplified model of friction that applies well enough to cases where the surfaces are smooth and hard enough). So each object, since they weigh the same, would experience the same friction force.

    (This simple model breaks down when the surfaces are highly deformable, so that they can better fill the nooks and crannies between them. That's one reason why drag racers have wide tires.)

    All contact forces between objects are electromagnetic in origin. Meaning that the fundamental force involved is the electromagnetic interaction between the charges in each surface (as opposed to being nuclear or gravitational force).
  7. Dec 9, 2005 #6
    Bhaita jara khud bhi socho.
    I don't find hisexplanation of settling down as something that does not explain the reason when friction is considered due to charge. When a body is moving there is gain a constantbreaking of attraction between the two surfaces.
  8. Dec 15, 2005 #7
    I hardly understood what you want to say, I live in india and my English is very weak...
  9. Dec 21, 2005 #8


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    Homework Helper

    Actually he want to say

    1. limiting friction is proportional to N/A

    2. limiting friction is proportional to A


    limiting friction is proportional N

    that is why we generally say that the limiting friction is in dependent of the area in contect.

    Last edited: Dec 21, 2005
  10. Dec 23, 2005 #9
    Friction is ue to the adhesive force between atoms in the tiny joints. Due to this force two different particles tend to stick together.

    and as stated above friction is independent of area but dependant on the normal reaction that is generally weight of a body.

  11. Dec 26, 2005 #10
    It seems to me it is built into the way kinetic friction is defined. Kinetic friction is the static friction divided by the normal force. Since you're always starting with the static friction and dividing it by some other figure, you'll never end up with a result that's larger than what you divided.
  12. Dec 27, 2005 #11
    That is not a bad thought but you are forgetting that if the Normal Force is less than 1, and if static friction is indeed divided by normal force to obtain kinetic friction, then the kinetic friction could, by definition, be greater than static friction.

    By the way, isn't the static friction between two objects equal to the normal force X the coefficient of static friction. This would mean that dividing the static friction by the normal force would give you the coefficient of static friction which is definetely not equal to the kinetic friction between the two objects.
    Last edited: Dec 27, 2005
  13. Dec 27, 2005 #12
    I wrote this in another thread, but repeat here :rolleyes:

    If kinetic friction is greater than static friction, you'll have a contradiction. (but this won't explain they can't be the same.)

    Assume kinetic friction is greater than static friction. And you are pushing a box on a floor, gradually increasing the force. At a point the box begins to move when the force becomes greater than the static friction. However, it's still smaller than kinetic friction under our assumption, so it must not be moving. This is clearly a contradiction.
    Last edited: Dec 27, 2005
  14. Dec 27, 2005 #13
    You're right. What I explained above is actually the formula for the coefficient of kinetic friction not the one for kinetic friction.

    To get the Force of kinetic friction we have to take the above coefficient and multiply it by the normal force.
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