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Homework Help: Difference between kinetic and static friction.

  1. Oct 21, 2012 #1
    1. The problem statement, all variables and given/known data
    I know that static friction is normally almost always more than kinetic friction. But I do not know how to prove that. I have to find formulas to find static and kinetic friction, but when the object is at rest or constant velocity, F-µ(Fn)=0. It is the same equation for both, then, because the object is only at rest or in constant velocity. I cannot get a different µ, then.

    2. Relevant equations
    For both static and kinetic

    3. The attempt at a solution
    I am not sure what to do if µ=m/g, because the mass will always be the same, even if the object is at constant velocity or not, and so will the gravity. So how come the kinetic friction is supposed to be less than the static friction?
  2. jcsd
  3. Oct 21, 2012 #2


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    I believe it has to be determined experimentally.
  4. Oct 21, 2012 #3
    I understand that, I do have a formula of y=mx+b given to me by the graphs I have made with certain data, and these do vary from when static friction is used to when kinetic friction is utilized, but I do not know how to explain it well in an analisis.
  5. Oct 21, 2012 #4
    I have thought about that too. If you have a ..perfect surface against a perfect surface, in this case we cast aside all those imperfections and wrinkles and blabla whatever there is in the surface of a table or block or whatever. Why is it harder to move when it s still and alot easier when it s moving?
    Maybe because of the inertia of the body? If it s moving , Newton says that it will want to keep moving so in a way you don't have to apply that much force on it because it is ..so to speak..helping you a bit with its own. I don't know how accurate that is, I just tried to reason with it according to Newton's laws of motion.

    As far as I know the coefficient of friction is the force of friction/mgcosa(the force that is trying to counteract the gravitational pull) . And if you don t know the force of friction or the cofficient, you cannot simply find the coefficient with some sort of magic formula, you have to determine it by experiments.

    Or maybe it has something to do with some sort of electric forces acting up - but in that case the surface has to have some kind of tiny tiny wrinkles in it.
  6. Oct 21, 2012 #5
    The thing is, this is in the context of an experiment where F=(µg)m=y=mx
  7. Oct 21, 2012 #6
    would µ=F/m/g then? The applied force would also be directly proportional to the mass, too, because µ is, isn't it? Since F-Fnµ=0
  8. Oct 21, 2012 #7
    Static friction occurs not because of Newtons laws, but just because of the properties of materials. When an object is stationary on a friction surface, it is kind of "bonded" to the surface because tiny parts of the surface "get into the grooves" of the object, and vice verse. In this case, static friction acts like a threshold that you need to overcome to get the object moving. Once the object is moving, it's sliding on the surface instead of "settling in" to it, thus kinetic friction should be lower.

    To get an object to move at all, you must apply [itex] F > \mu_{static} F_{n} [/itex]

    Once moving, you only need apply [itex] F \geq \mu_{kin} F_{n} [/itex] to keep it going.

    Hope that helps
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