Static Friction Vs Dynamic/Kinetic Friction

  1. Just been trying to think about why static friction is generally higher than Dynamic (or Kinetic) Friction?

    In a really crude sort of way I could imagine that the momentum of the object has something to do with it... but this doesn't really seem satisfactory to me... I suppose I just would like a little more solid reasoning behind it.

    I know this isn't always the case, but for a dry case this seems to be the norm.
  2. jcsd
  3. See 'HyperPhysics' for an insight.

    "When carefully standardized surfaces are used to measure the friction coefficients, the difference between static and kinetic coefficients tends to disappear, indicating that the difference may have to do with irregular surfaces, impurities, or other factors which can be frustratingly non-reproducible. To quote a view counter to the above model of friction:
    "Many people believe that the friction to be overcome to get something started (static friction) exceeds the force required to keep it sliding (sliding friction), but with dry metals it is very hard to show any difference. The opinion probably arises from experiences where small bits of oil or lubricant are present, or where blocks, for example, are supported by springs or other flexible supports so that they appear to bind." R. P. Feynman, R. P. Leighton, and M. Sands, The Feynman Lectures on Physics, Vol. I, p. 12-5, Addison-Wesley, 1964."
  4. Chi Meson

    Chi Meson 1,767
    Science Advisor
    Homework Helper

    One other part of the "static friction" business is the fact that to go from "not moving" to "moving" requires acceleration. This means that whenever the maximum static friction is tested in the usual "school lab" (i.e. "measure the force that is required to cause the object to start motion) the force that accelerates the object into motion is measured in addition to the frictional force.

    This does not explain the obvious differences between static and kinetic coefficients that is apparent for sustances like rosin and wax/water. Ultimately you have to look into the molecular bonding and/or electron transfer that occurs between any two substances.
  5. brewnog

    brewnog 2,775
    Science Advisor
    Gold Member

    I always like the pop-eng terms "sticktion and friction", I feel they describe the effect quite well!
  6. Clausius2

    Clausius2 1,467
    Science Advisor
    Gold Member

    Hi folks!

    The friction phenomena is something heuristic nowadays. People don't know certainly what happens. There have been a lot of theories trying to explain it. But what told me when I took this lesson?

    The differences between static and dynamic friction have micromolecular explaining. If you have an static brick over a surface, and If you look at the interface with a zoom, you will see some surface irregularities being in contact. The real contact area will not be the same of the "apparent" contact area. This last one, makes possible that "mu" friction coefficient does not depend on it. If you put a lot of load above the brick, apparent area would be the same, but not real area. When movement is started, the temperature at the interface increases straightforward, making possible the "fluence" of the irregularities material, and decreasing the grip between both faces. But it does not continue so long, because above of certain velocity threshold, "mu" starts to increase. It is due to a microscopic welding of the two faces, due to the temperature activation of chemical reactions between both materials.

    Hope this helping.
  7. theres a really easy way of looking at the difference. When a object is moving and creating friction, it has a momentum and this momentum helps it move over the rough surface. When the object is stationary, to get it to move, you need to give the same force you did when it had momentum, but you also need to factor in that the object is at rest, and you need to get it going, making stiction a harder force than friction.
  8. Clausius2

    Clausius2 1,467
    Science Advisor
    Gold Member

    As I have explained, the difference between the static and dynamic friction coefficient has nothing to do with the momentum. It is a microscopical matter.
  9. Im not talking about the coefficient, Im talking about the concept of friction.
  10. Clausius2

    Clausius2 1,467
    Science Advisor
    Gold Member

    Don't get angry with me, but friction concept is the friction coefficient itself. Knowing how friction coefficient works is knowing the concept of friction.
    Friction gets in mechanics equations by means of this factor. If this factor does not exist, also friction does not. And friction factor has a microscopic explanation. When Newton formulated its 2nd Law, he did not know how to explain friction phenomena. He was capable only by means of employing the 2nd law itself. But the science of friction provides a self-contained explanation, without the need of momentum and inertia.
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