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NEED HELP HW ! I really need this, it's due sooN!

  1. Mar 10, 2007 #1
    NEED HELP!!!! HW!!!!!! I really need this, it's due sooN!

    Can someone please explain me how coefficeint of kinetic friciton and static friction works ?! ?!

    Look at these problems, I don't get ANY of these!!!


    check out the odd problems for me >< at least....

    Last edited: Mar 10, 2007
  2. jcsd
  3. Mar 10, 2007 #2


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    Staff Emeritus
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    What particular questions are you trying to answer? You need to show some effort before we can help you!
  4. Mar 10, 2007 #3
    gah sorry!

    trufthfully, I don't exactely know how to start...
    um.. it's just the equation is odd to me. The book said that static friction is supposed to be dimensionless...
    F<(or eqaul to) U(s)N..

    How in the WORLD does this even work?!
  5. Mar 10, 2007 #4

    Doc Al

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    The coefficient of friction is dimensionless, not the friction force itself.

    This equation tells you how to calculate the maximum value of static friction between two surfaces, which depends on the nature of the surfaces (given by the coefficient of friction) and how hard they are being squeezed together (given by the normal force).
  6. Mar 10, 2007 #5


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    If one has a box or block resting on a flat surface, one will find that the force (push) is greater than the force to sustain movement. This is because the static friction is greater than the kinetic friction, which implies the coefficient of static friction is greater than that of kinetic friction.

    The force of friction 'acts' tangentially to the contact surface (i.e. in shear) and is proportional to the normal force, e.g. weight (force of gravity) of an object. The proportionality constant is the 'coefficient of friction.'
  7. Mar 10, 2007 #6
    Lets say you apply a force to move a block. You start with a light force and the block doesnt move. You push a little harder and it still doesnt move. Finally, you reach a point where you have applied a great enough force to get the block moving. In the first 2 cases, the force applied was less than the maximum static friction force. The force of friction balanced the applied force in these cases. It can take on any value in this range. Then after you exceed the maximum static friction force, a net force is created which moves the block. Note that its easier to keep the block moving once you get it started. This is because the coefficient of kinetic (sliding) friction is less than the coefficient of static (starting) friction
  8. Mar 10, 2007 #7
    Good clear explanation, Turdferguson. And the reason it's dimensionless is because it's a ratio of two forces -- all the units cancel each other out.
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