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Clarification about friction equation

  1. Jul 9, 2007 #1
    Clarification about friction equation !!

    1. The problem statement, all variables and given/known data

    Suppose there is an object on the top of a ramp at a rest would these equation/s be correct to preform calculations considering friction applies

    [tex]\Sigma[/tex]F_y = n - (m * g * Cos[tex]\Theta[/tex]) = 0
    [tex]\Sigma[/tex]F_x = (m * g * Sin[tex]\Theta[/tex]) - ([tex]\mu[/tex]_s * n) = 0

    Suppose there is an object on the top of a ramp was sliding downwards would these equation/s be correct to preform calculations considering friction applies

    [tex]\Sigma[/tex]F_y = n - (m * g * Cos[tex]\Theta[/tex]) = 0
    [tex]\Sigma[/tex]F_x = (m * g * Sin[tex]\Theta[/tex]) - ([tex]\mu[/tex]_s * n) = (m * a) or
    [tex]\Sigma[/tex]F_x = (m * g * Sin[tex]\Theta[/tex]) - ([tex]\mu[/tex]_k * n) = (m * a)

    Note : ([tex]\mu[/tex]_k * n) would be located at ([tex]\mu[/tex]_s * n)


    [​IMG]


    My Question is are these formulas correctly represented so that i could solve any question that involves static and kinetic friction
     
  2. jcsd
  3. Jul 9, 2007 #2

    Dick

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    Science Advisor
    Homework Helper

    Yep. That's pretty much it. Except you can't consider static friction when an object is accelerating, better put a=0 in that case, as you did the first paragraph. Good luck with actually solving problems!
     
  4. Jul 10, 2007 #3
    Thanks !!! Understood....
     
  5. Jul 10, 2007 #4

    Doc Al

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    Staff: Mentor

    Careful with setting static friction equal to [itex]\mu_s N[/itex]. That's the maximum value of static friction--the actual value may well be less.
     
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