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Coefficient of kinetic friction problem

  1. Sep 15, 2008 #1
    1. The problem statement, all variables and given/known data

    On a level road with its brakes on, a car traveling at 86 km/hr stops within 93 m when the brakes lock and the car slides.
    - a) What is the magnitude of the acceleration of the car? 3.0967 which i already found.
    - b) What is the coefficient of kinetic friction between the tires and the pavement? ? ?



    2. Relevant equations

    kinetic friction = (coefficient of kinetic friction)( Normal force)


    3. The attempt at a solution

    For part B (the part i can't figure out)
    86km/hr = 24m/s
    24(9.8 gravity)= 235.2 Normal force
    24/235.2 = .102 = coefficient of kinetic friction which is wrong
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 15, 2008 #2

    mgb_phys

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    You have the right equation for friction but you have misunderstood what it means.
    The normal force is the weight of the car = mass * g
    The kinetic friction is the friction force stopping the car. You know this because the "coefficient of kinetic friction" has no units this kinetic friction must have the same units as Normal force for the equation to balance.
    You can find this from F=ma where a is the (de)acceleration of the car while it is slowing and m is the mass of the car again.
     
  4. Sep 15, 2008 #3
    how can i find the normal force if i don't have the mass of the car?
     
  5. Sep 15, 2008 #4

    mgb_phys

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    Write out the equation - 'm' appears on both sides.
     
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