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Homework Help: Finding velocity (deals with coefficient of kinetic energy)

  1. Nov 17, 2008 #1
    1. The problem statement, all variables and given/known data

    Police lieutenants, examining the scene of an accident involving two cars, measure the skid marks of one of the cars, which nearly came to a stop before colliding, to be 85 m long. The coefficient of kinetic friction between rubber and the pavement is about 0.70. Estimate the initial speed of that car assuming a level road.

    2. Relevant equations
    Ff=(coefficient of kinetic energy) Fn
    vf = vi + at

    3. The attempt at a solution
    i have no idea what to do since none of the formulas work with the knowns.
  2. jcsd
  3. Nov 17, 2008 #2


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    Homework Helper

    You should also know this formula

    V2 = 2*a*x

    It among others can be found here:

    You also know that the force on the car from friction is m*a = u*m*g

    Which means that a = u*g

    V2 = 2*a*x = 2*u*g*x
  4. Nov 17, 2008 #3
    what does x represent in that formula?
  5. Nov 17, 2008 #4


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    Homework Helper

    Doesn't the problem say that it decelerated over a distance of 85 m ?

    Wouldn't you want to know how much the car decelerated over that distance to give you an idea of what velocity it slowed from?
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