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Kinetic Friction of moving car hitting a stationary car

  1. Jul 24, 2009 #1
    1. The problem statement, all variables and given/known data
    A police officer investigating an accident estimates from the damage done that a moving car hit a stationary car at 24 km/h. If the moving car left skid marks 47 m long, and if the coefficient of kinetic friction is 0.68, what was the initial speed of the moving car? (The answer is in km/h.)


    2. Relevant equations

    F=ma
    Vx^2=Vox^2+2ax(deltax)



    3. The attempt at a solution

    I found acceleration by putting uk(mg)=max. I got 6.66 m/s^2 for acceleration. I plugged it into the kinematic equation Vx^2=Vox^2+2ax(deltax) and got 24.11 m/s for initial velocity. Then, I converted it into km/h and got 86.81 km/h. But, the answer is wrong and I don't know how to do it the correct way.
     
  2. jcsd
  3. Jul 24, 2009 #2

    rock.freak667

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    Homework Helper

    I'd use conservation of energy.

    Work done by friction = change in kinetic energy of the car

    Wf=ukN
     
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