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Problem on Kinetic Friction

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  1. Feb 20, 2013 #1
    1. A police officer investigating an accident estimates from the damage done that a moving car hit a stationary car at 25 km/h. If the moving car left skid marks 44 m long, and if the coefficient of kinetic friction is 0.68, what was the initial speed of the moving car?



    2. Relevant equations
    fk = μk N


    F = ma
    3. The attempt at a solution
    drew a free body diagram, converted 25 km/h to 6.94 m/s
     
  2. jcsd
  3. Feb 20, 2013 #2

    haruspex

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    Can you calculate the deceleration of the skidding car?
     
  4. Feb 20, 2013 #3
    i tried and i do not know how. i tried getting the time by multiplying some number bye 44 in order to give me 6.94 but I didn't get the exact number... it is approaching to 6.3something.
     
  5. Feb 20, 2013 #4
    Apply Newton 2 to your free-body diagram to get the acceleration of the car.
    Then you can calculate the initial velocity.
     
  6. Feb 20, 2013 #5

    Andrew Mason

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    How is the change in kinetic energy of the car (ie. from just before the skid starts until it strikes the car) related to the length of the skid mark and the force of friction?

    AM
     
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