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Investigators measuring a car skid mark to find velocity.

  1. Jul 17, 2009 #1
    1. The problem statement, all variables and given/known data
    At an accident scene on a level road, investigators measure a car's skid mark to be 93m long. It was a rainy day and the coefficient of friction was estimated to be 0.32.
    Use these data to determine the speed of the car when the driver slammed on (and locked) the brakes.

    2. Relevant equations
    Fd=-m*vinitial^2/2


    3. The attempt at a solution
     
  2. jcsd
  3. Jul 18, 2009 #2

    rock.freak667

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    So what's the problem then?
     
  4. Jul 18, 2009 #3
    the answer didnt work.
     
  5. Jul 18, 2009 #4
    it gave me -12..and aparently that isnt the right answer
     
  6. Jul 18, 2009 #5

    rock.freak667

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    What exactly did you do to get -12m/s?
     
  7. Jul 18, 2009 #6
    well the F here is the force which comes from Fn*CoFr=Ff where Fn equals Normal Force CoFr is co. of friction and Fr is force of friction. So if we subsitute F for Fn*CoFf, the masses on both sides cancel out. leaving us with CoFr*g*d=-vinitial^2/2. The by pluging in the values, i got -23 acctually which isnt the answer either.
     
  8. Jul 18, 2009 #7

    rock.freak667

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    Really? Calculate that again.
     
  9. Jul 18, 2009 #8
    oh lord..k its 24. thanks alot
     
  10. Aug 13, 2009 #9
    I got, by using Vf2 = Vi2 + 2ad and using the fact that F/m = (coefficient of friction)*g, 24.15 m/s.

    EDIT: Whoops, I'm sorry for the bump!
     
    Last edited: Aug 13, 2009
  11. Aug 13, 2009 #10
    Bolded fine as long as the road is horizontal. Normal Force is not always = mg
    The original poster was using work change in kinetic energy idea I think. No kinematics necessary.
     
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