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Homework Help: Kinetic energy: car stopping, finding speed of car

  1. Nov 25, 2007 #1
    Last edited: Nov 25, 2007
  2. jcsd
  3. Nov 25, 2007 #2
    W = -Fd

    W = [tex]\Delta[/tex]KE

    W = (1/2)mv2^2 - (1/2)mv1^2

    -Fd = -(1/2)mv1^2

    but F = [tex]\mu[/tex]s(mg)

    -[tex]\mu[/tex]s(mg)d = -(1/2)mv1^2

    [tex]\mu[/tex]sgd = (1/2)v1^2

    v1 = sqrt(2[tex]\mu[/tex]gd)

    The flaw in your method was that you were using newton's second law, F = ma. While it's true in this case that the only force affecting the car's velocity is static friction, and so this is equal to ma, you don't know what a is, so it's necessary to formulate it differently.
    Last edited: Nov 25, 2007
  4. Nov 25, 2007 #3


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    It is against forum rules to provide complete solutions to homework questions. But anyway, you've make a mistake in your answer.

    The rules can be found at: https://www.physicsforums.com/showthread.php?t=5374
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