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Homework Help: Friction question

  1. Apr 16, 2009 #1
    The average rate of acceleration for a breaking car was -12.2m/s2. If the car had a mass of 925 kg, find the frictional force and direction.

    a = -12.2m/s2
    m = 925 kg
    Fn = 9074.25 N

    This is what I've figured out. I don't know how to find the coefficient of friction. If I get the coefficient I think I can multiply it by the normal force to get the frictional force.
  2. jcsd
  3. Apr 17, 2009 #2


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    Science Advisor

    You're complicating the problem unnecessarily.

    The question asks only for the frictional force and direction. Try drawing a free-body diagram for the car and then write down the F=ma equations.
  4. Apr 17, 2009 #3
    F_f = u*F_n

    F_n = mg

    F_f = u *mg

    find u
  5. Apr 17, 2009 #4
    The problem only asks for F_f :smile:
    The free body diagram is the way to go. Fn and Fg cancel out. What other force or forces provide the acceleration?
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