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Homework Help: Quick sign question for uniform circular motion

  1. Feb 15, 2009 #1
    1. The problem statement, all variables and given/known data
    what is the maximum speed with which a 1500kg car can make a left turn around a curve of radius 50m on a level road without sliding?

    2. Relevant equations

    3. The attempt at a solution
    i already got the answer....but my question is:

    f_s points left and the equation sigmaF_r =
    f_s = mv^2/r

    why isn't it negative if the force in the free body is pointing left? is it because the car is turning left?
  2. jcsd
  3. Feb 15, 2009 #2


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    Homework Helper

    The F in F = mv^2/r is the force in the direction of the center of the turning circle. The friction force is in that direction, so it is positive. I don't think you want to confuse things by thinking about left and right when the essential direction is centripetal - toward the center.
  4. Feb 15, 2009 #3
    the book says the radial component is simply the magnitude of the vector....so in doing probs with uniform circular motion, where a is pointing towards the center....it is positive regardless of where it points in the free body diagram?
  5. Feb 15, 2009 #4


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    Homework Helper

    Yes, positive as long as it points toward the center.
  6. Feb 15, 2009 #5
    thank you
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