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Homework Help: Kinetic friction ratio problem

  1. Sep 19, 2013 #1
    "A driver makes and emergency stop and locks up the brakes of the car, which skids to a stop on dry concrete. Consider the effect of rain on this scenario. If the coefficients of kinetic friction for rubber on dry and wet concrete are μk (dry)=0.80 and μk(wet)=0.50, how much farther would the car skid (expressed in percentage of the dry-weather skid) if the concrete were instead wet?"

    I thought it might just be dividing dry/wet coefficients and multiplying by 100 to get the percentage. I don't think this is right, as the professor was implying there was more to solving it than that. He talked about (dry-wet)/dry to work out the answer, but it wasn't just the μ values. I feel like I missed something that should be obvious. Any help will be greatly appreciated!
  2. jcsd
  3. Sep 20, 2013 #2


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    Staff: Mentor

    Hi marmadmit. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Wild guessing is never the recommended approach.http://physicsforums.bernhardtmediall.netdna-cdn.com/images/icons/icon13.gif [Broken] Do you have some equations to throw around and see what you can connect up?

    So far, we seem to have force and distance, so you need equations to allow you to relate those.
    Last edited by a moderator: May 6, 2017
  4. Sep 20, 2013 #3


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    Staff Emeritus
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    What's the expression for the frictional *force* in terms of μ and the normal force? That will tell you how much larger the frictional force is in the dry case than in the wet case.

    Now, what does force tell you about the acceleration? Hint: there is some fundamental law of motion here that will help answer that question.

    Given the acceleration, how much distance will it take to come to rest? In other words, how does the stopping distance depend on acceleration? For this, you will need some *kinematics*.
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