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Friction force in Newton's law

  1. Oct 11, 2011 #1
    In the early afternoon, a car is parked on a street that runs down a steep hill, at an angle of 35.0o relative to the horizontal. Just then the coefficient of static friction between the tires and the street surface is 0.725. Later, after nightfall, a sleet storm hits the area, and the coefficient decreases due to both the ice and a chemical change in the road surface because of the temperature decrease" By what percentage must the coefficient decrease if the car is to be in danger of sliding down the street?

    I have no idea how to start....
    If I draw a FBD, this is all that I got

    mgcosθ = N
    mgsinθ - fs = 0

    and if the car is in the verge of sliding, fs will be max, fs = mgsinθ
    us * N = mgsinθ
    us * mgcosθ = mgsinθ

    us = tanθ ????
    Now I dont now where to go from here.........
  2. jcsd
  3. Oct 11, 2011 #2
    Your physics is ok.
    Now find the value of tan theta which is the coefficient required.
    Then find the difference between the two values of the coeffienct.
    Then find the % difference.
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