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Homework Help: Static, kinetic friction

  1. May 28, 2010 #1
    1. The problem statement, all variables and given/known data

    Assume that the driver begins to brake the car when the distance to the wall is d=107m, and take the car's mass as m-1400kg, its initial speed as vo= 35m/s and the static coefficient to be .50. Assume that the car's weight is distributed evenly on the four wheels, even during braking.
    a.) what magnitude of static friction is needed between the tires and road to stop the car just as it reaches the wall?
    b.) what is the max possible static friction?
    c.) if the coefficient of kinetic friction betweeen the sliding tires and the road is .40, at what speed will the car hit the wall.
    d.) to avoid the crash, a driver could elect to turn the car so that it just barely misses the wall. What magnitude of frictional force would be required to keep the car in a circular path of radius d and at the given speed vo?
    e.) is the required force that the maximum static friction so that a circular path is possible?

    2. Relevant equations
    v2=v initial-2ad

    3. The attempt at a solution

    f= 1400 x 5.724
    f= 8014 N

    b.) Fs max= usFn
    =.5 x 1400 x 9.8

    c.) ma= -ukmg
    a=-.4 x 9.8

    v2= 35^2+2x-3.92x107
    v=19.6 m/s

    mv^2/R= uk x N
    N= 40070 N
  2. jcsd
  3. May 28, 2010 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Your answers to a, b, and c look OK
    If the car is to stay in a circle, it is not sliding..use the static friction coeficient... Don't solve for N, N is known....you are looking for the the friction force required to keep it in the circle (mv^2/r)....Since F_f (max)= u_s(N), can you answer part e?
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