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Homework Help: Friction contradiction

  1. Feb 23, 2008 #1
    1. The problem statement, all variables and given/known data

    What is the smallest radius of an unbanked (flat) track around which a bicyclist can travel if her speed is 29 km/h and the coefficient of static friction between tires and track is 0.29?



    2. Relevant equations

    coefficient of static friction = (f of maximum static friction / Normal force)

    fnet = ma = m * v^2 / r

    3. The attempt at a solution

    If an object is moving, isn't their just kinetic friction? I get that the normal force is the acceleration, but how can there possibly be a maximum static friction at this point?

    Here's what I've got:

    29km/h = 8.0556 m/s

    .29 = fstatmax / a, therefore a = fstatmax / .29

    Fnet = m* a = m * (v^2 / r)
    Fnet = a = v^2 / r
    fstatmax / .29 = (8.0556)^2 / r

    Thanks for any help!
     
  2. jcsd
  3. Feb 23, 2008 #2
    In case of tyres, the bottom-most point is instantaneously at rest. That's why static friction.

    f=0.29Xmg.
    [tex]f=\frac{m{v^2}}{r}[/tex]

    solve to get r.
     
  4. Feb 23, 2008 #3
    Ah...
    thank you for the help!
     
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