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Homework Help: Circular Motion with static friction

  1. Nov 13, 2013 #1
    1. The problem statement, all variables and given/known data

    A bus passenger has her laptop sitting on the flat seat beside her as the bus, traveling at 10.0 m/s, goes around a turn with a radius of 25.0 m. What minimum coefficient of static friction is necessary to keep the laptop from sliding?


    V = 10 m/s
    r = 25.0 m

    2. Relevant equations

    Fc = mv ^ 2 / r

    Ff = Us x Fn

    3. The attempt at a solution
    mv^2/r = Umg
    U = v^2/gr
    U = 10^2/9.6(25)
    U = 100/245
    U = 0.41 *rounded up

    I think i got it right because i checked my lesson and this is the way they did it but i don't understand what is happening with the forces for me to get this solution, can someone please just clarify on whats actually happening and why this is what i'm supost to do (if it is infact correct).
    Thank you - Nicky
  2. jcsd
  3. Nov 13, 2013 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Hi Nicky, welcome to PF!

    It appears that you got the correct answer by plugging in the given values into the equation you found in your lesson, without understanding why you were using that equation. This is definitely not a good thing.
    You should become familiar with friction, newton's laws, free body diagrams, and cenrtipetal acceleration. An object moving in a curved path (like a circle) experiences an acceleration, v^2/r, toward the center of the circle (why?) which must be caused by a net force acting toward the center of the circle, per Newton's 2nd law F_net = ma. In this case, the only force acting on the laptop toward the center of the circle is the friction force, uN, where N is found by applying Newton's first law in the vertical direction. Since the friction force is the only force acting toward the center, it is the net force acting toward the center, or the so called centripetal force.
  4. Nov 13, 2013 #3
    Thank you so much, i just needed to picture it in my head, but now i understand how it works. I really appreciate you taking the time to help me out =).
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