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Homework Help: Centrifugal force coins

  1. Mar 9, 2014 #1
    Two coins are on a turntable that steadily speeds up, starting from rest, with a ccw rotation. Which coin flies off the turntable first?

    Coin 2 is further away than coin 1.


    I believe centrifugal acceleration is done by friction. So fs=us*m*g. Fc=mv^2/r.
    Then, v=sqroot(us*g*r).
    Suppose coin 2 is at radius 2(r1), (r1) being the distance from center of coin 1.
    angular velocity 1= r1(sqroot(us*g))
    angular velocity 2=2r1(sqroot(us*g))

    So, I conclude that coin 1 must be thrown off first. HOWEVER, the answer manual says it is coin 2, which is an obvious answer for me too. But, I can't seem to be able to prove it.
  2. jcsd
  3. Mar 9, 2014 #2
    Your expressions for angular velocity are not right.
    Check again.
    The velocity (v) is OK.
  4. Mar 9, 2014 #3
    Ok, I rearranged it. I now understand! v=wr indeed!
    Is there any easier way to think about this problem?
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