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Homework Help: Circular motion of a clock

  1. Jan 8, 2008 #1
    The problem statement
    a) for a clock with minute hand 20 cm long find the average accelration of the hand as it moves from the 4 position to the 8 position

    b)an amusement ride is a rotating cylinder where the person is plastered to the inside wall as the ride speeds up and the floor drops away. if the cylinder is 6.6m across and turns 10.362 times in 21.5s what coeffient of friction is required to keep a person with mass 55kg from slipping down

    The attempt at a solution
    data given
    40min-->2400 secs (40 min since 4-8=40min)
    1hour-->3600 sec

    so first i drew a circle with tangents from 4 and 8 labling one tangent v1 and the other v2
    and used this equation to find v
    v=(4pi^2r)/(t^2) then from there on im not sure how you get acceleration since the other formula i can think of is a=(deltaV)/(delta t)
    but i can't use that formula since i don't have the differences in velocity so im not sure where to go from that

    6.60m diameter -->radius=3.3 (6.6/2)
    freq=0.48 -->(10.362/21.5)
    got a fdb

    v mg

    so fnety=f-mg
    since the guy isn't falling
    therefore f=mg
    so theres 2 unknowns, how do you find Fn?

    Thanks in advance ^^
  2. jcsd
  3. Jan 8, 2008 #2


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    Science Advisor
    Homework Helper

    For a), yes, delta(v)/delta(t). The speed at the two times is the same. But the velocity is different because velocity is a vector. The direction changes. For b) your notation is somewhat opaque. Find the normal force N at the given rotation speed. It has to produce a central acceleration on the person of v^2/r, right? What do you get? Then, ok, mg is the downward force and mu_static*N is the upward force. Balance them.
    Last edited: Jan 8, 2008
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