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Homework Help: Circular Motion, Static Friction

  1. Jul 10, 2008 #1
    1. The problem statement, all variables and given/known data

    An astronaut is training in an earthbound centrifuge that consists of a small chamber whirled around horizontally at the end of a 4.5 m-long shaft. The astronaut places a notebook on the vertical wall of the chamber and it stays in place.

    If the coefficient of static friction is 0.70, what is the minimum rate at which the centrifuge must be revolving?
    Express your answer using two significant figures.
    w = ________ rad/s

    2. Relevant equations


    F_s= uN

    3. The attempt at a solution


    F_s= uN


    setting the two forces equal togehter


    v= sqrt(ugr)

    i got 5.6.

    where am i going wrong? thanks
  2. jcsd
  3. Jul 10, 2008 #2
    f_s=u(mg) isn't valid in this problem

    The normal force isn't mg because gravity is acting downwards. You need the a=v^2/r.

    I think you just had it backwards... You wrote
    instead of
  4. Jul 10, 2008 #3


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    Homework Helper

    Keep in mind that "normal force" means the force acting perpendicular to the surfaces in contact ("normal" means "perpendicular" in geometry). So, in this situation, the normal force is pointing at the center of the circle.

    This means that the centipetal force (which is a resultant force and not a physical force -- which is why we never draw it in a free-body diagram) is provided by the normal force for this sort of problem.
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