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Circular motion and static friction of earth

  1. Feb 8, 2009 #1
    1. The problem statement, all variables and given/known data
    A platform on earth rotates at constant angular speed 2 rad/s. A 60 kg man stands on the platform. Static friction between the platform and the man's feet prevent slipping.
    (a) Make a diagram showing the forces he exerts on the platform standing a distance r from the axis of rotation. r = 0, 1, 2
    (b) What direction does the man sense as "up"? Calculate the angle that "up" makes with the vertical using values of r as stated.
    (c) If plants were planted on the platform, in what direction would they grow?

    2. Relevant equations
    a = omega^2 r

    3. The attempt at a solution
    (a) In the x plane, force of friction acts as force of centripetal acceleration = m omega^2 r. What I don't understand is what is the reactive force?
    In the y plane, normal force opposes gravitational force = mg. If this is so, the man's weight should be constant, despite his distance from the rotational axis. Is (a) a trick question?
  2. jcsd
  3. Feb 8, 2009 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to Physics Forums.
    It doesn't matter what we call the reaction force merely that one exists. Since the table exerts a force on the man, the man must also exert a force on the table in the opposite direction (i.e. radially outward) and of the same magnitude. I am being intentionally vague regarding the 'name' of this reaction force to avoid a long discussion on the centrifugal force.
    No trick, you are indeed correct.
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