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Maximum angular speed

  1. Jun 7, 2009 #1
    1. The problem statement, all variables and given/known data
    In a cement mixer, cement, gravel, and water are mixed by tumbling in a slowly rotating drum. If the drum spins too fast, the materials stick to the wall instead of mixing. Assume the axis of rotation is horizontal. Calculate the maximum angular speed for which the materials will mix. Express the answer as the number of revolutions per minute. Radius = 0.68 m.

    2. Relevant equations

    3. The attempt at a solution
    I have tried using angular speed = v/r = theta/t . I assumed that t was equal to 1 minute and the angle was 2*pi. However, this did not work.
  2. jcsd
  3. Jun 7, 2009 #2


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    The maximum angular velocity for which the material still mixes is when the material just falls down when it reaches the "top", maximum height, of the rotating drum. Which force makes the material fall? When the material just about sticks to the drum which force is equal to the mystery force that makes the material fall?
  4. Jun 7, 2009 #3
    Okay, so I know that the force which pulls the mixture down is the gravitational force. I believe that the force which is the mystery force is the normal force. Thus, making g+N=0. I just before the mixture falls it would have a velocity of 0 or would it be the velocity the mixture is moving in based on the rev?
  5. Jun 7, 2009 #4


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    No the total velocity does not have to be 0 for the mixture to fall. Hint, centripetal force.
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