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Direction of friction between disks

  1. Dec 9, 2014 #1
    1. The problem statement, all variables and given/known data
    2ajb6md.jpg
    Disk D turns with a constant clockwise angular
    velocity of 30 . Disk E has a weight of 60 lb and is
    initially at rest when it is brought into contact with D.
    Determine the time required for disk E to attain the same
    angular velocity as disk D. The coefficient of kinetic
    friction between the two disks is 0.3 . Neglect the
    weight of bar BC.

    2. Relevant equation
    Summation of forces and moments.

    3. The attempt at a solution
    I am confused with the direction of friction. In my calculations, the friction points to the left because the velocity where the disk meets points to the right; which resulted to a wrong answer. However, the correct free body diagram is shown above.
     
  2. jcsd
  3. Dec 9, 2014 #2

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    That is the force that "what" exerts on "what else?" And what is the reaction force to that?
     
  4. Dec 9, 2014 #3
    So when I isolate disk D, the friction points to the left to counter the clockwise motion, and when I isolate disk E, it now points to the right. Is this right?
     
  5. Dec 9, 2014 #4

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  6. Dec 9, 2014 #5
    Thanks!
     
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