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Homework Help: Static friction on objects resting on a rotating surface

  1. Mar 18, 2010 #1
    1. The problem statement, all variables and given/known data

    Suppose I have a brick that's sitting at the edge of a disc that is rotating clockwise with uniform circular motion.

    I what direction(s) are the forces of static friction acting?

    2. Relevant equations

    a = v^2/r


    3. The attempt at a solution

    I know that a centripetal force from static friction acts on the brick to keep it moving in a circle along with the disc. Is there also a static friction force acting on the brick tangentially in a counterclockwise direction to prevent the disc from slipping under the brick?
     
  2. jcsd
  3. Mar 18, 2010 #2
    The brick is moving in a circle with constant speed. What would happen to the
    speed of the brick if there was a friction force that wasn't perpendicular to the direction of the velocity?
     
  4. Mar 18, 2010 #3

    ehild

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    No.

    ehild
     
  5. Mar 18, 2010 #4
    If a force were applied tangentially, the brick would move in non-uniform circular motion (either accelerating or decelerating tangentially).

    The trouble I have is with the fact that the force in question is friction; and that static friction can act without causing an object to move.

    I'm finding it hard to picture why the disc doesn't slip (tangentially) under the brick, if static friction only points towards the centre of the disc.
     
  6. Mar 18, 2010 #5

    ehild

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    A body performs uniform circular motion under the effect of the appropriate centripetal force. If the brick moves already together with the disk then the only in-plane force points towards the centre of the disc and equals to mv^/R.

    When the disk was in rest and started to rotate, the brick might have slipped at the beginning till the kinetic friction had accelerated it to the velocity equal to its place on the disk. At this starting period, the force had both radial and tangential components.

    ehild
     
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