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Got a question

  1. Nov 17, 2007 #1
    If a glass of water is resting on a table and there is a piece of paper or cloth or whatever under the glass, and you pull the paper/cloth really fast, the glass does not move. Can someone please explain the physics of this to me. THNX
     
  2. jcsd
  3. Nov 17, 2007 #2

    dynamicsolo

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    Well, the glass does move a little bit, less the faster you can yank out the cloth. This is a classic magician's trick, in which a tablecloth is snapped out from under a fully set table without causing anything to fall over the edge. (Everything will jump a bit, however...)

    Think about the friction between the glass and the cloth and then the glass and the table surface. You want to pull quickly to overcome static friction. You will still have kinetic friction between the glass and cloth, so the glass will begin to accelerate in the direction the cloth is being yanked. Before much time passes, though, the cloth will have been completely pulled away, leaving the glass now in contact with the underlying table surface. The kinetic friction in that case will generally be greater, but in any event, the friction will immediately act to decelerate the glass to a stop.

    If you pick some reasonable coefficients of kinetic friction between the surfaces, you can work on how fast you have to pull the cloth away to keep the glass from sliding more than a couple inches (about 5 cm.). This has to be done in a (smallish) fraction of a second. (It's definitely suggested that you practice this with non-breakables before demonstrating this in front of an audience...)
     
  4. Nov 18, 2007 #3
    so speed affects the force of friction right (but it doesn't change the coefficient)??
     
  5. Nov 18, 2007 #4

    dynamicsolo

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    The rate at which you pull out the cloth affects the time over which the friction will act, not the coefficient of friction or the magnitude of the force. The friction from the cloth will give the glass a certain positive acceleration from rest; the time interval of contact will determine the speed the glass reaches. Kinetic friction from the table surface will then decelerate the moving glass at a certain rate.
     
  6. Nov 18, 2007 #5
    ok makes sense, thanks
     
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