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Why coin drop down into the glass?

  1. Jul 28, 2005 #1
    When we put a coin on a boardcard and both the boardcard and coin put on a glass.
    when we hit the boardcard,why the coin drop down into the glass.??

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  2. jcsd
  3. Jul 28, 2005 #2


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    what?? I dont understand...
  4. Jul 28, 2005 #3
    Are you asking about the inertia of the coin or the surface tension of the water?\
  5. Jul 28, 2005 #4
    inertia...it seems to say nothing bout water at all.
  6. Jul 28, 2005 #5
    Well the short answer is that the force of friction between the coin and the board is not great enough to overcome the inertia of the coin

    I suppose he knew that though...?
  7. Jul 28, 2005 #6


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    Apparently not, or he wouldn't have asked. We have no idea how old this lad is, or what his level of education is.
    The inertial of the coin is very high, and as suodei said, overpowers the forces that hold it to the board. When the board is abruptly removed, the coin stays behind. It's the same principle as the old trick of pulling a tablecloth off while leaving the dishes behind. With a slower movement, the coin would stay with the card.
  8. Jul 28, 2005 #7

    Doc Al

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    Staff: Mentor

    As several have noted, the answer relates to the inertia of the coin. All objects that have mass require a force to get them moving (or change their state of motion). The only horizontal force on the coin is the friction between card and coin. If there were no friction, then you could slide the card away at any speed and the coin wouldn't move (until it started falling, of course).

    But there is friction. If you move the card slowly, then static friction will move the coin along with the card. But if you move the card quickly, the much lower kinetic friction will be acting: the coin will move, but not as quickly as the card. Now if you move the card real quick, that friction force won't have much time to accelerate the coin; it appears to fall straight down (actually it must move a little to the side as well).
  9. Jul 28, 2005 #8


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    Of course there is no answer if the question is not understood. :frown:
  10. Aug 1, 2005 #9
    Thank you Doc Al my problem is solved by your answer.
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