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Homework Help: Newtons laws: tablecloth trick

  1. Jul 11, 2010 #1
    For the following question:

    A magician pulls a tablecloth out from under some dishes. *How far do the dishes move during the .25 seconds it takes to pull out the tablecloth? The coefficient of kinetic friction between the cloth and the dishes is .12

    I think I got the answer right by considering the force of kinetic friction (to the right, normal up, weight down) so since kinetic friction is = to .12 * N then .12mg=ma giving .12g=a then I used displacement=.5at^2 and got .037 meters. *Now that I'm reviewing the problem I'm wondering why in the free body diagram you don't include the force of the tablecloth pulling out from under the dishes. *Sorry if this sounds like a dumb question but I looked online and though I can't check my answer in the back of the book, all other examples don't consider the table cloth pulling on the dishes so my question is why?
     
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  3. Jul 12, 2010 #2

    kuruman

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    If you draw your free body diagram correctly, the force that the tablecloth exerts on the dishes should be part of it. There are two pieces of the Universe that interact with the dishes, (a) Earth, which exerts a force straight down and (b) tablecloth which exerts a force that has two components (i) the normal force opposite to the Earth force and (ii) the force of friction which is perpendicular to the normal force and in the direction of motion of the dishes.

    What you should not include is the force that the magician exerts on the tablecloth; it is not part of your system, which is just the dishes.
     
  4. Jul 12, 2010 #3
    But as the magician pulls on the tablecloth doesn't that tablecloth also pull on the dishes?
     
  5. Jul 12, 2010 #4

    kuruman

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    Yes, that's what I said. The tablecloth not the magician is what exerts the force on the dishes. The magician is not in any way touching the dishes, is he? Therefore he does not exert a force on the dishes.
     
  6. Jul 12, 2010 #5
    Ok yes, I think I'm forgetting what a free body diagram is, lol. Only the forces that touch the object, with the exception of gravity, I assume, are included. Thanks for all your help!
     
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