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Force question on pulling worker

  1. Feb 13, 2008 #1
    A worker pulls horizontally on a rope that is attached to a 10-kg crate resting on a rough
    floor. The coefficients of static and kinetic friction are 0.5 and 0.3, respectively. The
    worker pulls with a force of 40 N. The frictional force exerted by the surface is...??

    the box is moving because fk<fs

    If the Fnet is not 50N, the applied force then
    Fnet= T - fk
    Fnet= 50N - fk

    and of cource fk= .3 * 10 * 9.8 = 29.4N

    Is this right??
     
  2. jcsd
  3. Feb 13, 2008 #2

    Doc Al

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    Staff: Mentor

    The first thing to figure out is: Does it move?
     
  4. Feb 13, 2008 #3
    Yes it does move because fk= umg and fs=umg and since us>uk fk<fs which means that the object moves and accelerates, right??
     
  5. Feb 13, 2008 #4

    Doc Al

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    I don't understand your reasoning. Instead, figure out the maximum force that static friction can deliver given the nature of the surfaces. Compare that maximum value with the applied force.
     
  6. Feb 13, 2008 #5
    ok...the fs=49N which is < the 40N push. So the box is not moving. So is the fk= 40N so fnet= 0 = fp - fk???
     
  7. Feb 13, 2008 #6

    Doc Al

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    Good. The maximum possible static friction force is 49 N, which is more than enough to handle the applied force of 40 N. So the box doesn't move.

    Since it doesn't move, fk (kinetic friction) is irrelevant.

    But the net force is zero, so what must be the friction?
     
  8. Feb 13, 2008 #7
    40N


    thank you very much
     
  9. Feb 13, 2008 #8

    Doc Al

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    Perfecto!
     
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