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Homework Help: Force and distance

  1. Feb 13, 2008 #1
    A 5.0-kg stone is pushed out onto a level horizontal surface with a speed of 2.0 m/s. If
    it comes to rest after moving a distance of 20 m, the average frictional force acting on
    the block must have been...???

    Since it slows down the fk is > than the Fpush.

    Fnet= fk-fpush
    ma = umg - fpush
    ma + fpush = fk

    vf^2= v0^2 + 2as
    0= 4 + 40a
    -4/40 = a = -.1m/s/s

    So the stone is slowing down .1m/s every second which will take 20s.

    Since the a = -.1m/s/s and f=ma
    then wouldn't the fk = 5*-.1= -.5N
    Or would you do fk=mg= 5*9.8= 49N

    Any help would be great. Thank you.

    Stephen Doty
  2. jcsd
  3. Feb 13, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    No. The push that set the stone moving is irrelevant. All that matters is that somehow the stone is moving at the given speed and begins to slow down. The only force acting on the stone is friction.

    This is good!

    What must the force equal to produce this acceleration?
  4. Feb 13, 2008 #3
    So fk = 5*-.1= -.5N
  5. Feb 13, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Right. The average friction force is 0.5 N.

    (If you are familiar with energy methods, you can solve this problem that way as well.)
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