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Work done by kinetic friction force without coefficient?

  1. Oct 8, 2014 #1
    1. The problem statement, all variables and given/known data
    A person pulls a box (m=10 kg) horizontally with +2m/s2 acceleration by applying 45 N force. The displacement of the box is 8 meters from initial position.
    a) How much work is done by applied force?
    b) How much work is done by kinetic frictional force?

    2. Relevant equations
    Wf = Fcosθs
    F=ma
    Wfk = -Fks ???
    Fk = μN ???

    3. The attempt at a solution
    I solved for part a, which was 360J.
    I don't know how to solve for part b, which it seems like I would need the friction coefficient?
    I tried Wfk = -(10)(2)(8) = -160 J, but that is incorrect.
    How do I find Fk to solve for the work done by friction?
     
    Last edited: Oct 8, 2014
  2. jcsd
  3. Oct 8, 2014 #2

    jack action

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    Answer this question: How much of the applied force is used to accelerate the box?
     
  4. Oct 8, 2014 #3
    Um, 45 N of force was applied to it. Would force of kinetic friction be -45 N then? Or am I just getting myself confused?
     
  5. Oct 8, 2014 #4
    Jack has a point.
    Yes, you applied 45 N,
    but apparently that's not the force used to accelerate the box since its mass is 10 Kg and its accelerations is 2 m / s / s. The net force is what you should be looking at. Rethink. :):)
     
    Last edited: Oct 8, 2014
  6. Oct 8, 2014 #5

    jack action

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    [tex]\sum F = ma[/tex]
    [tex]F_{total} - F_{friction} = ma[/tex]
     
  7. Oct 8, 2014 #6
    Write Newton's second law for the box.
     
  8. Oct 8, 2014 #7
    Alright, so Ftotal = 45 N, I am looking for frictional force, and ma = 20.
    45-Ffk = 20
    Ffk=25
    Then I have Wfk = -(25)(8) = -200 J?
    Is this the correct answer?
     
  9. Oct 8, 2014 #8

    jack action

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    Look again at your OP and you already found 360 J and 160 J. How does your 200 J fits in there?

    Just for the fun of it, can you find the coefficient of friction?
     
  10. Oct 8, 2014 #9
    Are you saying I could have subtracted the 160 from 360? What value does the 160 J represent that I solved for? Gosh, I feel stupid.
     
  11. Oct 8, 2014 #10
    Never mind, I got it all covered. My brain is working again. It's amazing what stress can do to the brain. Thanks for the help everybody.
     
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