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Friction of an object being pulled up/down

  1. Feb 18, 2009 #1
    1. The problem statement, all variables and given/known data
    Find the normal force on an object, 80kg, which is being pulled by a force of 4000N. A) Up at an angle of 50 degrees, B) Down at an angle of 70 degrees


    2. Relevant equations
    Fn = Fwcosx
    Fp = Fwsinx

    3. The attempt at a solution

    A)
    Fp = Fwsinx
    Fp = 4000sin50 = 3064N
    (80*9.8) - 3064 = -2280N = 2280N

    B)
    Fp = 4000sin70 = 3758.8N
    (80*9.8) + 3758.8 = 4542.7N

    Is this correct?
     
    Last edited: Feb 18, 2009
  2. jcsd
  3. Feb 18, 2009 #2

    Tom Mattson

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    I'm confused. Is the object being pulled up an inclined plane, or is it on a flat surface and the applied force acts at the given angles?
     
  4. Feb 18, 2009 #3
    Pulled up on an inclined plane.
     
  5. Feb 18, 2009 #4

    Tom Mattson

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    In that case your "Relevant Equations" are wrong. You need to draw a free body diagram and apply Newton's 2nd law to it. That's how you do every inclined plane problem.
     
  6. Feb 18, 2009 #5
    Ah....I believe I was doing the other scenario then. In that case, I would simply do Fn = 80 * 9.8 * cos 50 = 503.9 N and Fn = 80 * 9.8 * cos70 = 268.14N, correct?
     
    Last edited: Feb 18, 2009
  7. Feb 21, 2009 #6

    Tom Mattson

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    The other scenario doesn't make much sense. If you pull on an 80 kg object with 4000 N at 50 degrees, the object will be lifted right off the ground. Hence, no normal force.
     
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