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Box Pushed Up an Incline Ramp: Basic Newton's Law Questions

  1. Jul 22, 2015 #1
    1. The problem statement, all variables and given/known data
    A 90 kg box is pushed by a horizontal force F at constant speed up a ramp inclined at 28°, as shown. Determine the magnitude of the applied force.
    1. when the ramp is frictionless.
    2. when the coefficient of kinetic friction is 0.18
    upload_2015-7-22_11-43-55.png
    2. Relevant equations
    F=mg
    FN=mg
    μ= Fk/FN

    3. The attempt at a solution
    a) mg=(90)((.8)= 882N=Fg

    sin28=Fgx/882N
    (0.469)(882)= Fgx
    414N=Fgx= magnitude of applied force

    b) FN=mg= 882N
    μ=Fk/FN
    0.18=Fk/882
    (882)(0.18)= Fk
    Fk= 158.76

    ** This is where I am unsure of my approach**

    Fapplied= Fgx+Fk
    =414N+158.76 N
    = 572N

    That seems like too much to me, but I'm not quite sure. Does it look like I have that equation correct ,or am I supposed to subtract the value I found for Fk from FgX, or something totally different. Any help would be very much appreciated
     
  2. jcsd
  3. Jul 22, 2015 #2
    You have not drawn FN correctly . Redraw correctly and try again .

    Hope this helps .
     
  4. Jul 22, 2015 #3
    oh okay, is it opposing the Fy aspect instead of Fg? I was confused when I was drawing that! Thank you for your response
     
  5. Jul 22, 2015 #4

    SammyS

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    Yes. The normal force opposes that component of gravity which would cause acceleration perpendicular to the surface ( the incline plane).
     
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