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Homework Help: Force help me please =)

  1. Apr 21, 2007 #1
    force!! help me please =)

    1. The problem statement, all variables and given/known data

    It's moving day and you need to push a 100kg box up a 20 degree ramp into the truck. The coefficients of friction for the box on the ramp are Us=0.9 and Uk=0.6. Your largest pushing force is 1000N. Can you get the box into the truck without assistance if you get a running start at the ramp? If you stop on the ramp, will you be able to get the box moving again?

    2. Relevant equations


    3. The attempt at a solution

    a) Fk=(0.6)(100)(9.8)


    yes, my maximum pushing force is 1000N. 335N<1000N

    b) Fs=Usmg

    Yes. I will be able to get the box moving again because I have enough force to overcome the static force.

    thank you!!!!!!o:)
  2. jcsd
  3. Apr 21, 2007 #2

    Doc Al

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    Staff: Mentor

    Careful here! Fk = Uk N, where N is the normal force. Since the mass is on an incline, the normal force is not just mg.
  4. Apr 21, 2007 #3
    ok... so i got

    a) Fk=ukmgsin20
    = 201.11 N

    so if i get a running start i would be able to move the box up w/o assistance.

    b) Fs= Usmgsin20
    = 301.66 N

    If i stop at the ramp, i will be able to get the box moving agian.. 301.66N<1000N

    is that it??? thanks!
  5. Apr 21, 2007 #4
    N is mgcos 20:rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl:
  6. Apr 21, 2007 #5
    can i ask why? i thought it's sine...:blushing:
  7. Apr 21, 2007 #6
    N is normal reaction ,component of gravity perpendicular to the surface
    when you are taking the component of gravity parallel to the surface as mgsin20
    how are you missing out that perpendicular component will be mg cos20
    just draw a free body diagram and resolve the forces....
  8. Apr 21, 2007 #7
    hmmm...... i still dont quite get it. i know how sine and cosine are. but i dont quite get the diagram... sorry!
  9. Apr 21, 2007 #8


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    Staff Emeritus
    Science Advisor

  10. Apr 21, 2007 #9
    well...i can't help u with this.i don't have any software to make a diagram for you....may be someone else can help...
  11. Apr 21, 2007 #10
    ok thanks austronuc and pcdagr8! i get that part now!

    a) Fk=Ukmgcos20
    = 552.5N

    b) Fs=Usmgcos20

    so this is what i have right now.
  12. Apr 22, 2007 #11
    good job:wink:
  13. Apr 22, 2007 #12
    yay! i got it? thanks!
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