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Help needed- Load being pulled along an incline surface

  1. Oct 13, 2012 #1
    1. The problem statement, all variables and given/known data
    A man is dragging a 68 kg trunk up the loading ramp of a movers truck, the ramp has a slope of 20deg and the man pulls upward with a force F whose direction makes an angle of 30deg with the ramp. The coefficient of kinetic friction between trunk and the ramp is 0.28. What force F is necessary to accelerate the trunk up the loading ramp at a rate of 0.55m/s^2?

    Image: first one
    http://imageshack.us/photo/my-images/61/p6nj9.jpg/


    2. Relevant equations



    3. The attempt at a solution
    My solution:
    I broke the gravity components into perpendicular and parallel so
    as a verticle equilibrium i got:
    mgcos20deg=Fn+Fsin30deg

    and as horizontal equilibrium i got:
    Fcos30deg=μFn+mgsin20deg

    I rearranged the horizontal equilibrium for F to find the Fn in the first equation
    like the following:
    F= ((μFn+mgsin20deg)/cos30deg)
    then:

    mgcos20deg=Fn+((μFn+mgsin20deg)/cos30deg)(sin30deg)
    By adding into all the given information i got 425.81N=Fn

    I plugged that into the friction formula Ff=μFn to find friction which i got was 119.2268N

    then i used this equation to use the acceleration given and find the F:
    Fnet=Fcos30deg-mgsin20deg-Ff
    ma=Fcos30deg-mgsin20deg-Ff
    F= 444.04 N

    But when i am checking the answer in the verticle equilibrium from before:
    mgcos20deg=Fn+Fsin30deg
    626.21N=425.8N+222.02N
    626.21N=647.82N

    what im i doing wrong? Pleas check and help me!
     
  2. jcsd
  3. Oct 13, 2012 #2

    tiny-tim

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  4. Oct 13, 2012 #3
    Re: Duplicate thread!

    I am aware for the duplicate sir/madame,

    however i would like to share my solutions with you guys..
    so you guys can see whats my mistake?
     
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