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Homework Help: What is force if acceleration is .75? (inclined plane)

  1. Dec 10, 2011 #1
    1. The problem statement, all variables and given/known data
    A horizontal force F is exerted on a 20kg box to slide it up a 30 degree incline. The friction force retarding the motion is 80N. How large must F be if the acceleration of the moving box is to be .75 m/s^2?


    2. Relevant equations
    All the forces on the box in the x and y direction have to equal zero

    F=ma
    F-Ff-Fn = ma


    3. The attempt at a solution

    Using F-Ff-Fn,

    F-80-20*9.8*sin(30) = 20*.75
    F-178= 15
    F= 193 N

    Which isn't even close to what the answer is supposed to be, which is F=220 N
     
  2. jcsd
  3. Dec 10, 2011 #2

    gneill

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    Staff: Mentor

    If the box is accelerating then there must be a net force. Newton's first law.
     
  4. Dec 10, 2011 #3
    How does that change my equations?
     
  5. Dec 10, 2011 #4

    gneill

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    Staff: Mentor

    Draw a Free Body Diagram for the box. There must be a net force that produces an acceleration of 0.75m/s2 (acting up-slope). So what forces act along the slope?
     
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