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Energy & Momentum Thinking Problem

  1. Sep 1, 2012 #1
    1. The problem statement, all variables and given/known data
    Two boxes each of mass 12kg are raised 1.8m to a shelf. The first one is lifted and the second is pushed up a smooth ramp. If the applied force on the second box is 48N, calculate the angle between the ramp and the ground.


    2. Relevant equations
    W = Eg = mgh for first (lifted) box
    Trig to calculate angle

    3. The attempt at a solution
    I first calculated the gravitational energy on the first box that is lifted:
    Eg = (12kg)(9.8m/s^2)(1.8m) = 211.7N
    Then, knowing that, used trig to calculate the angle between ramp and ground:
    Sin (theta) = opposite (211.7N)/hypoteneuse(48N)
    and, naturally, this does not compute, so where did I go wrong?
     
  2. jcsd
  3. Sep 1, 2012 #2

    Doc Al

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    Staff: Mentor

    That's an energy, not a force. Its units are Joules, not Newtons.
    Not sure what you are trying to do. Instead, examine the forces acting on the box as it is pushed up the ramp. (Assume the force applied is just enough to slide it up the ramp.)

    Hint: What's the component of the box's weight parallel to the ramp?
     
  4. Sep 1, 2012 #3
    Whoops, it's been a while. Anyways, I got:
    Fnet = Fa - Fg(parallel)
    = 48N - (12)(8.8)sin(angle)
    = 48N - 117.6Nsin(a)
    sin(a) = 48/117.6
    angle = 24.1 degrees?
     
  5. Sep 1, 2012 #4

    Doc Al

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    Staff: Mentor

    Good! :approve:
     
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