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Tensions with boxes on a ramp.

  1. Sep 9, 2008 #1
    1. The problem statement, all variables and given/known data
    Each box weighs 40 lbs, the angles are measured relative to the horizontal. The surfaces are smooth (no friction). Determine the tension in rope A and the normal force exerted by box B by the incline.

    2. Relevant equations
    W = mg
    ΣF_x = 0
    ΣF_y = 0

    3. The attempt at a solution

    I found the tension of rope C and then normal force exerted on box D by setting up a system of equations.

    Solved, the tension equals 15.1 lb, and the normal force exerted is 33.1 lb.

    Now I'm having trouble determining the normal force of B and the tension of rope A.

    Attempting the solution:
    ΣF_x = 0 = 15.1*cos(45) + N*sin(70) - T_a*cos(70)
    ΣF_y = 0 = -15.1*sin(45) + N*cos(70) - T_a*sin(70) - 40

    Solved using systems of equations, we found the normal force to be .0015lb (unreasonable), and the tension of A to be 31.2 lb (also unreasonable?).

    I can't figure out where I went wrong to get this bad answer. Anyone spot some mistakes? Thanks.
  2. jcsd
  3. Sep 9, 2008 #2
    Is there supposed to be a picture attached?
  4. Sep 9, 2008 #3
    Haha, yes, there is. I completely forgot to attach it. Why can't I edit my original post?

    Here's the pic (see attached)

    Attached Files:

    Last edited: Sep 9, 2008
  5. Sep 9, 2008 #4


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    You are only permitted to edit your posts for a maximum of one hour after they were originally posted.
  6. Sep 10, 2008 #5


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    Homework Helper

    Hi DyslexicHobo,

    The tension of rope A is upwards and so I believe the corresponding term in the y equation needs to be positive to match the other terms.
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