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Determine the magnitude of F.

  1. Sep 2, 2012 #1
    1. The problem statement, all variables and given/known data

    In order to raise the lamp post from the position shown, the force on the cable must create a counterclockwise moment of 1500 lb*ft about point A. Determine the magnitude of F that must be applied to the cable.

    2. Relevant equations

    I can find the magnitude of a vector when I know the vector. Not in this problem.

    3. The attempt at a solution

    I know a fair amount to get started, such as definitions, but I have no idea how to actually do calculations.
     

    Attached Files:

  2. jcsd
  3. Sep 2, 2012 #2

    PhanthomJay

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    Are you familiar with the cross product rule for determining moments?
     
  4. Sep 3, 2012 #3
    I'm familiar with the cross product, but not sure how to use it to determine moment.
     
  5. Sep 3, 2012 #4

    PhanthomJay

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    M = r X F =rFsintheta, wher r is the magnitude of the position vector between the base of the post and the point of application of the force(the length of the post), F is the magnitude of the applied tensile force, ant theta is the included angle between the force and position vectors. You'll have to use the law of sines and cosines to first find theta.
     
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