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Homework Help: Rigid bodies in equillibrium

  1. Mar 24, 2009 #1
    1. The problem statement, all variables and given/known data
    A wrecking ball weighing 4800N is supported by a uniform boom of wieght 3600N. A support cable runs from the top of the boom to the tractor. the angle between the support cable and the horizontal is 32degrees and the angle between the boom and the horizon is 48degrees.
    Find the magnitude of the force exerted on the lower end of the boom by the hinge.

    2. Relevant equations
    Sum of Fx=0
    Sum of Fy=0

    3. The attempt at a solution
    I have already established that the total torque on the rope is = 16 000N
    So then I am thinking that this might be right:

    Sum of Fx=0=H-Tsin58degrees

    Sum of Fy=0=V-4800-3600-Tcos58degrees

    I just cannot wrap my head around the angles vs the magnitude. eg. vertical force of weight of boom is 3600N but should I not include the angle somehow?
    And then the 58degrees is taken from 16+42 at the top of the boom - it that correct? Or should it be 32degrees? Really confused by all these angles.
  2. jcsd
  3. Mar 24, 2009 #2


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    Homework Helper

    anelmarx: You have three unknowns in your above post, but you posted only two equations (which are correct). You need one more equation, summation of moment. Assume the boom length is L. The weight of the boom acts downward at the boom midspan. Section-cut the slanted cable at the top of the boom, if you wish, and place the slanted cable tensile force T at this section cut. Now sum moments of the entire system. Solve simultaneously for the three unknowns. Your angles are OK. T is not 16 000 N. Try it again. You are almost there.
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