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Homework Help CONFUSED Torque

  1. Oct 8, 2007 #1
    1. The problem statement, all variables and given/known data

    A uniform steel beam of length 5.50 m has a weight of 4.50 103 N. One end of the beam is bolted to a vertical wall. The beam is held in a horizontal position by a cable attached between the other end of the beam and a point on the wall. The cable makes an angle of 25.0° above the horizontal. A load whose weight is 12.0 103 N is hung from the beam at a point that is 4.60 m from the wall.

    (a) Find the magnitude of the tension in the supporting cable.
    (b) Find the magnitude of the force exerted on the end of the beam by the bolt that attaches the beam to the wall.

    2. Relevant equations

    torque = Force * distance
    sum the vertical forces and set to zero
    sum the horizontal forces and set to zero.
    sum the moments about any point and set to zero.

    ok i seriously am confused how to solve this...cause we just learned about torques..but
    EFx = 0
    Tcos25 = 0..but this doesnt make sense cause this means that T= 0???
    Ffy = 0
    Tsin25 - 4.5E3N = 0

    then once you get T somehow... and then magically F appears..hmm im soo confused HELP PLEASE!!!!
  2. jcsd
  3. Oct 8, 2007 #2


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    Staff Emeritus
    Science Advisor

    The beams mass acts at its center of gravity, which is the midpont if the cross-section is uniform.

    There is a mass hanging at 4.6 m from the wall.

    The tension in the cable acts at 25° from horizontal, so there is a vertical component acting upward (normal to beam axis) and a horizontal component pulling the beam into the wall (hint for part b). The vertical tension component acts upward at the end of the beam.

    The sum of the moments about the connection at the wall must equal zero for static situation.

    Is the connection of the beam and wall a cantilever or pinned connection? That is important, since a fixed or cantilever joint will bear some of the momemt, where as a pinned joint will not.
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