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Sir Lancelot Equilibrium Bridge Problem

  1. Nov 11, 2008 #1
    1. The problem statement, all variables and given/known data

    Sir Lancelot rides slowly out of the castle at Camelot and onto the 12.0-m-long drawbridge that passes over the moat. Unbeknownst to him, his enemies have partially severed the vertical cable holding up the front end of the bridge so that it will break under a tension of 5.80 * 10^3 N. The bridge has mass 200 kg and its center of gravity is at its center. Lancelot, his lance, his armor, and his horse together have a combined mass of 600 kg.

    2. Relevant equations

    If so, how far from the castle end of the bridge will the center of gravity of the horse plus rider be when the cable breaks?

    3. The attempt at a solution

    I hate to say this, but I have no idea where to start. I tried to solve for the distance by:

    Torques = (600)(9.8)(x) - (200)(9.8)(6) = (5.8 * 10^3)(12)

    It says my tension value is wrong for what is holding the bridge up. The tension is located 12m from the opening of the castle, but apparently I need to solve for my tension in another way.

    Any suggestions?
    Last edited: Nov 11, 2008
  2. jcsd
  3. Nov 11, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Why the negative sign between the first two terms? (Those torques are in the same direction--they should have the same sign.) Correct that error and you should be fine.
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