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Equilibrium Question on Bridge

  1. Oct 23, 2005 #1

    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 [tex]5.80 \times 10^{3}\;{\rm N}.[/tex] 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.

    1. Will the cable break before Lancelot reaches the end of the drawbridge?

    Yes. 600 + 200 = 800 (kg). 800 * 9.80 = 7840 N > 5800 N.

    2. How far from the castle end of the bridge will the center of gravity of the horse plus rider be when the cable breaks?

    I'm thinking [tex]\sum \tau = (600)(x) - (200)(6.00) = 0[/tex].

    x=2 m.

    Is this correct?
  2. jcsd
  3. Oct 23, 2005 #2


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    At what angle is the Tension directed? Is there a picture you can post?
  4. Oct 23, 2005 #3
    Sure. I don't think there should be an angle involved; I may be wrong.

    Attached Files:

  5. Oct 23, 2005 #4
    My answer was wrong because I did not figure in the tension and I did consider units.

    [tex]\sum \tau = (600)(9.80)(x) + (200)(9.80)(6) = (5.38*10^3)(12)[/tex]
    [tex]x=9.84[/tex] m.
  6. Oct 23, 2005 #5


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    The picture is not clear for me, i only see the reaction force by the articulation support and the weight of the bridge and of the knight.
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