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Three dimensional torque questions

  1. Mar 30, 2008 #1
    how would you do torque problems, except in three dimensions?

    my knowledge is limited to basically planks or other bars that can be considered to be two d.

    for example, a 6m, 10kg plank has 2 meters hanging off the side of the pirate ship. how far does the 50kg pirate have to walk from the side of the ship to fall into the water?

    well that's simple.

    but now lets assume that the plank has a third dimension. let's say it's a square with a certain depth. what if he's walkign closer to one edge than the other?

    if we're looking at this situation from above.

    the square contains the points (3,3), (-3,3), (-3,-3), and (3, -3). the center of mass is at the origin. the ship's edge is along the line x=1. the pirate is walking along the line y=-2.

    how does that affect how far he has to walk?

    this isn't homework. just general curiosity. :)
  2. jcsd
  3. Mar 30, 2008 #2
    Just intuitively, I would think that if the plank has symmetry about its CG and it is perpendicular to the normal force of gravity, its "thickness" won't matter.
  4. Mar 30, 2008 #3
    true, but that wasn't what i was wondering about either way. i shouldn't've mentioned thickness. it's besides the point... sorry about that
  5. Mar 30, 2008 #4
    Statics. Say you are still dealing with a force straight down. That keeps it easy.
    In your example the force can only act around x=1 no matter where you place the force along Y. But if the plank were only supported at a point (1,1) then your force can act around x=1 and y=1. So, you have torque or a moment around both.
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