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Question on tourques and static equilibrium!

  1. Jun 17, 2008 #1
    How do you know when to use the sum of all forces acting on a body compared to the sum of all tourques acting on a body when solving for these types of questions involving ladders, hanging signs, etc...
  2. jcsd
  3. Jun 17, 2008 #2

    Doc Al

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    Staff: Mentor

    It really depends on the specific quantity you are asked to solve for and how many unknowns there are. Sometimes you need both; sometimes one will do. Whenever there's an extended body, be ready to consider torques.
  4. Jun 17, 2008 #3
    Ill go ahead and give you the question I have been working on:

    A ladder which is 4 meters long masses 40 kg and has its centre of gravity 1.5 m up along its length, leans against a frictionless wall and rests on a frictionless floor. To keep it from slipping, it is tied to the wall with a rope which is attached to the ladder at its center of gravity.

    and between the ladder and the rope is the center of gravity Fg [ down]

    the angle is 53 degrees. Find all forces acting upon the ladder..

    from here i am stuck...i could use tourque and put my pivot point on where the rope and gravity intersects, but then im left with the force of the floor [up] and the force of the wall
    Last edited: Jun 17, 2008
  5. Jun 17, 2008 #4

    Forces causes linear acceleration, torques cause angular acceleration. In this case, both net torque and net force need to sum to zero.

    I'd start by drawing a diagram--in fact, it might help to just draw a regular free body diagram and ignore where the forces act (don't deal with torque). See if you can figure out any of the unknown forces from that.​
  6. Jun 17, 2008 #5

    Doc Al

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    Staff: Mentor

    Well, that's just one of the conditions for equilibrium (Net Torque = 0). Sometimes one is enough, but usually it's not. Make use of the the other conditions: Net Force = 0.

    Here you have three unknown forces to find, so you need at least three equations. (Hint: Consider vertical and horizontal force components separately.)

    Don't be stingy with the equations. No extra charge for using more than one! :wink:
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