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Equilibrium of a Rigid Body

  1. Mar 30, 2014 #1
    1. The problem statement, all variables and given/known data
    Find the unknown mass on a lever (see-saw). I have three masses. To the right of the fulcrum, the mass is 166 g and it is 25.15 cm from the fulcrum. There are two masses to the left. One is 166 g and is 16.95 cm from the fulcrum. The unknown mass is 3.33 cm from the fulcrum. The lever is balanced on the fulcrum. This is a lab set up using a meter stick, fulcrum (5) 100 g masses, 4 weight hangers (16 g each), 3 hook masses (50 g each)

    2. Relevant equations
    ∑F=0 => ∑Fx=0 & ∑Fy=0
    ∑τ(about any axis)=0 => ∑τclockwise = ∑τcounter-clockwise

    3. The attempt at a solution
    I'm having trouble starting the problem because the unknown mass is on the same side with a known mass. I understand how to do this for one mass on each side but throwing another variable into the equation has me stumped. I do know that to solve for this equation i have to use the information about torque. The torque is zero due to equilibrium
  2. jcsd
  3. Mar 31, 2014 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    This is more of an algebra problem than anything else.

    You have the equations of static equilibrium in the OP. Why haven't you started using the KNOWN masses and distances and substituted them into these equations? You can use 'm' or 'x' for the unknown mass and you are given its distance from the fulcrum. You won't get anywhere just staring at a blank piece of paper.
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