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Homework Help: Rotational Statics

  1. Mar 29, 2014 #1
    1. The problem statement, all variables and given/known data
    A gymnast with mass m1 = 46 kg is on a balance beam that sits on (but is not attached to) two supports. The beam has a mass m2 = 118 kg and length L = 5 m. Each support is 1/3 of the way from each end. Initially the gymnast stands at the left end of the beam.
    What is the force the left support exerts on the beam?

    2. Relevant equations
    Sum of Torques = 0
    Sum of Forces = 0
    F = ma
    T = rf

    3. The attempt at a solution
    I set the second beam as the axis of rotation since that is an unknown quantity.
    m1 = man
    m2 = beam
    N1 = first support
    N2 = second support

    The equations I got are:
    m1*g + m2*g - N1 - N2 = 0
    -2L*m1*g/3 + LN1/3 - L*m2g/6 = 0

    Using the second equation, I solved for N1 = 2Lm1g+Lm2g/2 = 7406.55 which is wrong

    What am I doing wrong?
  2. jcsd
  3. Mar 30, 2014 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Check your algebra for solving this equation:

    -2L*m1*g/3 + LN1/3 - L*m2g/6 = 0
  4. Mar 30, 2014 #3
    Thank you. I was able to solve it correctly. I'll have to watch out for algebra mistakes.
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