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Please help with PHYS-1 Problem Forces/Static/Equilibrium

  1. Oct 30, 2016 #1
    A gymnast with mass 56.5 kg stands on the end of a uniform balance beam as shown in the figure below. The beam is 5.00 m long and has a mass of 250 kg (excluding the mass of the two supports). Each support is 0.530 m from its end of the beam. In unit-vector notation, what are the forces on the beam due to support 1 and support 2?
    12-p-076.gif

    2. Relevant equations
    F=ma
    Torque=I*alpha

    and where there is no acceleration, the sums are equal to 0.

    Setting up the forces as the following:
    F1 = support 1
    F2 = support 2
    F3 = gravitational force in center of beam
    F4 = force of the gymnast at the end

    I used Newton 2 for the sum of the forces in equilibrium (acceleration = 0)...
    F1+F2+F3+F4=ma=0 where F3 = (250kg*g), F4=(56.5kg*g)
    F1+F2+306.5g=0
    1.) F2=-F1-306.5g
    Then Newton II for sum of the torques.... Where L is the length of the beam, and point I will rotate is located at the point where the gymnast is, so I can simplify the calculations.

    2.)F1*4.47m +F3*2.5m+F2*0.053m=0

    Next, I plugged 1( solved F2) into (2)

    Simplifying, I get F1=1150.52N



    Please help, and thank you in advance for you time and generosity.
     
    Last edited: Oct 30, 2016
  2. jcsd
  3. Oct 30, 2016 #2
    For one, your work indicates that the support posts are positioned L/6 from the ends, but that isn't what is given in the problem. But it looks like your set up of the problem is good. I just think you made some errors in working through your equations. Based on the geometry of the problem, F1 is going to be less than F2. So yeah, your 2588 N for F1 is going to be too high.
     
  4. Oct 30, 2016 #3

    Student100

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    What're you doing here?

    You're writing it in terms of g? Why're all the signs positive? Did you draw a force diagram?
     
  5. Oct 30, 2016 #4
    I finally figured it out...
     
  6. Oct 30, 2016 #5
    Yes, I just implicated g= -9.8m/s^2
    Thank you both for your replies
     
  7. Oct 30, 2016 #6
    Yeah, I was going to comment on that also. I prefer to define the forces in the direction that I know (or think) they will be in reality. In this problem, you can know ahead of time, based on the geometry, the direction of all of the forces. So I would have written F1 + F2 = F3 + F4.

    Also, when I do torque calculations, rather than summing all of the forces to equal 0, it is easier for me to do CW = CCW, based on the direction that I defined all of the forces. But that is just my personal preference. I just like positive numbers better than negative numbers. :)
     
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