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Physics equilbirium

  1. Jan 20, 2009 #1


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    a person stands a distance of 0.300 meters from the right end of a 2.00 meter long uniform platform that is supported by two posts, one at each end. The board has a mass of 20.0 kg and the person's mass is 75.0 kg.

    a. determine the force exterted on the board by the post on the left end.

    b. determine the force exerted on the board by the post on the right end.

    Sum moments about any point. I will choose the Right end.

    -.3*75g -1*20g+Fleft*2=0

    That gives you Fleft end.

    Now, sum forces vertically
    solve for fright.

    i don't think i'm doing it right
    can someone help
  2. jcsd
  3. Jan 20, 2009 #2
    Hi key, well I think you simply over complicated things actually, have a look back at you first equation, isn't that already solvable, you did exactly the right thing by taking moments about the right post you eliminated the right reactive force from you equations leaving you with a single variable :D which is solvable on its own, and repeat to find the right hand force, hope that helps :D
  4. Jan 20, 2009 #3


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    As Galadirith has noted, you can sum moments about the left post to solve for F-right, and then this serves as a check for your value you obtained for F_right when you summed forces in the y direction, which is also correct. It's good to have a backup equation for a check; if the value obtained for F_right is different when using the sum of moment equation than it is when using the sum of forces equation, you made an error somewhere.
  5. Jan 20, 2009 #4


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    I got 21.25 for F left and -73.75 for F right.
    Is that right?
  6. Jan 21, 2009 #5
    Hi key, well i think you are basically there but a couple of things to say. Firstly the value reactive force at the right post you gave that as -73.75, i would suggest have a look at that again, and think does it seem logical that the reactive force is negative? Also you have given numerical values in terms of g, basically in you original equations you correctly set up the equation for moment about the right post, with their forces expressed in terms of their respective masses and the acceleration due to gravity represented by g.

    In order for you final values of the forces to be correct it would either be 21.25g N or 208.25 N, hopefully you can see why and verify that for yourself, the same also applies to the right post. :D
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