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Equilibrum question help here?

  1. Jun 28, 2012 #1
    1. The problem statement, all variables and given/known data

    We have a bar with the weight of 500 N which stands in A and B.In this bar an object which weights 1500 N has been hung.The length of the bar AB is 2 meters and AD is 0.8 meters.Find FA and FB just like in the picture.

    http://s1171.photobucket.com/albums/r549/Elaia06/?action=view&current=sdsd.jpg


    3. The attempt at a solution

    The resultant momentum is zero so

    FA*0+1900*0.8+500*1+2*FB=0
    But FB in my book is 850 N.This answer does not fit my answer.
     
  2. jcsd
  3. Jun 28, 2012 #2

    Doc Al

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    Staff: Mentor

    (1) Why 1900?
    (2) Distinguish clockwise and counterclockwise moments: they have different signs.
     
  4. Jun 29, 2012 #3
    it is 1900 sorry , I have written it the wrong way in the data.
    FA*0-1900*0.8-500*1+2*FB=0
    Still FB is not 860..
     
  5. Jun 29, 2012 #4
    Your drawing is not clear enough.
    Maybe you make the drawing, take a snapshot and insert image.
     
  6. Jun 29, 2012 #5

    Doc Al

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    Staff: Mentor

    Are you sure? When I solve for FB using your original data (weight of object = 1500N) I get FB = 850 N. Which matches the answer in your book (at least according to your first post).
     
  7. Jun 29, 2012 #6
    i have written it wrong ,i was sleepy last night :P
    It should be 860 N, the Fb.
    Yes I get 850 N too.The drawing is just like in my book.
     
  8. Jun 29, 2012 #7
    Not related to this exercise :Ah and by the way, If we have a bar with two forces acting in the begining and the end of it, what is the relationship of the weight of the bar with the forces? I mean, is the distance from a force to the point of weight equal to the distance of the other force with the weight or not?
     
  9. Jun 29, 2012 #8

    Doc Al

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    If the bar is uniform, the weight acts at its center. The support forces at each end will (of course) be equidistant from the center.
     
  10. Jun 29, 2012 #9
    what is a uniform bar? Sorry but english is not my native language :P
     
  11. Jun 29, 2012 #10

    Doc Al

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    Staff: Mentor

    A bar whose weight is evenly distributed along its length.
     
  12. Jun 29, 2012 #11
    Yes this is an uniform bar, thanks :)
     
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