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Clockwise or counterclockwise rotation about a fulcrum

  1. May 12, 2009 #1
    1. The problem statement, all variables and given/known data
    The diagram shows a uniform plank of length 5m and weight 225N that rests horizontally on two supports, with 1.1 m hanging over the right support. To what distance x can a person who weighs 450 N walk on the overhanging part of the plank before it just begins to tip?

    l-x-l

    \0/
    l
    /\
    _________________
    ^ ^
    l-1.1m-l

    2. Relevant equations
    T=Fl
    [tex]\Sigma[/tex] T = 0
    [tex]\Sigma[/tex] F = 0

    3. The attempt at a solution
    Take upward direction as positive and clockwise direction of motion as positive
    Name the leftmost fulcrum L and the right fulcrum R
    Fy = FL + FR - FWplank - FWPerson
    Fy = 0
    Thus FL + FR - 225N - 450 N = 0
    Thus FL = 675N - + FR

    This is where I get stuck. My equation kept boiling down to rubbish and when I checked the web for a solution, I found out I need to make the torque produced by Force L a negative number with respect to fulcrum R. However, won't the upward force of L produce a clockwise rotation? The rotation at R caused by force L is the only place that I seem to be messing up as other equations are exactly as the memo indicated. But I have taken clockwise to be positive (as the memo also indicated) and yet the memo says that my torque at L is negative.
     
  2. jcsd
  3. May 12, 2009 #2
    Sorry-one correction- FL=675N - FR
     
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