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Plank Question, please help/Final tomorrow!

  1. Dec 11, 2014 #1
    1. The problem statement, all variables and given/known data
    A horizontal, 10-m plank weighs 100 N. It rests on two supports that are placed 1.0 m from each end as shown in the figure. How close to one end can an 800-N person stand without causing the plank to tip?

    2. Relevant equations


    3. The attempt at a solution
    This is kind of difficult to me, please help me understand this

    I know the person weighs 800 and there is X distance

    so I have 800N from the weight of the person plus 10 N from the weight of the plank on the right side of the fulcrum

    800X + 10 N = 90 N (the weight of the rest of the plank)
     

    Attached Files:

  2. jcsd
  3. Dec 11, 2014 #2

    jedishrfu

    Staff: Mentor

    Isn't the formula F x Dist so that for this case (800 + 10) * X and the length on the other side would be?
     
  4. Dec 11, 2014 #3
    the length of the other side would be 90 meters which equates to 90N
     
  5. Dec 11, 2014 #4

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Not good.
    On the left we have 90 N * 4.5 m (as we had in October...)
    On the right we have
    from the person: 800 * (1-X) if X is the distance from the end of the plank
    from the plank: 10 N * 0.5 m (Haru's "the rest of the plank")

    Now balance the lot and you're done.

    Good luck with the exam :)
     
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