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Homework Help: Plank and torque, problem with finding center mass

  1. Jan 29, 2012 #1
    1. The problem statement, all variables and given/known data

    A uniform plank of length 5.0 m and weight 212 N rests horizontally on two supports, with d = 1.41 m of the plank hanging over the right support. To what distance, x, can a person who weighs 433 N walk on the overhanging part of the plank before it just begins to tip?

    2. Relevant equations

    Tnet = 0
    T = r * F

    3. The attempt at a solution
    I already found the answer, but there's something that bugs me.

    Torque of plank = Torque of person
    212N * 1.09m = 433N * distance

    The right support is the fulcrum point.

    What I don't understand is why the plank's weight acts on is 1.09m. I know the idea to get the midpoint from the two supports then subtract by the plank that is hanging over the right support, 5.00m / 2 - 1.41m = 1.09m, but I don't understand why this is the case.

    This is what I initially did, 5.00m + 1.41m = 6.41m, then I divided this value by 2 to get 3.205m, and this is the center point of the plank from either end point. This point will be 1.795m from the right support, and I used this to calculate the torque of the plank. Could someone enlighten me why this was wrong? And why the correct way is .. correct?
  2. jcsd
  3. Jan 29, 2012 #2


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    Gold Member

    The beam is 5.0 m in overall length. The distance between supports is 3.59 m. You have misinterpreted the beam as being 6.41 m long and the distance between supports as being 5 m. This is not what is given.
  4. Jan 30, 2012 #3
    ah thank you, I need to read it more clearly.
  5. Jan 30, 2012 #4
    It is a good idea to draw a diagram showing all the forces acting.
    The plank is uniform so the weight of 212N is acting at the centre of the plank (2.5m from each end). This should help to see the distancesa from forces and pivots.
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