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Torque plank problem

  1. Nov 7, 2007 #1
    A 5 meter uniform plank of mass 100kg rests on the top of a building with 2m extend over the edge of the building. how far can 50kg person can pass the edge of the building on the plank, before the plank begin to tip.

    torque=Fd
    i dont know where to get acceleration to calculate the force
     
  2. jcsd
  3. Nov 7, 2007 #2

    Chi Meson

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    No need for an acceleration. Find the point where the torques are balanced. The weight of the plank acts as though it is applied at its center of mass. The distance is from the pivot point (edge of building) to the center of mass.
     
  4. Nov 7, 2007 #3
    the point where torch is balance is 2.5m
    i dont really understand what you say
     
  5. Nov 7, 2007 #4
    The center of the plank is where the gravity acts on the mass. Since the plank is 5m, the center is 2.5m.

    So you have one force.
     
    Last edited: Nov 7, 2007
  6. Nov 7, 2007 #5
    how so?
     
  7. Nov 7, 2007 #6

    Chi Meson

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    Draw a picture. Find the pivot point (point of rotation), it's the edge of the building. Find the distance from this point to the center of the plank (d1). Then call the distance from the pivot point to the person "d2." F1d1 =F2d2
     
  8. Nov 7, 2007 #7
    mass of plank times acceleration due to gravity.
     
  9. Nov 7, 2007 #8
    100(9.8)2.5=40(9.8)d
    d=4.25?
     
  10. Nov 7, 2007 #9

    Chi Meson

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    Does that seem logical? a five meter plank, with 2 meters extending out over the edge of a building, and you can walk on it up to 4.5 meters beyond the edge of the building?

    Draw it I said. The center of the plank is 2.5 m from the end of the plank, but how far is the center of the plank from the edge of the building, if 2 meters extends over the edge?

    Draw it.


    Draw it. Go on, why don't you?
     
  11. Nov 7, 2007 #10
    i draw, i got .5 right?
     
  12. Nov 7, 2007 #11

    Chi Meson

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    .5 meters, That's d1, right?
     
  13. Nov 7, 2007 #12
    100(9.8).5=50(9.8)d
    d=1
     
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