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Rigid object in equilibrium?

  1. Jan 23, 2014 #1
    1. A horizontal, 10-m plank weighs 100 N. It rests on two supports that are placed 1.o m from each end. How close to one end can an 800-N person stand without causing the plank to tip?
     

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  3. Jan 23, 2014 #2
    Hi gcombina :smile:

    so far how you have attempted the problem?
     
  4. Jan 24, 2014 #3
    I know is a torque
    that is all I know and I don't know where to start
     
  5. Jan 24, 2014 #4

    haruspex

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    Your diagram is not the most useful since it shows the person right at one of the supports. Redraw it with a bit more overhang for the person to stand on. Draw in the relevant forces. No need to draw a stick figure, just indicate the force of the weight of the person. What equations do you know for static equilibrium?
     
  6. Jan 24, 2014 #5
    for static equilibrium? I know ΣF = 0
     
  7. Jan 24, 2014 #6
  8. Jan 24, 2014 #7
    1. SO, I got 4 forces
    F1 - W + F2 - W = 0
    F1 - 100N + F2 - 800N = 0

    i DONT EVEN KNOW WHY AM I DOING WHAT I AM DOING
    I AM TRYING TO FIND THE DISTANCE OF THE PERSON

    IM SO LOST, SORRY
     
  9. Jan 24, 2014 #8
    what does the fact that the body is in static equilibrium indicate to you?
     
  10. Jan 24, 2014 #9
    that there are no external forces??
     
  11. Jan 24, 2014 #10
    mmm no.
    what do you know about torques in static equilibrium ?
     
  12. Jan 24, 2014 #11
    I know torque = force x level arm

    when a rigid body is in equilibrium then it means that the sum of torques = 0
     
  13. Jan 24, 2014 #12
    try to write both
    1)∑F=0
    2)∑τ=0
    for your system
     
  14. Jan 24, 2014 #13
    I got it, thank you!
     
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