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Seesaw equilibrium physics problem

  1. Oct 7, 2009 #1
    1. The problem statement, all variables and given/known data
    Two children are sitting on opposite ends of a uniform seesaw of negligible mass, if a 35 kg child is 2.0 m from the pivot point, how far from the pivot point will her 30 kg playmate have to sit on the other side for the seesaw to be in equilibrium. please help

    2. Relevant equations

    Im not sure what to use??

    3. The attempt at a solution
    Not sure, thats why i am here
     
  2. jcsd
  3. Oct 7, 2009 #2

    rock.freak667

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    Re: Torque/Equilibrium

    Do you know how to calculate a moment of a force? Do you know what the sum of the moments about a point must be equal to for there to be equilibrium?
     
  4. Oct 7, 2009 #3
    Re: Torque/Equilibrium

    im sorry im not familiar to these terms, do you have an equation i can use
     
  5. Oct 7, 2009 #4

    rock.freak667

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    Re: Torque/Equilibrium

    The moment of a force is the Force multiplied by the distance from the point of rotation. Where in the see-saw do you think is a good place to take moments about? (i.e. where is the best place to measure the rotation from?)
     
  6. Oct 7, 2009 #5
    Re: Torque/Equilibrium

    would it be the pivot point?
     
  7. Oct 7, 2009 #6

    rock.freak667

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    Re: Torque/Equilibrium

    Yes it would, so what are the forces acting and what distances are they from the pivot point?
     
  8. Oct 7, 2009 #7
    Re: Torque/Equilibrium

    would the forces be mass and gravity and, distance 2m from pivot point
     
  9. Oct 7, 2009 #8

    rock.freak667

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    Re: Torque/Equilibrium

    Mass and gravity is the weight of the people.

    So lets call the distance of the 30kg child from the pivot point d. So what is the child's moment about the pivot point?

    Similarly what is the moment of the 35kg child about the pivot point?
     
  10. Oct 7, 2009 #9
    Re: Torque/Equilibrium

    oh so i take 35/2=17.5
    30/4=7.5
    17.5/7.5=2.3m
     
  11. Oct 7, 2009 #10

    rock.freak667

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    Re: Torque/Equilibrium

    No 35kg is how many newtons if g=9.81 N/kg? Convert 30kg to N as well.

    The moment of child 1 about pivot = Child's weight*distance from pivot.
     
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