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Teetertotter word problem

  1. Nov 16, 2011 #1
    1. The problem statement, all variables and given/known data

    suppose that a heavy person and a light person are balanced on a teeter totter made of plank wood. each person moves toward the fulcrum 25cm. why will the lighter persons end go down??
    2. Relevant equations


    not needed
    3. The attempt at a solution
    something about forces and gravity??
     
  2. jcsd
  3. Nov 16, 2011 #2

    phinds

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    Do you know the math to write the balancing equation for the two sides of the seesaw?
     
  4. Nov 16, 2011 #3
    math? i dont think so.
     
  5. Nov 16, 2011 #4

    phinds

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    OK, I'm a bit puzzled as to how you can be working on such a simple problem and not know the math. What grade are you in?
     
  6. Nov 16, 2011 #5
    im a senior. i dont understand why we need math. were not finding a number.
     
  7. Nov 16, 2011 #6

    phinds

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    No, you're not trying to get a number, you're trying to get an ANSWER, and the quantitative asnwer is a number and if you don't understand how to get the quantitative answer, you don't understand the answer.

    Do you have no idea how a fulcrum works mathematically?
     
  8. Nov 16, 2011 #7
    um not really. its like the piviot point. thats about all i know
     
  9. Nov 16, 2011 #8

    phinds

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    If you had a seesaw with a 100 pound kid sitting out 10 feet from the center (the fulcrum), how much effort do you think it would be to keep him in the air if you could put your hand on the (strong, but weightless) seesaw a mile out on the other end?

    How about 100 feet out?

    How about 1 foot out?

    What does thinking about this tell you?
     
  10. Nov 16, 2011 #9
    that the amount of force depends on the distance
     
  11. Nov 16, 2011 #10

    phinds

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    good. Can you quantify that?
     
  12. Nov 16, 2011 #11
    umm i can try.
    m1*d1=m2*d2
     
  13. Nov 16, 2011 #12

    phinds

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    excellent.

    SO ... if we have, for example, a 100lb kid 10 feet out and a 200 lb man 5 feet out, then they balance, according to your equation (which is correct).

    Now do the math if they both move in 1 foot.
     
  14. Nov 16, 2011 #13
  15. Nov 16, 2011 #14

    phinds

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    well, actually they are NOT equal, are they? So what does it mean in terms of who goes up and who goes down?

    What do those numbers represent?

    [by the way, check your math]
     
  16. Nov 16, 2011 #15
    sorry 800 haha. those number represent the force? It means the fat kid would go down?
     
  17. Nov 16, 2011 #16

    phinds

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    Yes it represents force. Which side is exerting the most force? Does the most force cause the side that has it to go up or does the OTHER side go up?
     
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