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How far does it move before coming to rest again?

  1. Jul 6, 2007 #1
    1. The problem statement, all variables and given/known data


    Two people are standing on a 2.3 m long platform, one at each end. The platform floats parallel to the ground on a cushion of air, like a hovercraft. One person throws a 6.9 kg ball to the other, who catches it. The ball travels nearly horizontally. Excluding the ball, the total mass of the platform and people is 128 kg. Because of the throw, this 128 kg mass recoils. How far does it move before coming to rest again?
    2. Relevant equations



    3. The attempt at a solution

    i tried the problem by drawing a diagram i got 0.12m but that answer's not right, and i have no clue why!!
     
  2. jcsd
  3. Jul 6, 2007 #2

    Doc Al

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    Staff: Mentor

    Is this one of those on-line homework systems? Perhaps it didn't like you rounding off.
     
  4. Jul 6, 2007 #3
    Yeah it is, but it might just be the wrong answer..I don't know if I did it right. Is that around what you got?
     
  5. Jul 6, 2007 #4

    Doc Al

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    Staff: Mentor

    Tell me how you solved it.
     
  6. Jul 6, 2007 #5
    i got the x _center of mass and set them equal to each other.

    and got 134.9kgx_b=294.4kg

    so x_b=2.182

    and so x_r=2.182-2.3=-0.118

    i wonder if that would be a better answer than 0.12?
     
  7. Jul 6, 2007 #6
    Wow, ok, that answer was right. Rounding has gotten me into a lot of trouble haha.
     
  8. Jul 7, 2007 #7

    malawi_glenn

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    I would say since the problem gives you two significant figues as the smallest, the answer must be with two significant figures too.

    2.3 m, 6.9 kg, 128 kg

    The answer must be 0.12, not 0.118
     
  9. Jul 7, 2007 #8
    that's what i hadput first but it said that was wrong... :( then i tried the 3 digit sig figs one and that worked..it always like the 3 digit sigfig answers better haha...
     
  10. Jul 7, 2007 #9

    malawi_glenn

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    I should complain A LOT ;)

    UNLESS it is given in the problem:

    2.30 and 6.90
     
  11. Jul 7, 2007 #10
    haha...my uncle's from malawi. and yeah that has happened a lot of me and this bad system of online homework
     
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