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Physics HW- Falling Bodies

  1. Nov 6, 2013 #1
    1. The problem statement, all variables and given/known data

    A diver steps off a platform 30m above the water. With what speed does he hit the water?


    2. Relevant equations
    v=intial v+at

    3. The attempt at a solution

    I reached an answer of 2.47 seconds, but I can't see how I got there anymore! Is this answer correct?

    Thank you very much!
     
  2. jcsd
  3. Nov 6, 2013 #2

    tiny-tim

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    Hi Medgirl314! :smile:

    You need to learn the standard constant acceleration equations.

    In this case, you know vi and a and s, and you want vf

    so the equation to use is … ? :wink:
     
  4. Nov 6, 2013 #3
    Thanks for the reply!

    I think I used several equations, but they faded a bit. Blasted erasable pens. Since it's a fairly new concept, my physics teacher doesn't want me to have them memorized yet. I think I used y=y0+v0+1/2 at^2 and t^2=2y/a .

    The answer seems plausible. Did I get it right?

    Thanks again, and thanks for the link!
     
  5. Nov 6, 2013 #4

    tiny-tim

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    (try using the X2 and X2 buttons just

    Hi Medgirl314! :smile:

    (try using the X2 and X2 buttons just above the Reply box :wink:)
    rubbish!

    there's only 3 basic equations, memorise them and everything will be much simpler!
    you mean y=y0+v0t + 1/2 at2

    (and the second one is just another version of the first one, with zero constants)
    (the question asked for a speed, and your answer is a time :confused:)

    on this forum, you need to show your calculations, not just the answer :wink:
     
  6. Nov 6, 2013 #5
    Oh! I'm sorry! My answer was actually 24.2 m/s. I gave you a different part that I used to find that answer. I think I used t=√2y/a, and got t=√2.3 m/9.8 m/s^2, giving a time of 2.47 seconds. Then I used v=vo+at , plugging in my numbers to get v=0+9.8 m/s^2(2.42 s), and then got my answer of v=24.2 m/s. Is this better? Thank you!
     
  7. Nov 6, 2013 #6

    tiny-tim

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    HI Medgirl314! :smile:

    (never use . for multiplication … use x or * or brackets :wink:)
    yes, that's correct (you meant 30 not 3)

    but quicker would have been to use vf2 = vi2 + 2as :wink:
     
  8. Nov 6, 2013 #7
    Thank you! I must have used a dot for multiplication on my paper and didn't notice that it was 30, not 3.0!
     
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