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Homework Help: John falls...

  1. Dec 29, 2015 #1
    1. The problem statement, all variables and given/known data
    To escape the fire which has just declared itself in the building, James jumps through the window. One second before reaching the mattress, he traveled half of the height of fall.

    What is the total time of the fall in seconds ?

    What is the total height of the fall in meters ?

    We shall take g = 9,81 m/s ²

    3. The attempt at a solution
    2 or 3 secondes ?

    I don't need g... Strange...
    my attempt...

    0.5 d= 4.9 (t-1)²
    t = 3.4142
    d = 57.118
    Last edited: Dec 29, 2015
  2. jcsd
  3. Dec 29, 2015 #2
    Why do you say 2 or 3 seconds? Do you think he is slowing down or speeding up? In what proportions?
    (You do need g for rép2 but not for the first part. And do not delete and repost please.)
  4. Dec 29, 2015 #3
    Strange for rep1 because the answer is x,xxxx...

    And no, he accelerates...

    One second is the half thus 1 * 2 but it isn't that... and i don't understand why...

    somebody has deleted my post, not me !
  5. Dec 29, 2015 #4
    Maybe it was deleted because the heading was so non-descriptive.

    I suggest you make a graph of distance versus time and mark the last second before impact.
  6. Dec 29, 2015 #5
    But there are no numbers on my graph...
    I dont't have the distance traveled during 1 second.
  7. Dec 29, 2015 #6
    The units on the vertical scale do not matter for how the graph looks like. The first part of the problem only involves the time axis. It is the same on the Moon as on Earth.
  8. Dec 29, 2015 #7
    I don't understand... One second is the only element in this statement and the answer is decimal...

    No Google search is necessary ?
  9. Dec 29, 2015 #8
    Try googling for "John, window"...

    Better: call the total time T. The distance is a quadratic function of time, on the Moon, on Earth. So:
    [itex](T-1)^2 = T^2/2[/itex]
    Do the algebra.
  10. Dec 29, 2015 #9
    T = 3.41421356

  11. Dec 29, 2015 #10
    and for rep2, d = 57.118 is it correct ?
  12. Dec 29, 2015 #11
    No, that is not correct. It is incomplete.
  13. Dec 29, 2015 #12
    it is 3:35 AM here, i'm tired.

    see you tomorrow and big thank you for your patience !
  14. Dec 29, 2015 #13
    d = 1/2 gt² ?
    No ?

    I have 57.118... meters
  15. Dec 29, 2015 #14
    Poor John (or James?).
    May he rest in peace.
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