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Vertical Kinematics

  1. Mar 3, 2008 #1
    1. The problem statement, all variables and given/known data
    A kangaroo jumps to a vertical height of 2.8m. How long was it in the air before returning to Earth?

    2. Relevant equations
    delta y= Vot + 1/2at^2

    3. The attempt at a solution
    I had to get t by itself so I rearranged the equation to say: t= delta y/.5(9.8) all under a square root. First off, I'm not sure if I set up the equation correctly, and secondly, I am not sure if just the top (delta y) goes under the square root or both the top and bottom(or the whole side of the equation) goes under the square root.
  2. jcsd
  3. Mar 3, 2008 #2

    Doc Al

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    Staff: Mentor

    Not sure of your approach here. What's Vo?

    Try this: Start at the top. Figure out how long it takes for the kangaroo to fall a distance of 2.8m. What would Vo be in that case? How would that time relate to the total time the kangaroo was in the air?
  4. Mar 3, 2008 #3
    Vo in this case would be zero, I believe...and if it would be zero, then it would eliminate the first part of the equation (Vot).
  5. Mar 3, 2008 #4
    Oh, sorry, I misread your question I think. I'm not sure if I know how to figure that out.
  6. Mar 3, 2008 #5


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    Homework Helper

    Looks ok to me :smile:

    Vo = 0, and y = (1/2)at^2, so t^2 = 2y/a, so t = ± √(2y/a).

    You seem to have the technique right - why are you unsure?
  7. Mar 3, 2008 #6
    well, I was just wondering if I were to take the square of the top and then divide it by the bottom, or if I divided the top by the bottom and then took the square root. I divided the top by the bottom, and then took the square root to get .57. But then I think I would need to multiply it by 2 to get 1.14 seconds. I'm not exactly sure.
  8. Mar 3, 2008 #7

    Doc Al

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    Staff: Mentor

    That's right.

    Sure you do. That's exactly what you're doing! (I don't see any Vot in your final equation.)

    You divide first, then take the square root--just like you did. And the reason you need to multiply by two, is that you only solved for the time it takes to fall from the highest point (or reach the highest point). That's only half of the total time.
  9. Mar 3, 2008 #8
    Thank you!
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