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Homework Help: How Long Something Stays in Air Calculation. Help Needed !

  1. Sep 13, 2011 #1
    1. The problem statement, all variables and given/known data
    Jumping straight up, how long will a kangaroo remain in the air if it jumps through a height of 3.0m?


    2. Relevant equations
    d=vf(t) - 1/2 a (t)2

    I Think?


    3. The attempt at a solution
    Defined all variable
    d=3.0 m
    a=9.81 m/ss
    vf=0m/s2

    then:
    d=vf(t) - 1/2 a (t)2
    3.0=0 - 4.905(t)2
    0.61=(t)2
    t=Square root of 0.611
    t=0.78

    I got the wrong answer. But when I multiplied my answer by two I got the right answer. Don't know why so I'll need some help here.

    And I know I skipped some calulation steps in my answer, I have them written on paper.
     
  2. jcsd
  3. Sep 13, 2011 #2
    Just thought about it. Is it because what goes up must come down? So it needs to be 2 times the time, I just calculated for the time to the top of the parabola. Am I right?
     
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