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Homework Help: Equation for airtime given velocity?

  1. Jan 22, 2007 #1
    Equation for air time given velocity?

    1. The problem statement, all variables and given/known data
    Given a certain velocity going straight up in a vacuum on Earth from y = 0 m, how long does a mass remain in the air? Create an equation.
    (So, for example, if you throw a baseball up at 20 m/second, how long will it take for it to fall back to your hand?)

    2. Relevant equations
    Unsure...d = v*t + 1/2at^2?

    3. The attempt at a solution
    I know that this should be really elementary, but I must be really sleep-deprived because I'm just not seeing how to extrapolate it from my standard kinematics equations.
    For the record, this isn't the whole question, just the part of a lab I'm hung up on...so please don't think I'm trying to get you guys to do my job for me.
    Last edited: Jan 22, 2007
  2. jcsd
  3. Jan 22, 2007 #2

    Chi Meson

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    You have the correct equation. Just consider this: the d is a vector. It represents the "displacement," or the straight line distance between the initial and final positions of the mass.

    Now assume that the mass falls back to the its initial position. What is d =?
  4. Jan 22, 2007 #3
    Oh. Okay. So for the sample;
    0 = 20*t + 1/2(-9.8)t^2
    0 = 20t + -4.9t^2
    0 = 20t - -4.9t^2...
    ...apply Quadratic Formula at this point?
    Last edited: Jan 22, 2007
  5. Jan 23, 2007 #4

    Chi Meson

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    No, it's easier than that. Subtract 20t from both sides. then...
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