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Velocity Calculation Contradiction

  1. Jul 25, 2011 #1
    1. The problem statement, all variables and given/known data
    I have to design an experiment where I am supposed to project an object horizontally from some point above ground level, measure the distances, calculate the average horizontal distance dx, then use that to calculate the initial and final velocities yatta yatta.
    Anyway I made the height 2 m, got 4.62 m as the average horizontal distance and I want to get Vfy (y component of final velocity)

    2. Relevant equations
    Vf^2 = Vi^2 + 2ad
    a = (delta)V/t
    V = d/t
    where: Vf=final velocity, Vi=initial velocity, a=acceleration(9.8 m/s^2), t=time interval, d= distance

    3. The attempt at a solution
    I can't complete the calculations in the experiment because I can't get one value for Vfy

    Vfy^2= Viy^2 + 2ad
    Vfy = (sqrt)[0 + 2(9.8m/s2)(2m)]
    = 6.261 m/s
    great, now to find time:
    a = (delta)V/t
    t = (delta)V/a
    = (6.261m/s)/(9.8 m/s2)
    = 0.639 s

    but when I use the time in this equation:
    V = d/t
    = (2 m)/(0.639s)
    = 3.13 m/s
    WHAT?! The same velocity is now half as small. How did this happen? Which one is the right value?
    Something tells me I should just go along with the first one.

    Oh by the way, Hi! :)
    Last edited: Jul 25, 2011
  2. jcsd
  3. Jul 25, 2011 #2
    V = d/t is only valid when velocity is constant.
  4. Jul 25, 2011 #3
    Lol I keep forgetting things like acceleration. Thanks btw :)
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