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Homework Help: Projectile problem w/ unkown velocity/time

  1. Dec 8, 2008 #1
    1. The problem statement, all variables and given/known data
    An athlete executing a long jump leaves the ground at a 25° angle and lands 6.30 m away.
    What was his take off speed?
    2. Relevant equations
    (In all equations, X can be substituted for Y in entire equation)
    Vy= Voy + AyT
    Vy^2 = Voy + 2AyY
    Y=VoyT + (1/2)AyT^2

    3. The attempt at a solution
    The answer is 8.98 m/s. I can't figure out how to do this problem, and need to understand this for an upcoming final. I have worked several other similar problems, but none without a known velocity and/or time.
    Any insight into how to even begin would be greatly appreciated.
    Last edited: Dec 8, 2008
  2. jcsd
  3. Dec 8, 2008 #2
    Sorry I edited this because I think i made a mistake don't worry I didn't give up on you.

    I'm sorry again I keep getting an answer very close and is probably just off because of my rounding. Though it's been a while since I have done projectile motion and I would hate to show you the wrong way with a final coming up.
    Last edited: Dec 8, 2008
  4. Dec 8, 2008 #3


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    Homework Helper

    The x component of velocity is Vo*Cosθ

    His total distance would be given by Vo*Cosθ*t

    So ...

    X = 6.3m = Vo*Cos25*t

    Now you still need time so ... look to the skies.

    Vo*Sinθ = g*t and t to max height = Vo*Sinθ/g

    But total time = 2*t

    Now we can solve. t = 2*Vo*Sin25/g = 6.3/Vo*Cos25

    Rearranging Vo2*2*Sin25*Cos25 = 6.3*g

    Recognizing that 2*Sinθ*Cosθ = Sin2θ

    Vo2 = 6.3*g/Sin50
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