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Homework Help: Find initial velocity and time in the air- projectile motion

  1. Apr 17, 2012 #1
    1. The problem statement, all variables and given/known data

    A soccer ball is kicked at 38.0° with respect to the horizontal and travels 64.0m before striking the ground.
    a) what is its initial velocity?
    b) how long was it in the air?

    2. Relevant equations

    Kinetic equation d=Vi*t+ 1/2*at2
    dh=-V2*sin2θ /g
    Δt=-2Vsinθ / g

    3. The attempt at a solution

    a) dh=cos(38)*64m= 50.4m

    Find t=√(39.4m/(0.5*9.8))=7.96s

    plug this value into the d=Vi*t+ 1/2*at2 equation and solve for Vi if d=64m


    b) V=dh/Δt= 50.4m/7.96s=6.33m/s

    Then solve for Δt=-2Vsinθ / g= 0.80s

    I would like for someone to just double check my answers and that I used the appropriate equations and that the number of significant figures are being respected! Thanks so much in advance!
  2. jcsd
  3. Apr 17, 2012 #2
    Sorry, last answer is bad, it looks okay. I would consider the highest y it takes and use energy conservation(kinetic energy) to obtain the initial velocity
    Last edited: Apr 17, 2012
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