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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

    dv=1/2*at2
    dh=Vh*Δt
    Kinetic equation d=Vi*t+ 1/2*at2
    dh=-V2*sin2θ /g
    sinθ=opp/hyp
    cosθ=adj/hyp
    Δt=-2Vsinθ / g

    3. The attempt at a solution

    a) dh=cos(38)*64m= 50.4m
    dv=sin(38)*64m=39.4m

    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

    Vi=3.14m/s

    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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