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Projectile motion problem (find angle theta)

  1. Mar 2, 2013 #1
    1. The problem statement, all variables and given/known data
    A projectile is launched from the ground with an initial velocity of 140m/s at an angle of θ above the horizontal. (assume AR is negligible and the ground is level)
    The time of flight is 18.7seconds and its range is 1.98*10^3
    show that the launch angle θ is approximately 41 degrees

    2. Relevant equations
    I'm unsure which equation Im supposed to use for a problem like this


    3. The attempt at a solution
    voh = 140cosθ

    ∴ cosθ = voh/140


    but Im not given enough information to find the initial horizontal velocity so this equation won't work. I also am sure that I have to use the time of flight and the range since its been given but I can't think how to rearrange things to make it work.
    Can anyone point me in the right direction for what I'm supposed to be doing please?
     
  2. jcsd
  3. Mar 2, 2013 #2

    rude man

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    What must voh be if the projectile flew for 1.98e3 m and took 18.7sec. to do so?
     
  4. Mar 2, 2013 #3
    thanks for the reply rudeman!

    So voh = 1.8e3m/ 18.7s = 105.88 m/s

    then θ = cos-1 (105.88/140)

    ∴ θ = 40.86 degrees ≈ 41 degrees
     
  5. Mar 2, 2013 #4

    rude man

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    Yay team!
     
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