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Homework Help: More projectile motion difficulties

  1. Oct 2, 2008 #1
    1. The problem statement, all variables and given/known data
    A soccer ball is kicked with a speed of 7.50 m/s at an angle of 30.0° above the horizontal. If the ball lands at the same level from which it was kicked, how long was it in the air? (Neglect air resistance.)

    2. Relevant equations

    3. The attempt at a solution
    I really don't know where to start, all of the equations are confusing me.
    Last edited: Oct 2, 2008
  2. jcsd
  3. Oct 3, 2008 #2
    This question is quadratic. What you need to find is how long the ball was in the air from when Vy=initial to Vy = 0. Your acceleration should be -9.80 m/s/. Finding the time, or the y-max (maximum height) will give you more insight as to how to solve the problem.

    Vf = Vt + a(dt)? try it!
  4. Oct 3, 2008 #3
    Sorry, how do I get y-max. I can't even begin to fill in this equation cuz I have only the acceleration
  5. Oct 3, 2008 #4
    Okay. Vf = 0 (since it only goes so high if it's kicked), and Vi = 7.50 m/s. You can use deltaY= Vi(t)+1/2at^2, where your a = -9.80 m/s. Solve using the quadratic formula for t. Once you get t, you can multiply t by 2 since it takes the same amount of time for the ball to fall downward as it does going upward.
  6. Oct 3, 2008 #5
    oh, wait. no. Get your Dy, by drawing a straight line through the middle of the pathway of the ball. use trigonometry to calculate the y part of the right triangle to get your Vi. then use the Vf=0, and then sub it into the equation to get t.
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