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2D Kinematics - Finding Initial Velocity

  1. Aug 23, 2011 #1
    1. The problem statement, all variables and given/known data
    The "hang time" of a punt is measured to be 3.50 s. If the ball was kicked at an angle of 69.0° above the horizontal and was caught at the same level from which it was kicked, what was its initial speed? (Neglect air resistance.)

    2. Relevant equations
    All Kinematics Equations


    3. The attempt at a solution
    Acceleration (y) = -9.81 m/s^2
    Acceleration (x) = 0 m/s^2
    Time = 3.5 s
    Vi,x = (Vi)cos69
    Vi,y = (Vi)sin69

    Now I don't think I have enough information to solve the problem, but I know the problem isn't written incorrectly. I don't know where to go with the information I have because I can't solve for anything.

    Help please? Thanks in advance.
     
  2. jcsd
  3. Aug 23, 2011 #2
    You know the "hang time" therefore you can deduce the time it takes to reach the highest point. At that point there is something that happens with your Vy. If you use that with Vi,y = (Vi)sin69 and an appropriate kinematic equation you can solve the problem.
     
  4. Aug 23, 2011 #3
    How can I deduce the time it takes to reach the maximum height? Is that just half of the hang time?
     
  5. Aug 23, 2011 #4
    The ball is caught at the same height from which it was kicked and the acceleration is constant. Energy is conserved. Do you see any reason for the way up to take more then the way down?
     
  6. Aug 23, 2011 #5
    Oh, no I don't. Okay, thank you. I found the correct solution. Thank you!
     
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