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Homework Help: Projectile Motion - A little Lost

  1. Sep 9, 2004 #1
    I've been trying to figure this problem out and I'm a little confused.

    Here is the problem.

    A quarterback is set up to throw the football to a receiver who is running with a constant velocity Vr directly away from the quarterback and is now a distance D away from the quarterback. The quarterback figures that the ball must be thrown at an angle A to the horizontal and he estimates that the receiver must catch the ball a time interval Tc after it is thrown to avoid having opposition players prevent the receiver from making the catch. In the following you may assume that the ball is thrown and caught at the same height above the level playing field. Assume that the y coordinate of the ball at the instant it is thrown or caught is y=0 and that the horizontal position of the quaterback is x=0.
    Use for the magnitude of the acceleration due to gravity, and use the pictured inertial coordinate system when solving the problem.

    Now, I need to find Find V0x , the initial horizontal component of velocity of the ball. Express your answer for V0x in terms of D, Tc, and Vr.

    I found a formula relating the receivers position to his distance, velocity and time. D +Vr*Tc

    Also I know the position of the ball to be V0x*Tc.

    Obviously to I need to combine those somehow to get this answer. Thats where I am a little stuck.

    I also need to find V0 the speed with which the quarterback must throw the ball in terms of D, Tc, Vr, and G.

    Finally, i need the Angle A assuming the qb threw it at V0. They claim this should contain an inverse trig function and i need it in terms of V0x, V0y and V0.

    I am hopelessly lost and have been trying to get this for quite some time. I care not about getting the answers right, just knowing how I am supposed to go about it! Any help would be greatly appreciated.
  2. jcsd
  3. Sep 9, 2004 #2
    Okay, I figured out the first part of the porblem. I just needed to set the position function for the quarterback equal to the position for the ball. Now the rest, still working on it. Help! The speed component, that is basically the magnitude of the vector correct? So how do I relate . . . . .
  4. Sep 9, 2004 #3
    Wow, things are starting to click. The learning process I presume? Well, I got the initial speed. Yay! Now just to find the angle . . . Feel free to pitch in.
  5. Sep 9, 2004 #4
    Got it! Okay sorry for wasting the thread space. All I had to do was arctan(V0y/V0x) and voila! Guess it's better that I figured it out on my own anyway. Ahh the learning process.
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