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AP Physics 2-D trajectory problem

  1. Oct 1, 2007 #1
    I'm not in the class, but a friend of mine asked me to help him with this problem, and I just can't figure it out! It seems like it's simple, but whenever I try to do it, I can't find a solution:

    Problem statement
    A field goal kicker is 50m away from the 3.4m high goal post. If he kicks it at an initial velocity of 15 m/s, then what are the two angles at which he can kick the ball to JUST clear the goal post?

    What I've tried
    I've tried looking at it from as many angles as possible. Right now I'm just trying to solve for one angle, and then I'll repeat the process to find the other. I've tried using trig identities (I couldn't find anything that would be helpful). I tried to use conservation of energy, but couldn't get anywhere with that. I *think* I may be on the right track now: I picked another point, x meters away from the start of kick, where the ball would be 3.4m off of the ground. I can find that the velocity at this point is 13.84 m/s by using conservation of energy, and the time it takes to traverse the distance x is .118s. Even with this information, I can't seem to find the x or y-velocity at any given time.

    Here's a pic
    Last edited: Oct 1, 2007
  2. jcsd
  3. Oct 1, 2007 #2


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    Staff: Mentor

  4. Oct 1, 2007 #3
    Blah. Looks like I had some figures wrong. A projectile launched at 15 m/s can't even traverse 50m no matter what the angle of launch is...
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