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Homework Help: 2D Projectile Motion, kicking football

  1. Oct 15, 2007 #1
    Kicking a football 36m from the goal, which is 3.05m high. The ball leaves the ground with a speed of 20m/s at an angle of 53 degrees to the horizontal. By how much does the ball clear or fall short of clearing the crossbar?

    delta y = v0cos(theta)delta t

    The thing is, I don't know delta t, and the equation for delta t I have is delta t = delta x/ v0cos(theta), but the only way of knowing delta x also requires delta t...

    Can someone please walk me through the problem? (I can do the math myself but just what equations to use and such.) thanks a lot!
  2. jcsd
  3. Oct 15, 2007 #2
    To find delta t, use the equation:
    v= (initial velocity) + a(t)

    v= 0 at its maximum point
    you know the initial vertical velocity (vo sin(theta)
    and you know the accl due to gravity.
    then multiply t by 2, since it is symmetric, to get the total time.

    Sorry, I don't know how to do all the symbols and such.
  4. Oct 16, 2007 #3
    TY for your reply.

    I tried finding the time that way, but I ended up getting horribly negative numbers.

    I then tried finding delta t by equaling it to delta x/ v0cos(theta), where delta x = 36 and ended up getting t = 2.99s

    I used that in the delta y equation of delta y = v0sin(theta)delta t - (.5)(g)(delta t)2 and was able to solve it from there.

    This thread is solved but i'm not sure how to notate it as such...
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