Homework Help: Trajectory, does it clear a certain height problem help!

1. Jun 3, 2009

iamtrojan3

Trajectory, "does it clear a certain height" problem help!

1. The problem statement, all variables and given/known data
A place kicker must kick a football from a point 45.5 m from the goal, and the ball must clear the crossbar, which is 3.05 m high. When kicked, the ball leaves the ground with a speed of 19.0 m/s at an angle of 48.0° to the horizontal. By how much does the ball clear or fall short of clearing the crossbar?

2. Relevant equations
Dy = distance in y = Voy *t -1/2gt^2
Dx = ~~~ x = Vox * t
Vox = initial velocity in x
Voy = ~~~~ y

3. The attempt at a solution
I tried to express Y in terms of x...
since x = Vox * t .... t = x / Vox
then
Dy = Voy(x) / Vox - 4.9 (x^2/Vox^2) assume gravity's 9.8

Plug in 45.5 m for x
Voy = 14.12 m/s
Vox = 12.71 m/s

Dy = -12.23 m

So when the ball is 45.5 m long, its 12.23m underground. which means the ball is 12.23 - 3.05 = 9.18m short of crossing the bar.

Idk... seems simple enough but apprently the answer's wrong.

2. Jun 3, 2009

rl.bhat

Re: Trajectory, "does it clear a certain height" problem help!

With a speed of 19.0 m/s at an angle of 48.0° to the horizontal, find the range.

3. Jun 3, 2009

iamtrojan3

Re: Trajectory, "does it clear a certain height" problem help!

Ahhhhhhh i figured it out... since its "underground"... you add the 3.05 meters!
thanks for the helps anyways >.<