# Problem 4-21-Projectile - Part 4 - B

A football is placed on a line $25\; m$ from the goalpost. The placement kicker kicks the ball directly toward the goalpost, giving it a velocity of $21.0\; m/s$ at an angle of $47.0^\circ$ above the horizontal. The horizontal bar of the goalpost is $3.0\; m$ above the field. How far above or below the horizontal bar of the goalpost will the ball travel?

Accumulated Solution

$v_{0_x}= 21 \cos 47 = (21) \cos47 = 14.32 \;m/s \\ v_{0_y}= 21 \sin 47 = (21) \sin47 = 15.36 \;m/s$

$v = v_0 + at \\ s = v_0t + (1/2)at^2 \\ v^2 = v_{0^2}+2as$

Correct

1st equation- $v, t$ unknown
2nd equation- $s, t$ unknown
3rd equation- $v, s$ unknown
So the vertical motion cannot be used yet. We must look at the horizontal motion.

For motion at constant velocity in the $x$ direction:

(A)   $x = t/v$
(B)   $x = v/t$
(C)   $x = vt$