# Math Help - Projectiles

1. ## Projectiles

A ball is kicked from point A on a horizontal field with an initial speed of 24.5 m/s at an angle of 30 degrees above the horizontal. The ball first hits the ground again at point B.

Calculate the time of flight of the ball

Ok so i have worked out

Vx = 24.5cos30 = 21.218
Vy = 24.5sin30 - 9.8t = 12.25

now i'm assuming i have to do something with these numbers but I have no idea :S

2. Originally Posted by djmccabie
A ball is kicked from point A on a horizontal field with an initial speed of 24.5 m/s at an angle of 30 degrees above the horizontal. The ball first hits the ground again at point B.

Calculate the time of flight of the ball

Ok so i have worked out

Vx = 24.5cos30 = 21.218
Vy = 24.5sin30 - 9.8t = 12.25

now i'm assuming i have to do something with these numbers but I have no idea :S
$\Delta y = v_0 \sin{\theta} \cdot t - \frac{1}{2}gt^2$

the overall displacement in the y-direction is 0 ...

$0 = v_0 \sin{\theta} \cdot t - \frac{1}{2}gt^2$

$0 = t\left(v_0 \sin{\theta} - \frac{1}{2}gt\right)$

$v_0 \sin{\theta} - \frac{1}{2}gt = 0$

$t = \frac{2v_0 \sin{\theta}}{g}$

3. Originally Posted by skeeter
the overall displacement in the y-direction is 0 ...

This is what I was looking for thankyou!