# Projectiles

• May 4th 2009, 04:43 AM
djmccabie
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
• May 4th 2009, 05:15 AM
skeeter
Quote:

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

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

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

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

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

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

$\displaystyle t = \frac{2v_0 \sin{\theta}}{g}$
• May 4th 2009, 05:24 AM
djmccabie
Quote:

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

This is what I was looking for :D thankyou!