# Homework Help: Projectile motion

1. Oct 2, 2011

### sweet_girl123

a cannonball is fired with initial velocity v0 at an angle 30 above the horizontal from the height of 40m above the ground. the projection strikes the ground with a speed of 1.2*v0. find v0

2. Oct 2, 2011

### sweet_girl123

I tried by getting the time first
y = 1/2 gt^2

but I am lost after that can anyone plz help me out....

3. Oct 3, 2011

### VulcanWong

Energy consideration maybe another(also faster i think=)) method to get the answer.
Try to think about the whole system energy(involve both KE and PE).

Think about the VERTICAL motion(Vo sin 30), you will also get the answer by setting up a motion equation

Last edited: Oct 3, 2011
4. Oct 3, 2011

### I like Serena

Hi sweet_girl123!

Easiest way to do this is with energy conservation (as VulcanWong suggested).

The formula for that is:
$$g y_i + {1 \over 2} v_i^2 = g y_f + {1 \over 2} v_f^2$$
with:
$y_i, y_f$ the initial and final heights
$v_i, v_f$ the initial and final (total) speeds
$g = 9.8 {m \over s^2}$ the acceleration of gravity

Can you fill in the numbers?