# Homework Help: Prooving a projectile motion formula

1. Aug 22, 2011

### stonecoldgen

1. The problem statement, all variables and given/known data

Two rocks are thrown from a cliff of height h at the same speed V0. One rock is thrown up at angle (Theta 1) adn the other is thrown down at angle (Theta 2). Show that both rocks hit the ground at the same speed.

V= (V0^2+2gh)^(.5)

2. Relevant equations

d=vt+.5at^2

3. The attempt at a solution

well, logically it is really easy to solve. When the rock that is launched up comes back down to the level that the other rock is launched down out, it gets the same speed it was launched at (V0), and the other rock is V0 as well, so there you go.

I tried a solution with a quadratic formula but itt didnt work, somehow i see that if i proove the firsth equation in my list (formula 5 of the kinematic equations), from galileo's equation i might get something, but im not sure how to do that.

2. Aug 22, 2011

### PeterO

When the are "launched", they start from the same height - so same Potential energy per kg, and start at the same speed, so same kinetic energy per kg, so when they both get to ground level, how will their kinetic energy per kg compare, and what does that mean?

3. Aug 23, 2011

### c03rcion

can you include a picture?