# Homework Help: 2D projecticle motion problem

1. Feb 12, 2008

### bengt2665

1. A stone is thrown horizontally from the top of a 20 m high tower. It strikes the ground at an angle of 45 degrees. With what speed was it thrown?

I'm having trouble with this problem, any help would be greatly appreciated.

This is what I have:

I know that Vox = Vo cos 45 and that Voy = Vo sin 45.
Given this, we know that cos 45 = sin 45 so Voy = Vox.

I think the trick here is to calculate Vox with what we have (the height of 20m) to obtain Voy but I can't seem to move on from there.

Other stuff I can think of that might help but I can't seem to connect with the problem:
the force of gravity in the y direction, the equation Xf = Xo + Vo(t) + 1/2at^2.

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

2. Relevant equations

3. The attempt at a solution

2. Feb 12, 2008

### rl.bhat

In projectile motion Vox remains the same throughout.
You can fing Vy by using Vy^2 = Voy^2 + 2gh
When it strikes the ground at an angle of 45 degrees, Vy = Vox