Homework Help: Find the Speed of the ball

1. Sep 27, 2010

kari82

A ball is thrown horizontally from the top of a 20-m high hill. It strikes the ground at an angle of 45°. With what speed was it thrown?

I thought that we could use h=(Vi^2sin^2alpha)/2g. However, using that formula im not able to get to the correct answer which is 20m/s. What am I doing wrong?

Thanks!

2. Sep 27, 2010

rock.freak667

You know the formula s=s0+ut-0.5gt2 where s = displacement and u= initial velocity.

Using this formula and considering vertical motion, the ball hits the ground at s=0. So what is the total time of flight? (If it was thrown horizontally, its initial vertical velocity is?)