Agirl throws a rock horizontally, with a velocity of 10 m/s, from a bridge. it falls 20 m to the water below. how far does the rock travel horizontally before striking the water, assuming negligible air resistance?

Before you begin, you need to first figure out what your problem gives you and what you need to look for.
Given:
vix=10m/s
dy= 20m

What you are looking for is horizontal distance or dx.

The formula for vertical distance is:


Where:
dx= horizontal distance
vix= initial horizontal velocity
t=time

But when you look at your problem, you do not have time. Now for time, you need to use the formula:



g in this equation is a constant. It is the acceleration due to gravity which has the value of 9.8m/s^2.

Now we first need to get time:





Now that you have time, all you need to do is insert that into our equation for horizontal distance. 





The horizontal distance of the rock will be 20.20m.