# Homework Help: Vertical velocity

1. Jul 13, 2007

### MIA6

1. The problem statement, all variables and given/known data
A bridge rises 321m above the Arkansas river. Suppose you kick a rock horizontally off the bridge. The magnitude of the rock's horizontal displacement is 45.0 m. Fine the speed at which the rock was kicked.

2. Relevant equations
Vi=Vx. Dx=45m. Dy=-321m

3. The attempt at a solution
I know how to find the speed, but my question is on my book, it says that there is no initial vertical velocity. WHy? i think when you kick a rock off the bridge, there is a initial vertical velocity?! so why the vertical initial velocity is 0? hope you can explain it to me.

Thanks.

2. Jul 13, 2007

### Astronuc

Staff Emeritus
The rock is kicked horizontally (impulsively) off the bridge so that it has a constant horizontal velocity only as it leaves the bridge. It then starts falling vertically with zero initial velocity, as though it was simply released from rest. It then obviously starts accelerating with gravity.

In the time that it take to fall from the point of release to the ground, it also travels 45 m horizontally.

3. Jul 13, 2007

### MIA6

ok, i get it. so the ball travels 321m vertically in free fall?

4. Jul 13, 2007

### Astronuc

Staff Emeritus
Yes. Constant acceleration, g, with no initial velocity (and perhaps ignoring wind resistance).

5. Jul 13, 2007

### MIA6

but i have a question, because i know that 321m is a path that vertical straight down, but the path that the ball travels is half of the parabola, so if we meansure the curve and the straight line, they won't be the same.