ActionScript 3.0 :: Attaching A Function To Every AddChild() Object?

Code:
var rockSpawn:rock;
function doCollision():void
{
trace("collision detected at "+collisionLoc+", Time: "+flash.utils.getTimer());
rockSpawn = new rock;

[code]....

This function spawns a rock object from the library at a given location, and then moves it to the right every frame until it reaches the edge of the screen, at which point it's deleted. The problem: when a rock is spawned, it moves to the right, but when a new rock is spawned, the previous one stops dead. Each rock moves progressively faster, as if each rockSpawn.x++ is stacking to make the object move 1 pixel faster each time the function is called.

How would I modify this code to spawn separate instances of a projectile that behave the same? Also, how would I reference them for say, collision detection? That is:

Code:
if (a rockSpawn instance collides with the movieClip ship)
{
do stuff;
}