Well, dx’s code shows you how to pick a random start position. Just make the ._y value above the stage. Then you need some sort of motion function that changes the _y positions of each movie clip by whatever speed you’ve assigned to it (probably want those to be bounded random numbers as well). There are lots of tuts out there for making things move on screen.
Gravity is pretty simple. Ok, say you have a ball MovieClip (ball) and you want to give it some gravity. I’m assuming that the ball will have some velocity (vx, vy : velocity on x and y axis) and a gravity constant (gravity). Here’s the code:
var gravity:Number = 0.03;
this.onEnterFrame = function():Void {
//apply gravity to y velocity (pulls down)
ball.vy += gravity;
//move the ball
ball._x += ball.vx;
ball._y += ball.vy;
}