Ball physics in AS3

1

ball physics in AS3 here is the code:-
var dx:Number;
var dy:Number;
var gravity:Number;
var dy_decrement:Number;

ball.addEventListener(Event.ENTER_FRAME,enter_frame);
init();

function init()
{
dx = 2.5;
dy = -20;
gravity = .5;
dy_decrement = dy;
ball.x = ball.width;
ball.y = stage.stageHeight-ball.height/2;
}

function enter_frame(e:Event)
{

if(ball.y > stage.stageHeight)
{
    ball.y = stage.stageHeight - ball.height;
    dy_decrement +=3 
    dy = dy_decrement;
}

else if(ball.x > stage.stageWidth)
{
    init();
}

else
{
    dy+=gravity;
    ball.x += dx;
    ball.y += dy;
}

}