var k:Number = 0.1;
var damp:Number = 0.96;
var g:Number = 3;
var targetY = Stage.height/2;
var vy = 0;

ball_mc._y = 50;

onEnterFrame = function() {
disty = targetY - ball_mc._y
vy =vy*damp + disty*k + g;
ball_mc._y += vy;

clear();
lineStyle(5,0xFFCC66,100);
moveTo(Stage.width/2, 40);
lineTo(ball_mc._x, ball_mc._y);
};