var gravity = 10;
var elasticity = 0.9;
var floor = 250;
var Vy = 0;
ball_mc._y = 40;

ball_mc.onEnterFrame = function() {

Vy = Vy + gravity;
this._y += Vy/4;

if (this._y > floor) {
this._y = floor;
Vy = -Vy*elasticity;
}
};