private function init():void
    {
     stage.scaleMode = StageScaleMode.NO_SCALE;
     stage.align = StageAlign.TOP_LEFT;
     
     balls = new Array();
     for(var i:uint = 0; i < numBalls; i++)
     {
      var radius:Number = Math.random() * 50 + 20;
      var ball:animation_ch11_Ball = new animation_ch11_Ball(radius);
      ball.mass = radius;
      ball.x = Math.random() * stage.stageWidth;
      ball.y = Math.random() * stage.stageHeight;
      ball.vx = Math.random() * 10 - 5;
      ball.vy = Math.random() * 10 - 5;
      addChild(ball);
      balls.push(ball);
     }