private function onEnterFrame(event:Event):void
    {
     vy += gravity;
     ball.x += vx;
     ball.y += vy;
     var left:Number = 0;
     var right:Number = stage.stageWidth;
     var top:Number = 0;
     var bottom:Number = stage.stageHeight;
     
     if(ball.x + ball.radius > right)
     {
      ball.x = right - ball.radius;
      vx *= bounce;
     }
     else if(ball.x - ball.radius < left)
     {
      ball.x = left + ball.radius;
      vx *= bounce;
     }
     if(ball.y + ball.radius > bottom)
     {
      ball.y = bottom - ball.radius;
      vy *= bounce;
     }
     else if(ball.y - ball.radius < top)
     {
      ball.y = top + ball.radius;
      vy *= bounce;
     }
    }