private function moveBall():void
    {
     ball.vy += gravity;
     ball.x += ball.vx;
     ball.y += ball.vy;
     if(ball.x + ball.radius > stage.stageWidth)
     {
      ball.x = stage.stageWidth - ball.radius;
      ball.vx *= bounce;
     }
     else if(ball.x - ball.radius < 0)
     {
      ball.x = ball.radius;
      ball.vx *= bounce;
     }
     if(ball.y + ball.radius > stage.stageHeight)
     {
      ball.y = stage.stageHeight - ball.radius;
      ball.vy *= bounce;
     }
     else if(ball.y - ball.radius < 0)
     {
      ball.y = ball.radius;
      ball.vy *= bounce;
     }
    }