private function moveBall(ball:animation_ch08_Ball, targetX:Number, targetY:Number):void
    {
     ball.vx += (targetX - ball.x) * spring;
     ball.vy += (targetY - ball.y) * spring;
     ball.vy += gravity;
     ball.vx *= friction;
     ball.vy *= friction;
     ball.x += ball.vx;
     ball.y += ball.vy;
    }
   }
  }