JQ自定义运动问题?初级

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

<script type="text/javascript" src="jquery-1.7.2.js"></script>

<title>Examples</title>

<style type="text/css">

	*{padding: 0;margin: 0;}

	li{list-style: none;}

	#box{

		width: 410px;

		height: 320px;

		margin: 30px auto;

	}

	#box li{

		width: 180px;

		height: 160px;

		overflow: hidden;

		border: 1px solid red;

		float: left;

		margin: 5px;

		position: relative;

	}

	#box li div{

		position: absolute;

		top: 0;

		left: 0;

	}



</style>

</head>

<body>

<script type="text/javascript">

$(function(){

	setInterval(function(){

		var num=Math.random()>0.5?160:-160;

		var num2=Math.floor(Math.random()*3);

		var ThisTop=$('#box ul li div').eq(num2).position().top;

		if(ThisTop==0){

			$('#box ul li div').eq(num2).stop().animate({'top':'-=160px'}, 300)

		}else if(ThisTop==-320){

			$('#box ul li div').eq(num2).stop().animate({'top':'+=160px'}, 300)

		}else{

			$('#box ul li div').eq(num2).stop().animate({'top':'+='+num}, 300)

		}

		//我想要的方法

		//$('#box ul li div').eq(num2).animate('top':function(){

		// if(){return ...} 为什么不行?还是我的格式有问题	

		// })





		



	},1500)

	







})

</script>

<div id="box">

	<ul>

		<li>

			<div>

				<img src="images/001.gif" alt="" />

				<img src="images/002.gif" alt="" />

				<img src="images/003.gif" alt="" />

			</div>

		</li>

		<li>

			<div>

				<img src="images/004.gif" alt="" />

				<img src="images/005.gif" alt="" />

				<img src="images/006.gif" alt="" />

			</div>

		</li>

		<li>

			<div>

				<img src="images/007.gif" alt="" />

				<img src="images/008.gif" alt="" />

				<img src="images/009.gif" alt="" />

			</div>

		</li>

	</ul>

</div>   

</body>

</html>