实现*落体运动需要理解的几个简单属性：
clientHeight：浏览器客户端整体高度
offsetHeight：对象（比如div）的高度
offsetTop：对象离客户端最顶端的距离
简单demo如下：
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>free_movement</title>
<style type="text/css">
#div1{
position: absolute;
height: 100px;
width: 100px;
background: red;
}
</style>
<script type="text/javascript">
window.onload=function () {
var btn=document.getElementById('btn');
var div1=document.getElementById('div1');
var Time=null;
var speed=0;
btn.onclick=function () {
startMove();
}
function startMove () {
clearInterval(Time);
Time=setInterval(function(){
speed+= 3;
var T = div1.offsetTop + speed;
if(T > document.documentElement.clientHeight - div1.offsetHeight){
T = document.documentElement.clientHeight - div1.offsetHeight;
speed *= -1;
speed *= 0.75;
}
div1.style.top=T+'px';
}, 30)
}
}
</script>
</head>
<body>
<input type='button' value='开始运动' id="btn">
<div id="div1"></div>
</body>
</html>
注：clearTnterval(Time)://防止多次点击事件的产生