js实现自由落体

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实现自由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

 <!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);   //clearTnterval(Time)://防止多次点击事件的产生

                 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>