1、方块下落：

  1. <style>
  2.     canvas{
  3.         width: 500px;
  4.         height: 300px;
  5.         border: 1px solid;
  6.     }
  7. </style>
  8. </head>
  9. <body>
  10. <canvas id="can" width="500px" height="300px"></canvas>
  11. <script>
  12.    var canvas = document.getElementById('can');
  13.    var ctx = canvas.getContext("2d");
  14. //    方块下落：
  15.    var height = 100;
  16.    var timer = setInterval (function (){
  17.     ctx.clearRect(0,0,500,300);
  18.     ctx.strokeRect(100,height,50,50);
  19.     height += 5;
  20.    },1000/30);
  21. </script>
  22. </body>

2、作业：自由落体：掉下去后弹起来