js动画之平抛运动

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>平抛运动运动</title>
	<style>
    body{margin: 0px;padding: 0px;}
		.animation{
			background-color: green;
			height: 20px;
			 20px;
			left: 0px;
			top: 0px;
            border-radius: 10px;
            position: absolute;
		}
	</style>
</head>
<body>
	<div  ></div>
</body>
 <script type="text/javascript">
    window.onload = function(){
    	var ele = document.getElementById("test"),
            WINDOW_WIDTH = window.innerWidth - 20;
            WINDOW_HEIGHT = window.innerHeight - 20;
            ele.timer = null;
            ele.onmouseover =function(){
                startAnimation(this);
            }
            
    	

    	function startAnimation(obj){
    		clearInterval(obj.timer);
            var V_x = 30,
                G_y = 1,
                t = 0;
    		obj.timer = setInterval(function(){
                t++;
                var _left = obj.offsetLeft,
                    _height = obj.offsetTop,
                    _S_x = V_x*t,
                    _S_y = G_y*t*t/2 ;
                    

    			
                if(_S_x >= WINDOW_WIDTH){
                     obj.style.left = WINDOW_WIDTH +'px';
                     obj.style.top = _S_y +'px';
                     clearInterval(obj.timer);
                 }else if(_S_y >= WINDOW_HEIGHT){
                    obj.style.left = _S_x +'px';
                    obj.style.top = WINDOW_HEIGHT +'px';
                    clearInterval(obj.timer);
    			}else{
                    obj.style.left = _S_x +'px';
                    obj.style.top = _S_y +'px';
                }
    			
    		},100)
    	}

    	
    }
 </script>
</html>