碰撞+重力

var iSpeedX=6;
var iSpeedY=8;
var timer=null;
function startMove()
{
    var oDiv = document.getElementById("div1");
    timer=setInterval(function(){
        iSpeedY+=3;
        var l = oDiv.offsetLeft+iSpeedX;
        var t = oDiv.offsetTop+iSpeedY;
        if(l>document.documentElement.clientWidth-oDiv.offsetWidth)
        {
            iSpeedX*=-0.8;
            l=document.documentElement.clientWidth-oDiv.offsetWidth
        }
        else if(l<0)
        {
            iSpeedX*=-0.8;
            l=0;
        }
        if(t>document.documentElement.clientHeight-oDiv.offsetHeight)
        {
            iSpeedY*=-0.8;
            iSpeedX*=0.8;
            t=document.documentElement.clientHeight-oDiv.offsetHeight;
        }
        else if(t<0)
        {
            iSpeedY*=-0.8;
            t=0;
        }
        if(Math.abs(iSpeedX)<1)
        {
            iSpeedX=0;
        }
        if(Math.abs(iSpeedY)<1)
        {
            iSpeedY=0;
        }
        if(iSpeedX==0&&iSpeedY==0&&t==document.documentElement.clientHeight-oDiv.offsetHeight)
        {
            clearInterval(timer);
            alert("停止");
        }
        oDiv.style.left=l+"px";
        oDiv.style.top=t+"px";
        document.title=iSpeedX;
    },30);
};