小球在引力的作用下自由运动

gistfile1.js

var ballList = [];
function createBall(){
    var ball = document.createElement("div");
    var r = 10+Math.random()*100;
    var density = 0.01; // 密度
    ball.redis = r/2; // 半径
    ball.name = "ball"+ballList.length;
    ball.velocity = {"x":0,"y":0}; // 速度
    ball.acceleration = {"x":0,"y":0}; // 加速度
    ball.mess = 4.0/3*Math.PI*ball.redis*ball.redis*ball.redis*density; // 质量
    with(ball.style){
        width = r+"px";
        height = r+"px";
        background="rgb("+Math.round(Math.random()*255)+","+Math.round(Math.random()*255)+","+Math.round(Math.random()*255)+")";
        opacity = 0.8;
        position = "absolute";
        webkitBorderRadius = r/2.0+"px";
        mozBorderRadius = r/2.0+"px";
        top = window.screen.availHeight/2+window.screen.availHeight/2*(Math.random()-0.5)+"px";
        left = window.screen.availWidth/2 + window.screen.availWidth/2*(Math.random()-0.5)+"px";
    }
    ball.moveTo = function(x,y){
        this.style.top = y-ball.redis+"px";
        this.style.left = x-ball.redis+"px";
    };
    ball.getPosition = function(){
        return {
            "x":Number(ball.style.left.slice(0,-2))+ball.redis,
            "y":Number(ball.style.top.slice(0,-2))+ball.redis
        };
    };
    ball.getForce = function(){
        return {
            "x":this.acceleration.x * this.mess,
            "y":this.acceleration.y * this.mess
        };
    };
    ball.updateVelocity = function(force){
        this.acceleration.x = force.x/this.mess;
        this.acceleration.y = force.y/this.mess;
        this.velocity.x = this.velocity.x + this.acceleration.x * 100/1000;
        this.velocity.y = this.velocity.y + this.acceleration.y * 100/1000;
    }
    return ball;
}
// 判断重合
function judgeCoincide(ball1,ball2){
    if(ball1.redis + ball2.redis - calcDistance(ball1,ball2) > 0){
        return true;
    }
    return false;
}
// 计算两球距离
function calcDistance(ball1,ball2){
    var p1 = ball1.getPosition();
    var p2 = ball2.getPosition();
    return Math.sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y));
}
// 计算两球引力
function calcGravitation(ball1,ball2){
    if(judgeCoincide(ball1,ball2)===true){
        return 0;
    }
    var G = 10; // 万有引力常量
    var distance = calcDistance(ball1,ball2);
    return G*ball1.mess*ball2.mess/distance/distance;
}
// 求单位向量
function calcUnitVector(ball1,ball2){
    var distance = calcDistance(ball1,ball2);
    p1 = ball1.getPosition();
    p2 = ball2.getPosition();
    return {
        "x":(p2.x-p1.x)/distance,
        "y":(p2.y-p1.y)/distance
    }
}
//计算小球引力合力
function calcAllForce(ball){
    var currentForce = {"x":0,"y":0};
    for(var i in ballList){
        if(ball.name == ballList[i].name){
            continue
        }
        var force = calcGravitation(ball,ballList[i]);
        var vector = calcUnitVector(ball,ballList[i]);
        currentForce.x += force * vector.x;
        currentForce.y += force * vector.y;
    }
    return currentForce;
}
function nextPosition(ball){
    var force = calcAllForce(ball);
    var currentPosition = ball.getPosition()
    ball.moveTo(
        currentPosition.x + ball.velocity.x * 100/1000,
        currentPosition.y + ball.velocity.y * 100/1000
    );
    ball.updateVelocity(force);
}
function moveAll(){
    for(var i in ballList){
        nextPosition(ballList[i]);
    }
}
window.onload = function(){
    //创建小球
    for(var i=0; i<10; i++){
        var ball = createBall();
        ballList.push(ball);
        document.body.appendChild(ball);
    }
    //    setInterval(moveAll,10);
    setTimeout(function(){
        moveAll();
        setTimeout(arguments.callee,10);
    },10)
};