自由落体函数

onload = function() {

//红色的球

var oBox = document.getElementById("box");

// y轴方向的运动

var ySpeed = -20;

var xSpeed = 5;

var count = 50;

var timer = setInterval(function(){

//速度

ySpeed += 2;

//更新位置

oBox.style.top = (oBox.offsetTop + ySpeed) + "px";

oBox.style.left = (oBox.offsetLeft + xSpeed) + "px";

//终止条件

//  页面高度减去球的高度

var maxHeight = document.documentElement.clientHeight - oBox.offsetHeight - 10;

if (oBox.offsetTop >= maxHeight) {

// 直接将球放在地面上

oBox.style.top = maxHeight + "px";

// 反向

// speed = -1 * speed;

ySpeed = -0.8 * ySpeed;

ySpeed = parseInt(ySpeed);

if (Math.abs(ySpeed) <= 1) {

count--;

if (count <= 0) {

clearInterval(timer);

}

}

}

}, 50);

}