完全按着老师的代码来做小球物理实验，为什么会出现这种效果，小球下落连成一片？代码没有报错

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<!DOCTYPE html>

<html>

<head>

</head>

<body>

var ball={x:512,y:100,r:20,g:2,vx:-4,vy:0,color:"black"}

window.onload = function() {

var canvas = document.getElementById("canvas");

canvas.width = 1024;

canvas.height = 768;

var context = canvas.getContext("2d");

setInterval(

function() {

render(context);

update();

)

}

function update() {

ball.x += ball.vx;

ball.y += ball.vy;

ball.vy += ball.g;

if(ball.y>=768-ball.r){

ball.y = 768-ball.r;

ball.vy-=ball.vy*0.5;

}

function render(cxt) {

cxt.clearRect(0,0,cxt.canvas.width,cxt.canvas.height)

cxt.fillStyle = ball.color

cxt.beginPath

cxt.arc(ball.x,ball.y,ball.r,0,2*Math.PI)

cxt.closePath

cxt.fill()

}

</script>

</body>

</html>

//img3.mukewang.com/59f7159a0001dfcc08130879.jpg

提问者：小鱼55555 2017-10-30 19:30