canvas问题

Question

绘制圆球匀速下落时每一次的速度比上一次更快，越来越快,不知道问题出在哪里。求解，谢谢各位了。var canvas = document.getElementById('canvas');var context = canvas.getContext('2d');var ball ={    x:0,    y:0,    r:15,    vy:12,    vx:12}var drawCircle = function (x,y,r,color){    context.clearRect(0,0,canvas.width,canvas.height);    context.lineWidth = '5';    context.beginPath();    context.arc(x,y,r,0,Math.PI*2,false);    context.strokeStyle = 'rgba(120,150,170,0.5)';    context.fillStyle = color;    context.closePath();    context.stroke();    context.fill()；}var  update = function (){    ball.y +=ball.vy;    if (ball.y >= canvas.height - ball.r)    {        ball.y = canvas.height - ball.r;        ball.x -= ball.vx;    }}this.onclick = function (event){    ball.x = event.clientX;    ball.y = event.clientY;    setInterval(function ()    {        drawCircle(ball.x,ball.y,ball.r,'blue');        update();    },100);}