这个程序对应于 NOC_2_1_forces.pde，
我们绘制了一个有重力加速度的球。越下落速度越快，但接触地面时会反弹，由于重力作用，越上升速度越慢。
程序运行界面
点击按钮 West Wind 对球施加从左吹来的风力作用。
可以从 https://github.com/jollywing/nature_of_code_examples_html5/blob/master/chp2_forces/wind_gravity_forces.html 下载该程序，用浏览器打开即可运行。
程序代码如下：
<!doctype html>
<html>
<head>
<title>Wind and Gravity force</title>
<meta charset="utf-8" />
<meta name="author" content="JollyWing(jiqingwu@gmail.com)"/>
<meta name="create" content="2015-01-06 Tue"/>
<style type="text/css">
#screen {border:1px solid #000; margin-left:auto; margin-right:auto;}
p.Scr {text-align:center;}
</style>
<script>
// get context
var scr;
var ctx;
var timeOut;
var position = new Object();
position.x = 5;
position.y = 5;
var velocity = new Object();
velocity.x = 0;
velocity.y = 0;
var gravityAccel = new Object();
gravityAccel.x = 0;
gravityAccel.y = 1;
var windAccel = new Object();
windAccel.x = 1;
windAccel.y = 0;
var ball = {
pos: position,
radius: 5,
vel: velocity,
erase: function(){
var top, left;
//caculate the topleft
left = this.pos.x - this.radius - 1;
top = this.pos.y - this.radius - 1;
// clear rect
ctx.clearRect(left, top, this.radius*2 + 2, this.radius*2 + 2);
},
draw: function(){
// set context property
ctx.strokeStyle="red";
ctx.fillStyle="yellow";
// draw
ctx.beginPath();
ctx.arc(this.pos.x, this.pos.y, this.radius, 0, 2*Math.PI);
ctx.stroke();
ctx.fill();
},
checkEdge: function(){
if(this.pos.x < 0){
this.pos.x = this.radius;
this.vel.x *= -1;
}
if(this.pos.x > scr.width){
this.pos.x = scr.width - this.radius;
this.vel.x *= -1;
}
if(this.pos.y < 0){
this.pos.y = this.radius;
this.vel.y *= -1;
}
if(this.pos.y > scr.height){
this.pos.y = scr.height - this.radius;
this.vel.y *= -1;
}
},
addForce: function(accel){
this.vel.x += accel.x;
this.vel.y += accel.y;
},
move: function(){
this.pos.x += this.vel.x;
this.pos.y += this.vel.y;
},
};
function initScr(){
scr = document.getElementById('screen');
ctx = scr.getContext('2d');
ball.draw();
}
function updateScr()
{
// erase ball
ball.erase();
// add gravity force
ball.addForce(gravityAccel);
// update ball position
ball.move();
// check edges
ball.checkEdge();
// draw ball
ball.draw();
timeOut = setTimeout('updateScr()', 100);
}
function startWalk()
{
updateScr();
}
function stopWalk()
{
clearTimeout(timeOut);
}
function westWind()
{
// add wind force to velocity
ball.addForce(windAccel);
}
</script>
</head>
<body onload="initScr()">
<p class="Scr">
<canvas id="screen" width="640" height="640"></canvas>
<br/>
<input type="button" value='Start' onclick='startWalk()'/>
<input type="button" value='Stop' onclick='stopWalk()' />
<input type="button" value="West Wind" onclick='westWind()'/>
</p>
</body>
</html>
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