hw4 helper

spring-mass-damper system

by 林 羿緯

HTML

<div id="info">Spring (hw4 helper) <br/>
    <a href="javascript:reset();">reset</a>
</div>
<div id="centerText"> test
</div>
<script src="http://cdnjs.cloudflare.com/ajax/libs/three.js/r70/three.min.js"></script>
<script src="https://code.jquery.com/jquery-2.1.4.min.js">
</script>

CSS

#info {
    position: absolute;
    top: 0px;
    width: 100%;
    padding: 10px;
    text-align: center;
    color: #ffff00
}
body {
    overflow: hidden;
}
#centerText {
  position:absolute;
  top:50vh;
  right:50vw;
  text-align:center;
  color:#ff00ff
}

JavaScript

var camera, scene, renderer;
var mesh, border;
var clock = new THREE.Clock();

var m = 1, k = 10;
var x = [];
var nowDate;
var vNow, vOld;
var then;

function reset() {
	x[0] = 10, x[1] = 0;
}

function deriv (x) {  // y_dot = f(x, y)
	var ff = [];
    ff.push (x[1]);       // x_dot = v;
    ff.push (-k*x[0]/m);  // v_dot = 1/m *(-kx)
    return ff;
}

function catchEnds() {
	
  if (vNow*vOld  < 0) { // velocity changes sign
	  //$('#centerText').text ( 'turn')
		let now = new Date()
	  $('#centerText').text ( 'period = ' + 2*(now - then)/1000)
		then = now;
	}
}
/*
function deriv (x) {
	var ff = [];
    ff.push (x[1]);                  // x_dot = v;
    ff.push (-(c*x[1] + k*x[0])/m);  // v_dot = 1/m *(-cx_dot -kx)
    return ff;
}
*/
function ODESolver (dt) {
   
	var n = x.length;
    var f = deriv (x);
    
    for (var i = 0; i < n; i++) {
        x[i] += f[i]*dt;
    }
    
    
    // fake version
    // let x[0] be a sine function
  /*  
    this.time = this.time + dt || 0;
    x[0] = 20*Math.sin(5*this.time);  // fake....
*/
}

init();
animate();

function init() {
    scene = new THREE.Scene();

    camera = new THREE.OrthographicCamera(-50, 50, 50, -50, -10, 10);
    camera.position.z = 10;
    scene.add(camera);

    mesh = new THREE.Mesh(new THREE.CircleGeometry(1, 10), new THREE.MeshBasicMaterial());
    scene.add(mesh);

    var material = new THREE.LineBasicMaterial({
        color: 0x0000ff
    });

    var geometry = new THREE.Geometry();
    geometry.vertices.push(
    new THREE.Vector3(-40, -0, 0),
    new THREE.Vector3(40, 0, 0));

    var border = new THREE.Line(geometry, material);
    scene.add(border);
    
    renderer = new THREE.WebGLRenderer();
    renderer.setSize(window.innerWidth, window.innerHeight);
    renderer.setClearColor(0x888888);

    document.body.appendChild(renderer.domElement);
    window.addEventListener('resize', onWindowResize, false);
    
	// INITIIALIZE ODE
    x.push (20);
    x.push (0);

	nowDate = new...