A CodePen by Adam. Tear-able Cloth - Javascript cloth simulation.

index.html

<canvas id = "c" > </canvas>
<p>
  Tear the cloth with your mouse.<br><br>
  Right click and drag to cut the cloth.<br><br>
  Reduce physics_accuracy if it's laggy, increase it for a better effect.
</p>
<a target="_blank" href="http://lonely-pixel.com/">my website (in progress, soon to be a javascript experiment showcase)</a>

script.js

/*
Copyright (c) 2013 Stuffit at codepen.io (http://codepen.io/stuffit)

View this and others at http://lonely-pixel.com

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
*/

// settings

var physics_accuracy = 5,
mouse_influence      = 20, 
mouse_cut            = 6,
gravity              = 900, 
cloth_height         = 30,
cloth_width          = 50,
start_y              = 20,
spacing              = 7,
tear_distance        = 60;


window.requestAnimFrame =
window.requestAnimationFrame       ||
window.webkitRequestAnimationFrame ||
window.mozRequestAnimationFrame    ||
window.oRequestAnimationFrame      ||
window.msRequestAnimationFrame     ||
function(callback) {
    window.setTimeout(callback, 1000 / 60);
};

var canvas,
	ctx,
	cloth,
	boundsx,
	boundsy,
	mouse = {
		down: false,
		button: 1,
		x: 0,
		y: 0,
		px: 0,
		py: 0
	};

window.onload = function() {

	canvas = document.getElementById('c');
	ctx    = canvas.getContext('2d');

	canvas.width  = 600;//window.innerWidth;
	canvas.height = 320;//window.innerHeight;

	canvas.onmousedown = function(e) {
		mouse.button = e.which;
		mouse.px = mouse.x;
		mouse.py = mouse.y;
		mouse.x = e.clientX || e.lvyerX;
		mouse.y = e.clientY || e.lvyerY;
		mouse.down = true;
		e.preventDefault();
	};

	canvas.onmouseup = function(e) {
		mouse.down = false;
		e.preventDefault();
	};

	canvas.onmousemove = function(e) {
		mouse.px = mouse.x;
		mouse.py = mouse.y;
		mouse.x = e.clientX || e.lvyerX;
		mouse.y = e.clientY || e.lvyerY;
		e.preventDefault();
	};

	canvas.oncontextmenu = function(e) {
		e.preventDefault(); 
	};

	boundsx = canvas.width - 1;
	boundsy = canvas.height - 1;

	ctx.strokeStyle = 'rgba(222,222,222,0.6)';
	cloth = new Cloth();
	update();
};

var Point = function(x, y) {

	this.x = x;
	this.y = y;
	this.px = x;
	this.py = y;
	this.vx = 0;
	this.vy = 0;
	this.pin_x = null;
	this.pin_y = null;
	this.constraints = [];
};

Point.prototype.update = function(delta) {

	if (mouse.down) {

		var diff_x = this.x - mouse.x,
			diff_y = this.y - mouse.y,
			dist   = Math.sqrt(diff_x * diff_x + diff_y * diff_y);

		if (mouse.button == 1) {

			if(dist < mouse_influence) {
				this.px = this.x - (mouse.x - mouse.px) * 1.8;
				this.py = this.y - (mouse.y - mouse.py) * 1.8;
			}

		} else if (dist < mouse_cut) this.constraints = [];
	}

	this.add_force(0, gravity);

	delta *= delta;
	nx = this.x + ((this.x - this.px) * .99) + ((this.vx / 2) * delta);
	ny = this.y + ((this.y - this.py) * .99) + ((this.vy / 2) * delta);

	this.px = this.x;
	this.py = this.y;

	this.x = nx;
	this.y = ny;

	this.vy = this.vx = 0
};

Point.prototype.draw = function() {

	if (this.constraints.length <= 0) return;
	
	var i = this.constraints.length;
	while(i--) this.constraints[i].draw();
};

Point.prototype.resolve_constraints = function() {

	if (this.pin_x != null && this.pin_y != null) {
	
		this.x = this.pin_x;
		this.y = this.pin_y;
		return;
	}

	var i = this.constraints.length;
	while(i--) this.constraints[i].resolve();

	this.x > boundsx ? this.x = 2 * boundsx - this.x : 1 > this.x && (this.x = 2 - this.x);
	this.y < 1 ? this.y = 2 - this.y : this.y > boundsy && (this.y = 2 * boundsy - this.y);
};

Point.prototype.attach = function(point) {

	this.constraints.push(
		new Constraint(this, point)
	);
};

Point.prototype.remove_constraint = function(lnk) {

	var i = this.constraints.length;
	while(i--) if(this.constraints[i] == lnk) this.constraints.splice(i, 1);
};

Point.prototype.add_force = function(x, y )  {

	this.vx += x;
	this.vy += y;
};

Point.prototype.pin = function(pinx, piny) {
	this.pin_x = pinx;
	this.pin_y = piny;
};

var Constraint = function(p1, p2) {

	this.p1 = p1;
	this.p2 = p2;
	this.length = spacing;
};

Constraint.prototype.resolve = function() {

	var diff_x = this.p1.x - this.p2.x,
		diff_y = this.p1.y - this.p2.y,
		dist = Math.sqrt(diff_x * diff_x + diff_y * diff_y),
		diff = (this.length - dist) / dist;

	if (dist > tear_distance) this.p1.remove_constraint(this);

	var px = diff_x * diff * 0.5;
	var py = diff_y * diff * 0.5;

	this.p1.x += px;
	this.p1.y += py;
	this.p2.x -= px;
	this.p2.y -= py;
};

Constraint.prototype.draw = function() {

	ctx.moveTo(this.p1.x, this.p1.y);
	ctx.lineTo(this.p2.x, this.p2.y);
};

var Cloth = function() {

	this.points = [];

	var start_x = canvas.width / 2 - cloth_width * spacing / 2;

	for(var y = 0; y <= cloth_height; y++) {

		for(var x = 0; x <= cloth_width; x++) {

			var p = new Point(start_x + x * spacing, start_y + y * spacing);

			y == 0 && p.pin(p.x, p.y);
			y != 0 && p.attach(this.points[x + (y - 1) * (cloth_width + 1)]);
			x != 0 && p.attach(this.points[this.points.length - 1]);

			this.points.push(p);
		}
	}
};

Cloth.prototype.update = function() {

	var i = physics_accuracy;

	while(i--) {
		var p = this.points.length;
		while(p--) this.points[p].resolve_constraints();
	}

	i = this.points.length;
	while(i--) this.points[i].update(.016);
};

Cloth.prototype.draw = function() {

	ctx.beginPath();

	var i = cloth.points.length;
	while(i--) cloth.points[i].draw();

	ctx.stroke();
};

function update() {

	ctx.clearRect(0, 0, canvas.width, canvas.height);

	cloth.update();
	cloth.draw();

	requestAnimFrame(update);
}

style.css

* {
	margin: 0;
  background:#333;
}

p {
  position:absolute;
  top:16px;
  width:auto;
  left:620px;
  color: #777;
  font-family:sans-serif;
  font-size:20px;
}

a {
  position:absolute;
  left:620px;
  top:160px;
  color:#fff;
  font-family:sans-serif;
  font-size:20px;
  
}