particle.js

var particle = {
	position: null,
	velocity: null,
	mass: 1,
	radius: 0,
	bounce: -1,
	friction: 1,

	create: function(x, y, speed, direction, grav) {
		var obj = Object.create(this);
		obj.position = vector.create(x, y);
		obj.velocity = vector.create(0, 0);
		obj.velocity.setLength(speed);
		obj.velocity.setAngle(direction);
		obj.gravity = vector.create(0, grav || 0);
		return obj;
	},

	accelerate: function(accel) {
		this.velocity.addTo(accel);
	},

	update: function() {
		this.velocity.multiplyBy(this.friction);
		this.velocity.addTo(this.gravity);
		this.position.addTo(this.velocity);
	},

	angleTo: function(p2) {
		return Math.atan2(p2.position.getY() - this.position.getY(), p2.position.getX() - this.position.getX());
	},

	distanceTo: function(p2) {
		var dx = p2.position.getX() - this.position.getX(),
			dy = p2.position.getY() - this.position.getY();

		return Math.sqrt(dx * dx + dy * dy);
	},

	gravitateTo: function(p2) {
		var grav = vector.create(0, 0),
			dist = this.distanceTo(p2);

		grav.setLength(p2.mass / (dist * dist));
		grav.setAngle(this.angleTo(p2));
		this.velocity.addTo(grav);
	}
};