liabru-matter-js/examples/gravityPlugin.js

(function() {

    var Body = Matter.Body,
        Common = Matter.Common,
        Vector = Matter.Vector;

    var MatterGravity = {
        name: 'matter-gravity',

        version: '0.1.0',

        for: 'matter-js@^0.10.0',

        uses: [
            'matter-attractors@^0.1.0'
        ],

        install: function(base) {
            base.Body.create = Common.chain(
                Matter.Body.create,
                function() {
                    MatterGravity.Body.init(this);
                }
            );
        },

        Body: {
            init: function(body) {
                if (body.gravity) {
                    body.attractors.push(MatterGravity.Body.applyGravity);
                }
            },

            applyGravity: function(bodyA, bodyB) {
                var bToA = Vector.sub(bodyB.position, bodyA.position),
                    distanceSq = Vector.magnitudeSquared(bToA) || 0.0001,
                    normal = Vector.normalise(bToA),
                    magnitude = -bodyA.gravity * (bodyA.mass * bodyB.mass / distanceSq),
                    force = Vector.mult(normal, magnitude);

                Body.applyForce(bodyA, bodyA.position, Vector.neg(force));
                Body.applyForce(bodyB, bodyB.position, force);
            }
        }
    };

    Matter.Plugin.register(MatterGravity);

    window.MatterGravity = MatterGravity;

})();