rx.concat_friction.js

    function initConcatFriction(Rx) {
        Rx.Observable.prototype.concatFriction = function (u, xS, yS, vS, aS) {
            
            if (typeof xS !== 'function')
                xS = function (m) { return m.x; };
            if (typeof yS !== 'function')
                yS = function (m) { return m.y; };
            if (typeof vS !== 'function')
                vS = function (m) { return m.velocity; };
            if (typeof aS !== 'function')
                aS = function (m) { return m.angle; };
            
            var latest = null;
            
            return this
                .doAction(saveLatest)
                .concat(Rx.Observable.defer(fakeDrags));
            
            function saveLatest(val) { latest = val; };
            function fakeDrags() {
                
                if (latest == null) return Rx.Observable.empty();
                
                var deceleration = Math.abs(u * 9.8);
                
                return Rx.Observable
                    .timer(0, 16)
                    .scan(latest, decelerate)
                    .takeWhile(hasVelocity);
                
                function decelerate(memo) {
                    var v = vS(memo),
                        a = aS(memo),
                        vx = v * Math.cos(a),
                        vy = v * Math.sin(a),
                        x = xS(memo) + vx,
                        y = yS(memo) + vy;
                    v -= deceleration;
                    return { x: x, y: y, angle: a, velocity: v };
                }
                
                function hasVelocity(drag) {
                    return drag.velocity > 0;
                }
            }
        }
    }

rx.scan_velocity.js

    
    function initScanVelocity(Rx) {
        Rx.Observable.prototype.scanVelocity = function(xS, yS) {
            if (typeof xS !== 'function')
                xS = function(m) { return m.x; };
            if (typeof yS !== 'function')
                yS = function(m) { return m.y; };
            return this
                .timeInterval()
                .scan(function(memo, latest) {
                    var event = latest.value,
                        interval = latest.interval,
                        mx = xS(memo.value),
                        my = yS(memo.value),
                        ex = xS(event),
                        ey = yS(event),
                        vx = (ex - mx) / interval,
                        vy = (ey - my) / interval;
                    event.velocity = Math.abs(
                        Math.sqrt((ex * ex) + (ey * ey)) -
                        Math.sqrt((mx * mx) + (my * my))
                    );
                    event.angle = Math.atan2(vy, vx);
                    return latest;
                })
                .skip(1)
                .select(function(wrapper) {
                    return wrapper.value;
                });
        }
    }

usage.js

$('.dragMe')
  .dragAsObservable()
  .scanVelocity()
  .concatFriction(0.45)
  .repeat()
  .subscribe(function(drag) {
    $('.dragMe').css({
      top: drag.global.x,
      left: drag.global.y
    });
  });