$(document).ready(function(){

    var $sidebar   = $("#nav"),
        $window    = $(window),
        offset     = $sidebar.offset();

    $window.scroll(function() {
        if ($window.scrollTop() > offset.top) {
            $sidebar.stop().animate({marginTop: $window.scrollTop()}, {easing: 'easeOutCirc'});
        } else {
            $sidebar.stop().animate({marginTop: 0}, {easing: 'easeOutCirc'});
        }
    });
    
});