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$(document).ready(function() {
  
  $(window).scroll(function () {
      //if you hard code, then use console
      //.log to determine when you want the 
      //nav bar to stick.  
      console.log($(window).scrollTop())
    if ($(window).scrollTop() > 280) {
      $('#nav_bar').addClass('navbar-fixed');
    }
    if ($(window).scrollTop() < 281) {
      $('#nav_bar').removeClass('navbar-fixed');
    }
  });
});


.navbar-fixed {
    top: 0;
    z-index: 100;
  position: fixed;
    width: 100%;
}