Make a nav bar stick

$(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');
    }
  });
});

html, body {
	height: 4000px;
}

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

#body_div {
	top: 0;
	position: relative;
    height: 200px;
    background-color: green;
}

#banner {
	width: 100%;
	height: 273px;
    background-color: gray;
	overflow: hidden;
}

#nav_bar {
	border: 0;
	background-color: #202020;
	border-radius: 0px;
	margin-bottom: 0;
    height: 30px;
}

.nav_links {
    margin: 0;
}

.nav_links li {
	display: inline-block;
    margin-top: 4px;
}
.nav_links li a {
	padding: 0 15.5px;
	color: #3498db;
	text-decoration: none;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="banner">
     <h2>put what you want here</h2>
     <p>just adjust javascript size to match this window</p>
  </div>

  <nav id='nav_bar'>
    <ul class="nav_links">
      <li><a href="url">Nav Bar</a></li>
      <li><a href="url">Sign In</a></li>
      <li><a href="url">Blog</a></li>
      <li><a href="url">About</a></li>
    </ul>
  </nav>
<div id='body_div'>
    <p style="margin: 0; padding-top: 50px;">and more stuff to continue scrolling here</p>
</div>