How to block movement of tr in jquery sortable ?

How to block movement of tr in jquery sortable ?

Hello

I am using jQuery sortable to sort my table. see my structure

  1.     <table id="sortable">
  2.     <tr class="not"><td>Elements</td</tr>
  3.     <tr><td>I am first</td</tr>
  4.     <tr><td>I am second</td</tr>
  5.     <tr><td>I am third</td</tr>
  6.     </table>

my jquery is

  1.     <script>
  2.       jQuery('#sortable tbody').sortable({
  3.      update: function(event, ui) {
  4.     
  5.     
  6.      },
  7.      cancel: '.not'
  8.            
  9.         });
  10.     </script>

Here I can move the

  1.     <tr><td>I am first</td</tr>
  2.     <tr><td>I am second</td</tr>
  3.     <tr><td>I am third</td</tr>

rows to before 

  1.     <tr class="not"><td>Elements</td</tr>

How can I block that movement ?