drupal_add_js weighting options structure
In order to set the weight of a js file relative to one of the other using drupal_add_js:
<?php
$jsEasing = $libPath.'jquery/jquery.easing.1.3.min.js';
drupal_add_js($jsEasing, array('type'=>'file', scope=>'header', 'group' =>JS_DEFAULT, 'weight' => 501));
?>