Javascript/Jquery- "$ is not defined- $function()" Error
<p>I am trying to run a javascript/jquery function that is triggered by clicking an option in the select box. While debugging for some reason firebug finds the error: "$ is not defined $(function()". The javascript is placed inside a file called core.js and referenced on index.php. Why is the script is not running at all? </p>

<p><strong>Javascript</strong></p>

<pre><code>&lt;script type="text/javascript"&gt;
    var formObject = {
        run : function(obj) {
            if (obj.val() === '') {
                obj.nextAll('.update').html('&lt;option value=""&gt;----&lt;/option&gt;').attr('disabled', true);
            } else {
                var id = obj.attr('id');
                var v = obj.val();
                jQuery.getJSON('/mod/update.php', { id : id, value : v }, function(data) {
                    if (!data.error) {
                        obj.next('.update').html(data.list).removeAttr('disabled');
                    } else {
                        obj.nextAll('.update').html('&lt;option value=""&gt;----&lt;/option&gt;').attr('disabled', true);
                    }
                });
            }
        }
    };
    $(function() {

        $('.update').live('change', function() {
            formObject.run($(this));
        });

    });
&lt;/script&gt;
</code></pre>

<p><strong>PHP/html</strong></p>

<pre><code>&lt;html&gt;
    &lt;select name="main" id="category" class="update"&gt;
    &lt;option value=""&gt;Select one&lt;/option&gt;
                &lt;? if (!empty($list)) { ?&gt;
                &lt;? foreach($list as $row) { ?&gt;
                    &lt;option value="&lt;?php echo $row['id']; ?&gt;"&gt;
                        &lt;? echo $row['name']; ?&gt;
                    &lt;/option&gt;

                    &lt;? } ?&gt;
                    &lt;? } ?&gt;

    &lt;/select&gt;
&lt;/html&gt;
</code></pre>
