This should fix it:
jQuery(document).ready(function($){
//you can now use $ as your jQuery object.
var body = $( 'body' );
});
Put simply, WordPress runs their own scripting before you can and they release the $
var so it won't collide with other libraries. This makes total sense, as WordPress is used for all kinds of web sites, apps, and of course, blogs.
From their documentation:
The jQuery library included with WordPress is set to the noConflict() mode (see wp-includes/js/jquery/jquery.js). This is to prevent compatibility problems with other JavaScript libraries that WordPress can link.
In the noConflict() mode, the global $ shortcut for jQuery is not available...