How to call Javascript inside sidebar?

Hi,

I want to have apop-up message when users click on a buton in sidebar. So I create a javascript file named prob.js:

// JavaScript Document
function disp_alert()
{
alert("Cool");
};

then I add this code to header.php:

<script src="/javascript/prob.js" type="text/javascript"></script>

after that, i add to the top of the sidebar.php file:
<?php get_header(); ?>
But nothing happens! I wonder how can I call it then!!

Thanks in advance!