$ is not defined error

$ is not defined error

hi guys i m new 2 jquery and i am using asp.net C# . i used accordian theme to display the main content of my page. so for that i included the necessary jquery files & css files in the head of my master file ..nd evn called the accordian function frm there. Jquery used to work perfectly fine for the previous pages creaded nd still works perfectly there .. but nw whn i m adding new pages ..firebug is throwing this error.
my head in master page looks like:

<head runat="server">
    <title>Performance Appraisal System</title>
    <meta name="keywords" content="" />
    <meta name="description" content="" />
    <meta http-equiv="content-type" content="text/html; charset=utf-8" />
    <link href="../Styles/JqueryUiCss/jquery-ui-1.8rc3.custom.css" rel="stylesheet" type="text/css" />
    <link href="../Styles/default.css" rel="stylesheet" type="text/css" />
    <script src="../Scripts/jquery-1.4.2.min.js" type="text/javascript"></script>
    <script src="../Scripts/jquery-ui-1.8rc3.custom.min.js" type="text/javascript"></script>

    <script type="text/javascript">
        $(function() {
            $("#posts").accordion({ header: "h2.head" });
            $("#posts1").accordion({ header: "h2.head" });
        });
    </script>
    <asp:ContentPlaceHolder id="head" runat="server">
    </asp:ContentPlaceHolder>
</head>