The value of the property '$' is null or undefined, not a Function object

The value of the property '$' is null or undefined, not a Function object

Hi all I am pretty new to using J query. I keep getting this error in IE but my code works just fine in code. How do i remedy it?

<head>
    <!-- Remember to include jQuery :) -->

    <!-- jQuery Modal -->

    <script type='text/javascript' >
        $(document).ready(function () {
            $("input:radio").change(function () {
                var selectedText = $("input:radio:checked").attr('id');
                if (selectedText === "button_1" ) {
                    $('#ex1').modal('show');
                }
            });
        });