How to Reference Form Elements

How to Reference Form Elements

I need to know how to reference a specific form element within a page with multiple forms and common element names, example below.

<form id="Form1">
<input id="UserID">
</form>

<form id="Form2">
<input id="UserID">
</form>

<form id="Form3">
<input id="UserID">
</form>

I thought something like this would work, but I can't get it to work.

$("#Form2.UserID").change();

Any help would be appreciated.