It is a high quality applet. I really like it. I believe you can create more and more good quality applets for physics teaching and learning. Please check out the examples in the EJS distribution. You will learn more technique to do what you want.
You always can post questions related to EJS, and I am really happy to be able to help someone like you.
The most enjoyable thins for teachers is: s/he find her/himself really can help students learn what s/he would like them to learn.  And student  later can learn it by her/himself.  :D