it looks like i have done similar work but to create knowledge on my own is really difficult, and i can't seem to find the parts where you explained how to get velocity from first principles. so i am making up the knowledge (codes) from my thoughts and experience.

I still have a lot to learn :)

i just saw the xml. thanks! my current xml also can drag.

i just found a applet that is similar to what i was thinking of.

i see Lisa Denise Murphy  at the University of Illinois has one made in 1997

let me study it