the tip worked very well!
thank you very much again for helping teachers all over the world in customizing computer models to suit our learning and teaching needs. :)
attached is the corrected version thanks to engrg1's (we are colleagues in the same workplace)
help after discussion.
will post the refined simulation up ASAP. at http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1924.0
the people who use Easy Java simulation are awesome!