[quote author=Fu-Kwun Hwang link=topic=13.msg10526#msg10526 date=1405688343]
I have create a simulation according to your requirement above.
I hope it is what you want. Enjoy it!

[/quote]

Dear Dr Hwang,

Thank for taking the time to make the simulation. I would look into the simulation in more detail. When I "play" with the simulation online, it quite enjoy your work. However, there is still something missing.
The missing parts are
- I do not seem have an option to make the particle start with a negative velocity showing that the string of particle can be pulled down.
- I also hope to show where are all the particles when it is a few times steps backward or forward when I pause the simulation.
I would look into the source to see if it can be added more features.
I like your dt2 option - it is very useful.

I would post my questions in after a few days. Once again thanks for the simulation.

Cheers