1. Please check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=178.0]Browian motion[/url] if you really want to add collision between particles.
[/quote]
Thanks! will explore

The above applet take care of collision between  all particles. However, adding above effect would not make much different result from the simulation. It will need more time to calculate possible collision between any two particles.
A loop with 400*400 checking for collision.
[/quote]
reference: http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1455msg5504;topicseen#msg5504

My friend (LeeTL) in school wanted a simulation to be used for students to play with, and according to the assumption number 2.All collisions [b]between gas molecules[/b] are elastic and all motion is frictionless (no energy is lost in collisions or in motion), I thought it would be good to show it in the simulation, instead of telling the students the assumptions are not modeled in our applet.

You can add [b]Maxwell distribution of speeds[/b] , however, it only make sense if you have enough number of particles.
Otherwise, it would not make the simulation better (it might be even worse).
If you increase number of particles to 4000, the computational time will increase 100 times compared to 400 particles.
[/quote]
I was thinking of using a tactics or trick like a time interval and keep adding, as long as i am able to approximately show molecules has different speeds, it should be fine, the Maxwell distribution does not need to be ideal, just to illustrate molecules at the same temperature, it is likely the molecules has different speed but the v[sub]rms[/sub] helps us to determine the temperature.

We can add more physics conditions into simulation, but does it make the simulation better for teaching purpose.
If you want to simulate physics effect, then it is another story.
[/quote]
Just the 6 assumptions of Kinetic theory of gases http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1455msg5504;topicseen#msg5504 as best as i can represent and model the assumptions.