Is it two questions? or  one question?
[quote]Question:how to make the simulation more like fall and rebounce ?[/quote]
The particle was moving in the horizontal direction. I do not feel any [b]fall[/b] effect?
May be you can draw another particle moving vertically (but you call it x , it will make user confused).
How about draw both vertical and horizontal at the same time.

and
[quote]i have this technical problem,
to make T[4] = 10 and T[6] = 10.
but it will screw up the rest of the calculation for acceleration etc....
any simple way to make the simulation more like a fall and rebounce on a hard surface ?
but i don't want to affect the original codes for the rest of the options.
[/quote]
I do not understand your question? What is T? Why it will affect acceleration.
Could you copy the code and let me know where is the code? (constrains/user defined functions,...)