Please don’t necropost (5 years ago with custom gravity simulation). Also, please use code-tags when posting code; the above is barely readable.
Note that your post isn’t really to do with the OP subject apart from having the word “gravity” in it.
Please create your own thread describing your problem; in this case what you mean by “air resistance”. The above is the most basic part of PhysX. Gravity is added as an impulse and linear drag tries to reduce gravity; all of which is done using numerical integration using the fixed time-step.