About kinetic theory of gases?

Question

Does it say "the gravity has no effect on molecules of a gas" or "it has no effect on movement of gas molecules"? If 1st one is right, how can earth have atmosphere around it when its gravity has no effect on air?also, in the derivation of equation for kinetic theory of gases,we use a cube as a container,then how is that equation applicable for other containers too?

Answer 1

It's not that gravity has no effect on the gases but that the effect is negligible on each individual particle due to their extremely low mass.

When working out things in physics we have to make some assumptions otherwise calculations would become extremely complicated, these assumptions usually filter out the things that have an extremely small  effect on the result, but allow us a close approximation to the answer.