I've read in quite a few textbooks, chemistry and physics, that we can approximate a gas as many molecules which are floating in space and bouncing off each other. I want to know on what basis we make this approximation. What forces do we ignore and why? Here are some of the questions I asked my prof: Somehow, you approximate these molecules to be beach balls simply floating around and bouncing, or at least, this was the picture I got. How and why is this justified? When these things are "bouncing off", what forces of interaction come into play? The teacher replied that Coulomb(static force law) Force is the only force we consider. How can we do this when the charges are in motion. I know the obvious answer is "the other part that comes out of the motion of the charges is negligible". Can someone justify? Apart from Coulomb attraction, he also said that we don't consider any other forces. Why not? As far as I understand, we cannot be sitting and calculating 1023 force equations which give you the motion of these particles. This is why I have to study this subject of statistical mechanics, wherein we use probability to describe our system. But how is this beach balls picture justified?