There is a chapter in Feynman Lectures on Physics called The Random Walk(41-4). I understand everything till the paragraph right after equation 41.18. I have no idea what he is trying to say. There is an equation 41.19, which is diff. eq. for object that is forced and is in a enviroment that causes resistance... that is not eq. of particle in a gas. Than he is saying that average of [itex]\left\langle x v_x \right\rangle[/itex] does not change because it doesn't remember where it was before.. I don't understand that formulation. I am also not sure why [itex]\left\langle x F_x \right\rangle = 0[/itex]. Could someone help me please ? Thank you very much.