My book (Halliday, 6th ed: Section 20-4), uses the momentum of the individual molecules in a gas to derive the pressure of the gas. They imagine the molecules hitting a wall. I'm a little rusty on my memory of conservation of momentum, so this equation is confusing me a bit: (delta)px = (-mvx) - (mvx) = -2mvx They then say that the molecule delivers +2mvx of momentum to the wall. Are they approximating the wall as much more massive than the molecule, so the molecule's speed is unchanged (velocity opposite)? So to get from +mvx to -mvx you would take mvx -mvx -mvx = -mvx to get the change in momentum? I'm thinking I'm probably right, but I just feel a little queezy about it. Like there is something I'm not getting. I guess what I don't understand is the sentence: "the momentum (delta)px delivered to the wall by the molecule during the collision is +2mvx." Could someone possibly start from scratch and show how all this comes from the conservation of momentum? I hope I made sense. Thanks a lot.