I'm an undergraduate taking a physical chemistry course, and I got to a part in my reading about the derivation of the ideal gas law. The passage is linked below. http://chem.libretexts.org/Textbook...f_Gases/16.01:_Gases_Behave_Ideally_if_Dilute My question has to do with the time interval selected. My understanding of calculating the force of a collision makes me think that the time interval delta t should be the time the particle is in contact with the wall, however they use it as the time it takes the particle to come back and collide with the wall again. Why do they use this? Is it some way to average out the force over time?