The molecules of a gas collide not only with each other but also with the walls of the container. In Chapter 14, where pressure was introduced, we suggested that the pressure in a gas is due to these collisions with the walls. The force due to one such collision may be unmeasurably tiny, but the steady rain of a vast number of molecules striking a wall each second exerts a measurable macroscopic force. The gas pressure is the force per unit area resulting from these molecular collisions.
Our task in this section is to calculate the pressure by doing the appropriate averaging over molecular motions and collisions. This task can be divided into three main pieces:
Calculate the momentum change of a single molecule during a collision.
Find the force due to all collisions.
Introduce an appropriate average speed.