When gas molecules hit the walls of the container and bounce back, a change in momentum occurs in a split second. This is obviously a very very fast action.
The end result of the above momentum is that the walls of the container experience a force.
Pressure is defined as the force that acts on a unit surface area. Therefore, all surfaces that are knocked by air will experience a pressure. In order for this to take effect all of the gases molecules in the container or free surface must be moving swiftly in a very short time and hit the surface repeatedly.
This pressure is called gas pressure.
Kinetic Theory of Gases
The basic assumption for the kinetic theory of gas is as follows:
Gas is composed of molecules.
Gas is composed of molecules.
Gas molecules are continually in random and independent motion in all directions at high and different speed.
The motion of gas molecules follows all of the Newton Laws of Motion.
All collisions between the gas molecules (i.e. one with another) and the walls of the container are assumed to be perfectly elastic. Therefore, momentum and kinetic energy are conserved during collision.
The volume of the molecules can be conserved compared to the volume occupied by the gas.
The force among the gas molecules can be neglected except during collision.
The time period of a collision can be neglected when compared with the time interval between two collisions.
