Pressure

The Pressure is a measure of the force applied per unit area on the boundaries of a substance or system. It is caused by the collision of molecules of the substance with the borders of the system. Since molecules strikes the walls, they apply forces which try to push the walls outward. The forces resultant from all of such collisions cause the pressure applied by a system on its surroundings. The pressure is often measured in units of lbf/in² (psi).