We must seek the two properties of a gas that are analogous to the tension, and mass density properties of a spring or rope. As discussed earlier, air molecules are in motion but move in a random fashion. They are constantly bumping into one another and into the walls of any container filled with air. Some fraction of the molecules are always rebounding from the walls and this creates a force on the wall.
Pressure is defined as force per unit area .
Pressure is often measured in pounds per square inch . Let's consider an example to get an intuitive feel for the physical property we call pressure. Consider a 1 pound book placed on top of your hand. You feel a pressure that is equal to the weight of the book divided by the surface area of your hand. As it turns out, it is the pressure of a gas that is the physical quantity analogous to tension in strings.