Pressure is the amount of force within a specific area. You measure air, steam, and gas pressure and the fluid pressure in hydraulic systems in pounds per square inch (psi). However, you measure water pressure in pounds per square foot. Youll find more about pressure measurements in chapter 10. To help you better understand pressure, lets look at how pressure affects your ability to walk across snow.

Have you ever tried to walk on freshly fallen snow to have your feet break through the crust when you put your weight on it? If you had worn snowshoes, you could have walked across the snow without sinking; but do you know why? Snowshoes do not reduce your weight, or the amount of force, exerted on the snow; they merely distribute it over a larger area. In doing that, the snowshoes reduce the pressure per square inch of the force you exert.

Lets figure out how that works. If a man weighs 160 pounds, that weight, or force, is more or less evenly distributed by the soles of his shoes. The area of the soles of an average mans shoes is roughly 60 square inches. Each of those square inches has to carry 160 60= 2.6 pounds of that mans weight. Since 2 to 6 pounds per square inch is too much weight for the snow crest to support, his feet break through.

When the man puts on snowshoes, he distributes his weight over an area of about 900 square inches, depending on the size of the snowshoes. The force on each of those square inches is equal to only 160 900 = 0.18 pounds. Therefore, with snowshoes on, he exerts a pressure of 0.18 psi. With this decreased pressure, the snow can easily support him.