Air Pressure

The folks at Newton's Apple wrote the following!

Back to cool questions

Can we measure the force with which the air is pressing on the earth? What does a drinking straw have to do with air pressure? How does a barometer work? What would happen to us in a vacuum?

Insights
Even though we can't feel it, air is constantly pressing down on us with a tremendous force--14.7 lbs. per square inch (100,000 newtons per square meters), to be exact! This was graphically demonstrated in 1654 when Otto von Gueicke, Burgmeister of the town of Magdeburg, Germany used a vacuum pump to remove almost all of the air from the space between two half-meter diameter hemispheres. The air pressure holding them together was so strong that two teams of horses couldn't pull them apart; when air was let back in, the hemispheres fell apart easily.

Air pressure is created by the weight of the earth's atmosphere. Although we can't see air, the gas molecules still have mass, and gravity acts upon it. The air pressure changes daily due to the heating and cooling of the earth's surface. When air gets warm, it expands, becoming less dense, and therefore pushes with less pressure. We can measure changes in atmospheric pressure by using a barometer. Some barometers use long glass tubes filled with mercury inverted in a dish. Air pressing down on the surface of the dish forces the mercury up the tube. Normal air pressure can support a column of mercury about 760 mm high. When atmospheric pressure drops, the force of the air pushing on the dish isn't as great, so the column of liquid falls and we have a "falling barometer." When the atmospheric pressure increases, the mercury rises, thus a "rising barometer."

We use air pressure all the time when we breathe. When our diaphragm moves down, air is pushed into our lungs from the outside, expanding the volume of the chest cavity. The diaphragm doesn't "pull" air in; it expands the volume of our lungs, and the air pressure fills the volume.

Connections
1. Before viewing the segment, have some of the students present a classroom demonstration using the Try This activity involving the bottle of soda or juice and a straw (see bottom of this page). Why can't you get the liquid out of the bottle using the straw when the top of the bottle is sealed?
2. What would happen if you tried to use a really long straw to pull water from the ground to the top of a five-story building? Would it work?
3. How do changes in atmospheric pressure relate to weather patterns?

Vocabulary
aneroid barometer a device for measuring air pressure which uses an air-tight box instead of a tube of liquid

atmospheric pressure the force per unit area exerted by the atmosphere at any point within the gaseous envelope surrounding the earth

force a push or pull that causes a body to accelerate or change shape

gravity the force that makes objects tend to move toward each other

mass the amount of matter a body or object contains; a measure of the inertia of a body or object

vacuum a space from which all of the air has been removed

Resources

3-2-1 Classroom Contact: Air is matter, air is there. (1991) New York: Children's Television Workshop. Videotape.
Ehrlich, R. (1990) Turning the world inside out. Princeton, NJ: Princeton University Press.
Gardner, M. (1981) Entertaining science experiments with everyday objects. New York: Dover.
Hewitt, P. (1991) Conceptual physics, 2d ed. New York: Addison-Wesley.