The folks at Newton's Apple wrote the following!
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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?
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.
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
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