What is the weight of the air in the average room?

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Answered by: Lawrence, An Expert in the Basic Concepts in Physics Category
What is the Weight of the Air in the Average Room?

The question of what is the weight of air in a room invariably makes most people stop and wonder about the person who raised the

question. Most people cannot imagine that air even has weight. You can't see air so how can it have weight? It does, though, because

all matter has mass, which in a gravitational field, gives rise to proportional weight. Although air is a mixture of gases (primarily

nitrogen, oxygen, argon, and a few others), and is therefore not very dense, it does, in fact, have weight. Then why don't you feel it?

You don't feel it because you were born into the normal atmospheric pressure and have lived in it and been surrounded by it all

your life so you are used to it.

How do we measure the weight of air in a room, though? We cannot simply get out a scale and expect it to register the weight of the air

in the room. The problem is that air is equally distributed throughout the room and exerts equal pressure on all surfaces in all directions.

For example, at sea level, every square inch of your skin is under the constant atmospheric pressure of 14.7 pounds. Again, you don't feel it because its always been there.

The way to figure the weight of air in a typical room, maybe 10 feet by 12 feet by 8 feet high, is to find the volume of the room which

is filled with the air and find the weight of each small unit of volume and then apply the definition of density which is equal to mass

divided by volume. Algebraic manipulation of that definition tells us that mass is equal to volume times density.

To find the volume of the room in cubic feet, use the formula for the volume of a rectangular solid and simply multiply the three

dimensions of the room together: 10 ft X 12 ft X 8 ft = 960 cubic feet.

To find the density of air, we use the Internet but we must specify the conditions of that air. Under NPT (normal temperature and pressure), air has a density of 1.204 kilograms per cubic meter or 0.075 pounds per cubic foot. The conditions are important because air changes its density depending on its temperature and pressure. We use NPT to describe conditions at sea level and about 68 degrees Fahrenheit.

Now that we have the density of the air as weight per cubic foot and the volume as the number of cubic feet of space in an average

bedroom, we just multiply the two together: 960 cubic feet times 0.075 pounds per cubic foot = 72 pounds of air in the average bedroom.

To truly put this in perspective, place a cardboard box on a scale and fill it with books until the box and the books register 72 pounds.

Now, if you are feeling fit, carefully lift that box of books from the scale and feel the weight of air in the room.

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