ELIF: if air does have weight, why doesn’t weighting a thing under the roof shows lower number than doing it outside?

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ELIF: if air does have weight, why doesn’t weighting a thing under the roof shows lower number than doing it outside?

In: Physics
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Air’s “weight” on another object is communicated by air pressure it exerts on it. Generally speaking, the air pressure inside a building will be nearly identical to the air pressure outside of one, because they’re not airtight. The air in a container (in this case a building) will attempt to reach equilibrium with the environment surrounding it. Because the pressure is the same, they it will press down on an object being weighed the same.

Like u/MorganLaBigGae pointed out, the difference is not between inside and outside (unless we’re talking about a vacuum) but between roof and ground level.
The difference is there, it is just too small for your bathroom scale to measure.

More precise instruments called barometer will show different air weight (air pressure) at different altitudes. This fact is even used to determine altitude in airplanes!

First, being in a building won’t really change air pressure. The Earth’s surface is flooded with air in the same way the sea floor is flooded with water. Unless you build something so airtight it becomes a vacuum, the gaps in the structure will equilibriate the pressure inside and out. But let’s imagine you’re weighing something at sea level vs weighing something at the top of Mt. Everest (1/3 as much air pressure). It still wouldn’t matter.

Most physical scales work by balancing an object of unknown weight against an object of known weight. Air pressure will put the same amount of weight the same on both sides of the scale, so whatever amount balances at sea level will balance on Everest.

Then digital scales typically don’t tell you the total weight of everything that’s on top of them. When you turn them on (or press the tare button), they display 0. They might be measuring pressure on the scale, but they set that amount as a baseline and only measure any additional pressure put on the scale after that point. If you put a bowl on a scale, tare it, then put 10 grams of water in the bowl, the scale will only read 10 grams. Thus a digital scale would tare out any initial differences in air pressure.