Please explain pressure/PSI


I saw a video the other day explaining air pressure where a teacher breaks a ruler by hitting it over a table underneath a sheet of newspaper. I get generally how that works. But then she said atmospheric pressure is 14 psi and so the whole sheet has 7000lb of pressure on it. That’s BS right? Like it clearly doesn’t have 7000 lbs pressed against it? Isn’t that the weight of the column of air above it?

In: 0

It’s not bull shit. It’s the weight of the fluid that is the atmosphere pressing down evenly across a surface.

With the example of breaking the ruler, *additional* pressure is being added on top of the atmospheric pressure, and it’d being added unevenly causing structure changes – the ruler breaking

yes, that is the weight of the column of air above it. therefore, the sheet of paper has 7000lb resting on it.
granted i didnt do the math, im just agreeing with the teacher on the math they used since im assuming the teacher did their math correctly.
now you want to know why we are able to move with thousands of pounds of air pressing down on us. we are just like fish in water. we move through the air. our bodies evolved within this vast amount of pressure and is able to withstand it pressing against us and down on us.
if you were to swim to the bottom of the ocean, you would be crushed by the pressure because water weighs quite a bit more than air and our bodies are not designed to stand up to that pressure.

PSI is pounds per square inch. It means for every inch of surface area, there are that many pounds of pressure. The 14 PSI of atmosphere is caused by the weight of the air, but if there are 500 square inches of surface area on an object, then 14 pounds /in^2 x 500 in^2 is 7000 pounds distributed across the entirety of the object.

It’s not heavy because it is pushing inward on the object from all directions. Like being at the bottom of a deep swimming pool, the water pressure might be enough to pop your ears going down, but it doesn’t feel heavy being down there. The pressure is pushing inward.

There are 2 forms of PSI rating that people will refer too. The one most people commonly refer to is PSIg. This is the difference between the atmosphere and whatever your containing. There is also PSIa which is the absolute pressure for any system. 0 PSIg = 1PSIa (Atmospheric pressure)

Yes that paper has 7000lbs of air on top of it even though for everyday things, that number is irrelevant because there is no pressure difference between the two. That number only comes into the play when there is a pressure differential generated across the paper sheet for that force to take effect. If you were to create the most perfect vacuum possible and use an unbreakable sheet of paper as the seal for that vacuum, it would experience 7000lbs of force across it. However air is tiny and likes to move so that vacuum is broken almost instantly that moment any counteracting force is applied against the paper. Same concept as water and drag. The faster you move, the less time water has to move out the way, and the more its force begins to apply.

It does have 7000lbs of force against it, because it has 15 pounds per square inch of pressure on it across 467 square inches, so any one square inch has 15 lbs of pressure. That pressure is the weight of the entire column of air all the way up to space. Air may not be heavy, but once you have a lot of it, it gets heavy. Imagine the newspaper is a suction cup, if you try to pick it straight up off the table, it would take 7000lbs of force to separate it from the table unless you introduce air underneath the suction cup. That air would push up with the 7000lbs of force, because that air would be at the same 15 psi, and the forces balance out, and you only have to overcome gravity to lift the suction cup