Physics for Dummies: At the Penalty of Sounding Real Stupid
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To keep it simple psi is a unit of pressure, and lbs is a unit of mass. 1 psi for instance simply means 1 lbs of force per square inch.
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
As Chris said, "pressure" is the key word here.
You inflate your car's tires to 44 pounds of air PRESSURE, not weight.
You inflate your car's tires to 44 pounds of air PRESSURE, not weight.
wuzak
Captain
It also depends on the size of the container.
As the size goes up the container will need a thicker wall and/or stronger material if the pressure remains the same.
btw, what do you call "absurdly high PSI"?
As the size goes up the container will need a thicker wall and/or stronger material if the pressure remains the same.
btw, what do you call "absurdly high PSI"?
- Thread starter
- #5
But I remember being told that based on the surface area of our bodies, we'd have a ton of pressure acting on us all the time. Our bodies are designed to withstand this pressure so we don't get squished into gak.
I found a cannister that looked close enough to the one I saw. It was around 48 cubic inches.
If I recall it was around 5000 PSI, but I found a figure of 4500 in a 48 cubic inch container for paintballing: Using a calculator to convert that to square inches, I got 13.2077.
You'd think that would be around 59434.65 pounds of compressed gas: It was nowhere near that heavy. It was in a class on the fundamentals of speech, but I did see the guy move the tank easily, and I can assure you it was nowhere near 59000 pounds.
fubar57
General
mikewint
Captain
Chris LBS or Pounds is a unit of FORCE. There is an English unit of mass called a SLUG though it is seldom used today.
At or near the Earth's surface gravity exerts a FORCE on all massed objects which accelerate the object towards the Earths center of mass. That FORCE varies inversely with the square of the distance between the two centers of mass. Commonly we call that FORCE the object weight which thus varies with distance from the Earth's center of mass. An objects MASS is constant anywhere in the universe. Kilogram is the SI unit of MASS while the NEWTON is the SI unit of FORCE. So a KILOGRAM of MASS weighs 9.8 NEWTONS at or near the Earth's surface or in the English system 2.2 POUNDS.
Well sort of. We are basically bags of water and water is non-compressible. BUT the air spaces in our bodies are highly compressible. So the only way that we can withstand the weight of the Earth's atmosphere is if the pressure exerted upon it (14.7 psi) is if that pressure is EQUAL in all directions. So while 14.7 psi is pushing on your front 14.7 is pushing on your back, top, and sides AND outward within all internal air spaces. So ALL objects have that same pressure/weight of air pushing on them equally in all directions or over an English ton 2117 pounds on every square foot of surface.
We often experience UNBALANCED air pressure, i.e., when the air molecules strike mostly from one direction. It is what we commonly call WIND and that unbalance in forces explains the destructive power of storm winds. Think of a ton of force pushing on EVERY square foot of ONE wall of your house or roof.
When you're in bed tuck the covers around you and suddenly try to raise your arms. or sit up. Raising the covers means pushing the air out of the way first.
Suck on a straw. We commonly say that we a sucking the liquid into our mouths while in actual fact the vacuum you create inside the straw is nothingness and nothing cannot provide a force to lift a liquid. Reducing the pressure in the straw allows the exterior atmospheric pressure to push the liquid up the straw. That also means that since the atmosphere only has 14.7 pounds of force you can only raise 14.7 pounds of liquid up a pipe using suction. That occurs at 33 feet. A 100 foot deep well has to have the pump at the bottom pushing the water up the pipe. In the same manner tall buildings have pumping stations every so many floors to push the water to the top step by step.
fubar57
General
LOL. Cite your copy/paste link Mike. Someone might think this is you
mikewint
Captain
I'll give you another demo: Take an empty aluminium soda/beer can and put a few tablespoons of water inside. Put on a hot plate/stove and bring the water to a boil. VERY quickly put the can with boiling water into a pan of cold water and watch what happens. Unbalanced air pressure is a very powerful force
michael rauls
Tech Sergeant
- 1,679
- Jul 15, 2016
Let me take a wild swing at this......the Lbs refers to the amount of pressure exerted on a given 2 dimensional area, usually a square inch, not literal wieght.
So 100 Lbs psi would expert the same amount of force as 100 Lbs of wieght resting on one square inch but the air itself, of course, wieghts almost nothing.
So 100 Lbs psi would expert the same amount of force as 100 Lbs of wieght resting on one square inch but the air itself, of course, wieghts almost nothing.
Our bodies are not designed to withstand the pressure we evolved permanently in that pressure. Moving out of it only slightly can be dangerous, some people can become seriously ill with altitude sickness at 15,000ft. You don't have to go very deep in water before the pressure has an effect. It holds us together as much more than it crushes us.
- Thread starter
- #12
M
michael rauls
That would require the pressure to be exerted in all directions (including up) evenly. That said, how come to put a given PSI of water inside a vessel, it weighs more? It seems there's a conceptual error here...
P pbehn The term is figurative, I know it's evolved.
P pbehn The term is figurative, I know it's evolved.
michael rauls
Tech Sergeant
- 1,679
- Jul 15, 2016
Very true. I spend alot of time in the high Sierras and I've seen some people get pretty sick from altitude sickness.
Fortunately niether me or my wife suffer from it.
AMCKen
Senior Airman
The main reason for 'altitude sickness' is the lower pressure of air giving less oxygen per cubic inch of inhaled air. The lower pressure itself, is not the problem.
Just an illustration of how little change is needed before some feel the effects of it.
That's hypoxia - Causes for Altitude sickness, particularly high altitude oedemas aren't well known. That's why the treatment for altitude sickness isn't simply to go on oxygen llike it is with hypoxia. https://www.altitude.org/altitude-sickness
Pressure is in all directions, otherwise a submarine wouldn't need a bottom, as a ship doesn't need a top.
Builder 2010
Staff Sergeant
Air doesn't really weigh very much. At sea level atmospheric pressure is 14.7 psi. That means a column of air, 1 sq inch in area and extending up the entire column of our atmosphere weighs 14.7 pounds. That's a column that's about 20 miles high. It's mostly nitrogen we're measuring. If the column was all CO2 it would weigh a bit more since it's heavier than air. In order to generate more pressure you have to add energy to the system.
Somebody please reel Mike in, and actually explain to him what he copy and pasted. I am on vacation and don't have time to bother with giving him a physics lesson.
mikewint
Captain
Again the bag of water that comprises us is, like all liquids uncompressible so water pressure only affects the air spaces within the body. The biggest are of course the lungs. The ribcage is a remarkable structure able to withstand considerable force. Herbert Nitsch made a free-dive to 214 meters (702 feet) subjecting himself to 22 atmospheres of pressure or 323 pounds of pressure per square inch with no ill effects.
Using a SCUBA gear and a pressurized gas mixture the human depth record is 332 meters or 1090 feet or 33.7 atmospheres which is 496 pounds per square inch of pressure.
Theoretically humans breathing an oxygen rich liquid would have the same depth/pressure tolerance as any fish. Mice breathing an oxygenated saline solution were subject to 160 atmospheres of pressure or one mile below sea level. They survived for several minutes but died from acidosis. The mamalian respiratory tract was not evolved for moving liquids and thus CO2 blood levels rose killing the rodents. 2352 pounds per square inch pressure caused them not a single problem.
Depth induced problems are problems caused by the pressure affecting the gasses in the lungs and the uptake of those gasses into the blood stream. As the gas pressures within the lungs increases so does the concentration of those gasses in the blood stream. At about 130 feet dissolved nitrogen levels in the blood rise to a point where the dissolved nitrogen begins to have a narcotic-like effect. Divers loose all perspective and judgement and are seriously inebriated. To prevent this nitrogen narcosis the nitrogen is replaced by an inert gas usually helium.
Secondly, while oxygen is required for life it is also toxic at high pressure levels above 1.6 ATA. So deep dives require very careful monitoring to oxygen concentration in the mixture to prevent this toxicity. Eventually the line between too little and too much becomes to fine to control.
You'd best stop and consider that "PER SQUARE INCH" line. I have zero problems with the rest of your post but consider that a square foot is 144 square inches. 14.7 pounds X 144 = 2117 pounds of air on EACH AND EVERY SQUARE FOOT OF EARTH"S SURFACE. Well over 5.5 QUADRILLION tons of air.
Chris please feel free to do so at any time. I am always willing to learn. What exactly are my errors?
