Air pressure inside restaurants
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@lucas1 said in Air pressure inside restaurants:
@Dragnslcr Look at the vid I posted.
I don't need to look at any faked videos that you post. I'm already well aware of how wrong you are here.
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@Dragnslcr How is my video faked?
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@Dragnslcr That's funny. Happens where I work whenever the front door opens. The door to the back slams shut. I must have faked that, though. You fucking idiot.
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@GodEmperor I am going to do a Yeti video next. Fuck it.
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@lucas1 said in Air pressure inside restaurants:
@GodEmperor I am going to do a Yeti video next. Fuck it.
Why not big-foot? ;)
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@GodEmperor I want to tour Asia before America.
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@lucas1 said in Air pressure inside restaurants:
@GodEmperor I am going to do a Yeti video next. Fuck it.
So you are going to put a Yeti costume on and go "REEEEEEEEEEEEEEEEEEEEEEEE!" ?
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@GodEmperor No Asian Hookers.
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@lucas1 said in Air pressure inside restaurants:
@GodEmperor No Asian Hookers.
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Well ... This was entertaining ... Same time next week?
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@Luhmann Anytime dude.
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Alright, I tried an experiment.
I took a piece of cardboard about 35cm x 39cm and attached a small spring in the middle, then attached a long cord to the spring. Then I tied a weight to the other end of the cord and dropped the weight from the same height so that it'd produce the same exact force in both tests.
I tried it with the sheet of cardboard resting on two surfaces. The first time, I set it on a smooth glass tabletop, so there would not be atmospheric pressure underneath it. The second time I set it on a wire rack so there was equal atmospheric pressure on both the top and bottom.
From that initial setup, I dropped the same candle from the same height to produce the force to lift the cardboard. The atmospheric pressure on top of the cardboard when it was placed on the glass tabletop made it slightly harder to lift off the tabletop. Once I slowed down the video and examined it frame-by-frame, I could observe this by the deformation of the spring. When the cardboard was placed on the wire frame, it was easier to lift, because it had atmospheric pressure on both sides. The spring was deformed less in that video.
So, in both scenarios lifting the sheet of cardboard required overcoming the air resistance, but the difference between the scenarios was that the first one also had to overcome atmospheric pressure holding the cardboard down initially.
And here's the video, in slow mo:
The difference was noticeable, but I don't think it would be enough to make or break the ruler in his experiment (no, I don't have a bunch of rulers that I want to try to break so I had to come up with a different experiment).
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@anotherusername So you created experiment Y to disprove experiment X.
Jokes aside, I wiill look at tomorrow. I hope I am totally wrong now because I might learn something.
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@lucas1 it's basically the same experiment; I just lifted a piece of cardboard instead of a sheet of newspaper (for what he was trying to demonstrate, this shouldn't make much of a difference), and I used a spring to measure the amount of force instead of trying to see if it'd break a ruler.
Here's the mp4 version as an upload, in case you can't get imgur to load. (The gif is >2MB so it won't upload here.)
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@anotherusername :I am too tired and drunk to argue,. I will look tomorrow. You maybe completely right.
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@lucas1 ok.
Like I said, the air pressure did cause a noticeable difference. I still think that the air resistance was the more significant factor, and probably would've broken his ruler either way. I could be wrong about that, but I don't want to break any rulers to try to find out.
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@anotherusername I have a new theory, fuck doors. ;-)
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@lucas1 said in Air pressure inside restaurants:
I noticed that he added a second sheet of paper when he broke the ruler, implying that he probably tore the paper with only one sheet.
Pressure is how much the fluid "presses" or squishes against everything. As such, it is in equilibrium with everything else in the room. The table and the newspaper and the air all exert the same amount of pressure on the ruler, which correspondingly exerts the same pressure back on them (or else they would crush it smaller until the ruler either did exert the same pressure back or until the ruler's structural strength met the force applied by the other pressures).
Resistance is how much the fluid fights against moving. The fluid's viscosity is a measure of the same thing. It might be considered to be primarily "internal friction", though it also affects the friction between a fluid and another object.
In this video (and in @anotherusername's video as well), the effects of both are displayed. The motion of the ruler (spring) against the paper (cardboard) attempts to raise the middle of the paper, but the air's resistance to move causes the edges of the paper to be sucked to the tabletop, creating a minor seal. The increasing force of the ruler to raise the middle of the paper creates a pressure differential, with higher pressure above and lower pressure below the paper. This is now essentially a suction cup of paper holding itself (and that end of the ruler) to the table. It is leaky, but the air's viscosity prevents it from filling the space underneath the paper quickly enough to counter the increasing force applied to the ruler, which means that either the paper will tear or the ruler will snap (or in the case of the spring, the spring will stretch way out).
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As for the restaurant door question from the OP, air conditioning, kitchen fans, and other air movers pull air out of the restaurant and create a slight pressure differential between the indoor and outdoor atmospheric pressures.
BTW, hospital rooms can be purposely set to have higher or lower air pressures than the hallways or outside, in order to prevent airborne infectious agents from moving from a contaminated (or possibly contaminated) area to a "clean" area. Patients with compromised immune systems generally have positive relative air pressure in their rooms, while patients with infectious diseases have negative relative air pressure in order to keep the disease from spreading.
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@djls45 yeah, although the point of mine was to compare the scenario where there's a flat surface underneath and the scenario where there's air underneath side-by-side; then you can actually see how much extra force was required to lift it when it had air pressure holding it down, in comparison to the force required to lift the one that did not. (In both scenarios it had to overcome air resistance. To see how much of an effect that had, you'd need to attach a weight, equal in mass to the cardboard, to the end of the spring, and then see how quickly it'd lift that.)
And I probably should've used a lighter weight to do the lifting, but oh well. It took way longer to set up than you'd probably think (moving a desk around and the standing clothes rack that placed directly above it to act as a pulley for the cord), so it is what it is.
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@djls45 said in Air pressure inside restaurants:
BTW, hospital rooms can be purposely set to have higher or lower air pressures than the hallways or outside, in order to prevent airborne infectious agents from moving from a contaminated (or possibly contaminated) area to a "clean" area. Patients with compromised immune systems generally have positive relative air pressure in their rooms, while patients with infectious diseases have negative relative air pressure in order to keep the disease from spreading.
Huh, TIL. That's pretty neat actually. Smart.
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@anotherusername
I think it was a well-designed demonstration.
There was a delay rather than extra force required to lift the sheet that was on the desk, although more force happened to be exerted as the spring stretched. The delay was because the viscosity of the air slowed the air from flowing between the cardboard and the desk to equalize the pressure, so the pressure of the air above held the cardboard down until it reached close enough to equilibrium.
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@djls45 Right, the air pressure only holds until the suction cup effect is broken; once air flows in underneath it's more or less the same pressure both above and underneath. So initially, it takes extra force to overcome the air pressure; the spring deforming (further than it deformed in the other example) illustrates this extra force.
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@anotherusername said in Air pressure inside restaurants:
So initially, it takes extra force to overcome the air pressure
Not quite. The suction cup effect is already deficient because air easily leaks under the cardboard. The extra force is irrelevant for that purpose.
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@djls45 the extra force was required because the cord was trying to lift it faster than air could leak underneath.
I guess basically it's the same thing. It resisted lifting upward until it had air pressure underneath; before that, air pressure was holding it down. Pulling upward caused air to flow underneath, but it did that slowly enough that you could clearly see that it was also being held down by something; that something was the air pressure above it.
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@anotherusername said in Air pressure inside restaurants:
the extra force
was requiredoccurred/was observable because the cord was trying to lift it faster than air could leak underneath.This is my only quibble with your statements. Other than that, I completely agree with you.
If there had been a strong seal, then it would be required, but there wasn't a strong seal, so it wasn't.
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Here are the two short videos I recorded, if anybody cares to see them... the first one shows the sort of arrangement that I had set up. The only thing different in the second is that I removed the desk and put a big stack of stuff there in its place, so the surface was the same height as the desk was.
They were too large to upload originally, so I downsampled them.
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@anotherusername said in Air pressure inside restaurants:
[Well-reasoned scientific explanation]
I'll give you two guesses as to who downvoted your post.
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@ben_lubar said in Air pressure inside restaurants:
@anotherusername said in Air pressure inside restaurants:
[Well-reasoned scientific explanation]
I'll give you two guesses as to who downvoted your post.
It was... @Jack-Fisher! No wait, @candlejack!
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@masonwheeler said in Air pressure inside restaurants:
It seems like half the time I go out to eat somewhere, it's difficult to open the door when I leave
Because you've just eaten fifteen pounds of moose meat and whale blubber! You're lucky folding the napkin doesn't give you a coronary.
Filed under: Or maybe that's just the restaurant we have in
?
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@lucas1 said in Air pressure inside restaurants:
@tufty Good I like to think that you are in pain. Don't worry I won't get a boner over it.
Gosh, aren't you just the milk of human kindness? Oddly enough, people who have jobs often have other things to do with their evenings and weekends than sit in front of their computer arguing trivia with morons.
You obviously don/t believe me, but hey. This is where we were, although we did it "in reverse", climbing the Combe du Grand Crêt and coming down the Combe de Paccaly, and here's the view of Tardevant and the Pointe Percée from the Trou :
@lucas1 said in Air pressure inside restaurants:
Whatever mate, last time I tried talking to you your "macbook" needed an update.
I think you'll find I said ...
Anyway, I have a reboot to do. Gee, thanks, Apple.
The "macbook" part comes entirely from your server logs. Well done you.
Now, @anotherusername has largely explained your ruler-snapping video, going as far as to actually conduct an experiment.
Your door slamming can also be explained, and probably in terms of airflow rather than resistance or pressure. Open your door, and put something in the frame to stop it closing completely, leaving the door very slightly (around a centimetre or so) open. Note if there is airflow in or out, probably the easiest way to do this is to hold a cigarette close to the crack in the door, but I'm sure you can work out some other way of doing this if you don't smoke. Now open your windows and repeat.
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@lucas1 said in Air pressure inside restaurants:
fuck doors
I doubt the restaurant owner will appreciate
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@Luhmann the waitstaff will be the ones cleaning it up... and they've cleaned up much worse already -- trust me.
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@anotherusername said in Air pressure inside restaurants:
the waitstaff will be the ones cleaning it up... and they've cleaned up much worse already -- trust me.
are you saying they will be ok with @lucas1 humping their door?
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@Luhmann said in Air pressure inside restaurants:
are you saying they will be ok with @lucas1 humping their door?That depends on whether it costs the restaurant its operating license.
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@dkf said in Air pressure inside restaurants:
That depends on whether it costs the restaurant its operating license.
Maybe it would be safer then to use the backdoor
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@Luhmann said in Air pressure inside restaurants:
Maybe it would be safer then to use the backdoor
True, and the restaurant would be able to use the extra ranch dressing too.
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@dkf said in Air pressure inside restaurants:
the extra ranch dressing too.
Isn't that the plongeur's job?
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@Luhmann said in Air pressure inside restaurants:
@anotherusername said in Air pressure inside restaurants:
the waitstaff will be the ones cleaning it up... and they've cleaned up much worse already -- trust me.
are you saying they will be ok with @lucas1 humping their door?And now we know exactly what lucas' job at Tesco as a "doorman" involved.
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@tufty said in Air pressure inside restaurants:
some other way of doing this if you don't smoke
A match, a cigarette lighter, a candle, or any other small, consistent, open flame would work.
The interesting thing is that if there is a small amount of airflow and the door opens away from the source of higher air pressure, it will tend to pull the door shut if it is just barely ajar, due to the Bernoulli effect, but push it further open if it's opened a lot more than that. If the door opens towards the source of higher air pressure, then it will always push it closed.
(Source: observing this phenomenon in several houses that used a swamp cooler for a/c in the summer)
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@anotherusername said in Air pressure inside restaurants:
@lucas1 ok.
Like I said, the air pressure did cause a noticeable difference. I still think that the air resistance was the more significant factor, and probably would've broken his ruler either way. I could be wrong about that, but I don't want to break any rulers to try to find out.
Easy to see if it's pressure or resistance: Do it sideways.
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What the hell? I leave a snarky comment and consider the thread done, only to come back a day later and find multiple people drawing graphs and performing real life experiments to demonstrate a 7th grade physics phenomena.
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@Rhywden said in Air pressure inside restaurants:
@anotherusername said in Air pressure inside restaurants:
@lucas1 ok.
Like I said, the air pressure did cause a noticeable difference. I still think that the air resistance was the more significant factor, and probably would've broken his ruler either way. I could be wrong about that, but I don't want to break any rulers to try to find out.
Easy to see if it's pressure or resistance: Do it sideways.
What the fuck would that prove?
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@CodEmperor said in Air pressure inside restaurants:
What the fuck would that prove?
That you had it sideways
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@tufty It involved me pulling cages off the lorry and putting them in the right place for people that were stacking the shelves.
It was a perfectly fine job while at University.
I also worked in McDonalds "flipping burgers" ... are you going to try to shame me on that?
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@lucas1 said in Air pressure inside restaurants:
I also worked in McDonalds "flipping burgers" ... are you going to try to shame me on that?
Nah, shoulda worked at a smoothie joint, at least there you could pretend it was healthy!
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@CodEmperor said in Air pressure inside restaurants:
@Rhywden said in Air pressure inside restaurants:
@anotherusername said in Air pressure inside restaurants:
@lucas1 ok.
Like I said, the air pressure did cause a noticeable difference. I still think that the air resistance was the more significant factor, and probably would've broken his ruler either way. I could be wrong about that, but I don't want to break any rulers to try to find out.
Easy to see if it's pressure or resistance: Do it sideways.
What the fuck would that prove?
Easy: Atmospheric pressure is the weight of the vertical column of air above the object measuring the pressure. Turn the page sideways and you've effectively nullified atmospheric pressure.
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@Rhywden Nope. Pressure of water at X depths doesn't disappear because you are going side to side.
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@lucas1 said in Air pressure inside restaurants:
@Rhywden Nope. Pressure of water at X depths doesn't disappear because you are going side to side.
Yes. And that's why you turn the whole thing sideways. Thus the forces exerted by atmospheric pressure are now perpendicular to the forces exerted by the ruler on the paper.
And thus don't play a role anymore.
