What-if 0006: "Glass Half Empty"

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Enygma846
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Re: What-if 0006: Glass Half Empty

Postby Enygma846 » Sat Aug 11, 2012 2:25 am UTC

Fieari wrote:This What-If is unique in that it provides a really cool end-result, while not being catastrophically destructive like the others. As such, I'd love to actually see this in real life, and I'm pondering how it could be done.

So... Construct a glass that is segmented with a thin-but-vacuum-resistant seal between the top and bottom halves. Fill the top half with water, and create a vacuum in the bottom half via a small hole and pump. Then, using a weight or something similar, shatter the seal between the two layers.

Would this work? Would it create the same physics situation as in the What-If?


What about something like this quick-and-dirty brainstorm and quicker-and-dirtier Google SketchUp, http://i45.tinypic.com/35m4jec.png? Call it a Munroe Glass, or somesuch.

_IF_ it would work, it would be reusable as a "physics-is-cool" demo and, minus the flying glass (potentially...), would be fairly safe.

So, running through things... The bottom is a screw-top glass vessel that is possibly custom-made, but could easily be 250mL Mason jars or dollar-store spice bottles. Next up is a mated screw-bottommed and screw-topped metal ring with a vacuum valve to allow a vacuum to be created. Between that and the third piece is a mylar sheet held taut when the third piece is screwed down. The third piece has a screw-bottom and along the side (opposite the valve as an attempt to help it not spin when/if it flies upwards) it has a spring-loaded arm with two bumps, a top one to hold piece four up and a bottom one to hold it down. Piece four is a heavy-ish cylinder that fits into three and is rimmed along the bottom with a blade.

So, screw the valve onto the glass jar, top with mylar and screw on the cup. Vacuum the air out of the jar, and fill the cup with water. Slide the cutter partially in and rest it on top of the prop. Attach a string to the prop and step back as far as desired, then pull. Cutter drops and is trapped as it cuts the mylar making the vacuum suddenly "appear". And then.... ?????

Now take that as it is, it took ALL DAY (well... a coffee break, part of lunch, perhaps a few minutes I should have been working.... you know, certain values of all...) to come up with it and I have no idea if it is feasible. But I'm sure someone here can say.

JetstreamGW
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Re: What-if 0006: Glass Half Empty

Postby JetstreamGW » Sat Aug 11, 2012 7:30 am UTC

bmonk wrote:And while I'm on pet peeves, when hypothetical asteroids strike the earth, why are they always centered on major cities, particularly New York or Los Angeles? Surely most of the earth's surface is water, and another large percentage is sparsely inhabited at best. What if a rock struck 100 miles inland from LA? Or 678 miles off Long Island? Or even in the middle of the Amazon jungle?


http://impact.ese.ic.ac.uk/ImpactEffects/


I like this website for those hypotheticals.

jpers36
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Re: What-if 0006: Glass Half Empty

Postby jpers36 » Sat Aug 11, 2012 4:08 pm UTC

Pfhorrest wrote:
yaliceme wrote:You know, I've always been bothered by the "engineer sees a glass twice as big as it needs to be" variant. What kind of engineer designs something that only accommodates *exactly* its intended load? At best, the glass has a safety factor of 2.

I guess it varies engineer to engineer. Reminds me of a scene in Star Trek: Deep Space Nine where a Federation officer is complaining to a Cardassian on board about there being only one backup for some vital system on the station, and the Cardassian is taken aback at the thought of the Federation overengineering backups for their backups...


Destiny, in the third season. I just watched it last weekend.

brenok
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Re: What-if 0006: Glass Half Empty

Postby brenok » Sat Aug 11, 2012 6:29 pm UTC

Moose Anus wrote:
bmonk wrote:And while I'm on pet peeves, when hypothetical asteroids strike the earth, why are they always centered on major cities, particularly New York or Los Angeles? Surely most of the earth's surface is water, and another large percentage is sparsely inhabited at best. What if a rock struck 100 miles inland from LA? Or 678 miles off Long Island? Or even in the middle of the Amazon jungle?

Buenos Aires is not quite in the Amazon jungle, but it's close enough.

Image

/Wasn't a natural trajectory


If you consider 1.500 miles close enough.

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Re: What-if 0006: Glass Half Empty

Postby wbeaty » Mon Aug 13, 2012 9:05 am UTC

Fieari wrote:So... Construct a glass that is segmented with a thin-but-vacuum-resistant seal between the top and bottom halves. Fill the top half with water, and create a vacuum in the bottom half via a small hole and pump. Then, using a weight or something similar, shatter the seal between the two layers.


Yep, that's how it's done in physics lectures. One version uses a sheet of mylar film cemented across the mouth of a pipe to standoff the ~100lbs. Stab the sheet and the water slams through. There would probably be an initial thin jet as the sheet tore. But that probably wouldn't make much difference, since the glass-breaking impact occurs when the last few remaining vacuum-pockets collapse to zero diameter, and the pressure skyrockets, fracturing the glass bottom.

One question I have: because empirically a glass bottom can be fractured by only a small 5mm vacuum pocket, what happens when the vacuum is 5cm wide instead? Maybe the 20joules, 40KPH "muzzle velocity" of the traveling water is only reduced by 10% in cracking the glass bottom. Narrow water-jets exiting through the cracked glass might be orders of magnitude faster. Might even break the skin of onlookers, injecting them with (dirty?) water.

Hit the top of a water-filled beer bottle with a rubber-coated sledge hammer moving at 100KPH, I think that would get us in the right region (forming a few-cm vacuum pocket before it collapsed.)
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AvatarIII
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Re: What-if 0006: Glass Half Empty

Postby AvatarIII » Mon Aug 13, 2012 9:30 am UTC

Max™ wrote:
AvatarIII wrote:
Max™ wrote:With large enough error bars one could argue the glass is completely full of water or completely empty of water.

Upon reflection one finds that the glass simply is, the quantity of fluid held within the glass does not change this.


you can't just randomly assign error bars,

Sorry, been reading up on climate modeling a lot lately, I've gotten used to randomly assigned error bars.

LOL
gmalivuk wrote:
AvatarIII wrote:you can't just randomly assign error bars
No, but for order of magnitude approximations the glass is full, since 0.5 is about 1.

sure, but ±0.125 of a glass is probably more appropriate, assuming a normal distribution, and using 1 standard deviation as the error.

Thorbard9
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Re: What-if 0006: Glass Half Empty

Postby Thorbard9 » Mon Aug 13, 2012 11:52 am UTC

Enygma846 wrote:
Fieari wrote:This What-If is unique in that it provides a really cool end-result, while not being catastrophically destructive like the others. As such, I'd love to actually see this in real life, and I'm pondering how it could be done.

So... Construct a glass that is segmented with a thin-but-vacuum-resistant seal between the top and bottom halves. Fill the top half with water, and create a vacuum in the bottom half via a small hole and pump. Then, using a weight or something similar, shatter the seal between the two layers.

Would this work? Would it create the same physics situation as in the What-If?


What about something like this quick-and-dirty brainstorm and quicker-and-dirtier Google SketchUp, http://i45.tinypic.com/35m4jec.png? Call it a Munroe Glass, or somesuch.

_IF_ it would work, it would be reusable as a "physics-is-cool" demo and, minus the flying glass (potentially...), would be fairly safe.

So, running through things... The bottom is a screw-top glass vessel that is possibly custom-made, but could easily be 250mL Mason jars or dollar-store spice bottles. Next up is a mated screw-bottommed and screw-topped metal ring with a vacuum valve to allow a vacuum to be created. Between that and the third piece is a mylar sheet held taut when the third piece is screwed down. The third piece has a screw-bottom and along the side (opposite the valve as an attempt to help it not spin when/if it flies upwards) it has a spring-loaded arm with two bumps, a top one to hold piece four up and a bottom one to hold it down. Piece four is a heavy-ish cylinder that fits into three and is rimmed along the bottom with a blade.

So, screw the valve onto the glass jar, top with mylar and screw on the cup. Vacuum the air out of the jar, and fill the cup with water. Slide the cutter partially in and rest it on top of the prop. Attach a string to the prop and step back as far as desired, then pull. Cutter drops and is trapped as it cuts the mylar making the vacuum suddenly "appear". And then.... ?????

Now take that as it is, it took ALL DAY (well... a coffee break, part of lunch, perhaps a few minutes I should have been working.... you know, certain values of all...) to come up with it and I have no idea if it is feasible. But I'm sure someone here can say.


Looks like it should work as designed. Don't know how you'd maintain a vacuum proof seal between the (metal?) vacuum pump connection and the glass jar at the bottom.

Additionally, all that extra material is going to make a huge amount more to go flying up to the ceiling. Maybe enclose the whole apparatus in a long clear tube with a shock absorber at the top to catch it all.

Syagria
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Re: What-if 0006: Glass Half Empty

Postby Syagria » Wed Nov 14, 2012 3:02 am UTC

What would this experiment result in if conducted in metal cups, or maybe even airtight cups (read- Steins.)? the shockwave would obviously not occur in the second case...


A shockwave will always occur; the difference is how the other surrounding materials react to it. Glass, being non-malleable in its solid form will react much differently than "metal" (I'm handwaving the fact that there are a multitude of different elemental metals and their alloys, each of which has their own properties).

You can test this experiment, or rather, a decent approximation of it. Here's how:

Get a can of Coke (or Pepsi, or whatever). Pop it open and drink it. While you're enjoying the soda, get a medium to large bowl and put an inch or two of water in it with several cubes of ice. You want this water nice and cold. Set this bowl aside. When you're done with the soda, put a little water in the can and place it right side up on a hot plate for several minutes until the water starts to boil. Using a pair of tongs (for goodness' sake, don't touch it with your bare hands), pick up the soda can. Turn it over so the open drinking hole is facing down, and place the upside down soda can in the cold water.

The effect will be nearly instantaneous. There will be a popping noise and the can will crumple.

Here's why it works: Heating the can also heats the air inside the can, increasing the kinetic energy of the air molecules inside. The increase in energy and speed of the air molecules causes more of them to escape out of the open can, reducing the number of air molecules inside the can. This will set up a pressure differential between the air inside the can and the air outside the can. While the mouth of the can is unobstructed, this is not a problem. When the can is flipped over and placed in the ice water, two things occur: the water removes the capability of free air exchange between the heated air inside the can and the cooler air outside the can, and second, the abrupt drop in temperature causes the heated air inside the can to cool rapidly. This rapid cooling causes the air molecules within the can to move more slowly, with lower average kinetic energy, causing a drop in air pressure inside the can. Since there is no opportunity for additional air to enter the can, this sudden drop in air pressure is, essentially, a vacuum, just as described in the glass half empty/half full bit. The force of the suction caused by the drop in the air pressure is so intense that it pulls in the sides of the can, causing it to crumple. If this were done with a glass soda bottle, the bottle would shatter.

This experiment can be repeated using a 55-gallon steel drum and a child's wading pool and it will produce the same results, on a much grander scale and with a much louder bang.

If you search on YouTube, you can find many videos of the coke can version.

pixeldigger
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Re: What-if 0006: Glass Half Empty

Postby pixeldigger » Wed Aug 28, 2013 8:55 pm UTC

Showsni wrote:You know, I've thought this for a long time, but surely it should be the pessimists who view the glass as half full, and the optimists as half empty.

If you view a glass as half full, that means that you think of the natural state of glasses as being empty. "Most glasses are empty, but this one happens to be half full." That's a pessimistic viewpoint!

If you view a glass as half empty, though, you regard the natural state of glasses as being full. "Most glasses are full, but this one happens to be half empty." Surely viewing most glasses in the world as full is the optimistic viewpoint!

It's not just me, right?


Actually, all that matters is if you are trying to FILL or Empty the glass.

Optimist :"I've been drinking water all day, and this glass is still only half empty"
Pessimist: " I've been pouring water into this glass all day and this glass is still only half full"

Still, the realist will just tell you its half a glass of water.
If it was a standard 8 oz glass, then you have 4 oz of water.
or visually . . . . . .
Spoiler:
halfglass.jpg

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davidstarlingm
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Re: What-if 0006: Glass Half Empty

Postby davidstarlingm » Wed Aug 28, 2013 9:22 pm UTC

Syagria wrote:Heating the can also heats the air inside the can, increasing the kinetic energy of the air molecules inside. The increase in energy and speed of the air molecules causes more of them to escape out of the open can, reducing the number of air molecules inside the can. This will set up a pressure differential between the air inside the can and the air outside the can. While the mouth of the can is unobstructed, this is not a problem. When the can is flipped over and placed in the ice water, two things occur: the water removes the capability of free air exchange between the heated air inside the can and the cooler air outside the can, and second, the abrupt drop in temperature causes the heated air inside the can to cool rapidly. This rapid cooling causes the air molecules within the can to move more slowly, with lower average kinetic energy, causing a drop in air pressure inside the can. Since there is no opportunity for additional air to enter the can, this sudden drop in air pressure is, essentially, a vacuum, just as described in the glass half empty/half full bit. The force of the suction caused by the drop in the air pressure is so intense that it pulls in the sides of the can, causing it to crumple. If this were done with a glass soda bottle, the bottle would shatter.

Is it really suction? Or is it just the compressive strength of the can giving way under the greater atmospheric pressure outside as opposed to inside?

rmsgrey
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Re: What-if 0006: Glass Half Empty

Postby rmsgrey » Wed Aug 28, 2013 11:00 pm UTC

davidstarlingm wrote:
Syagria wrote:Heating the can also heats the air inside the can, increasing the kinetic energy of the air molecules inside. The increase in energy and speed of the air molecules causes more of them to escape out of the open can, reducing the number of air molecules inside the can. This will set up a pressure differential between the air inside the can and the air outside the can. While the mouth of the can is unobstructed, this is not a problem. When the can is flipped over and placed in the ice water, two things occur: the water removes the capability of free air exchange between the heated air inside the can and the cooler air outside the can, and second, the abrupt drop in temperature causes the heated air inside the can to cool rapidly. This rapid cooling causes the air molecules within the can to move more slowly, with lower average kinetic energy, causing a drop in air pressure inside the can. Since there is no opportunity for additional air to enter the can, this sudden drop in air pressure is, essentially, a vacuum, just as described in the glass half empty/half full bit. The force of the suction caused by the drop in the air pressure is so intense that it pulls in the sides of the can, causing it to crumple. If this were done with a glass soda bottle, the bottle would shatter.

Is it really suction? Or is it just the compressive strength of the can giving way under the greater atmospheric pressure outside as opposed to inside?


What is suction, if not the apparent inward force caused by localised reduction of pressure?

pixeldigger
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Re: What-if 0006: Glass Half Empty

Postby pixeldigger » Thu Aug 29, 2013 11:50 am UTC

Its all in perspective,
if I'm OUTSIDE the can, it was crushed
If I'm INSIDE the can, it sucks.

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Klear
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Re: What-if 0006: Glass Half Empty

Postby Klear » Thu Aug 29, 2013 11:56 am UTC

pixeldigger wrote:If I'm INSIDE the can, it sucks.


Definitely.

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Re: What-if 0006: Glass Half Empty

Postby OrbitalFacePalm9001 » Fri Dec 13, 2013 8:35 am UTC

HEY, FREE GLASS!
>implying that you can't greentext on xkcd forums

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EpicanicusStrikes
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Re: What-if 0006: Glass Half Empty

Postby EpicanicusStrikes » Tue Jan 28, 2014 8:34 pm UTC

pixeldigger wrote:Actually, all that matters is if you are trying to FILL or Empty the glass.
Spoiler:
Image

Actually, all that matters is the last known state of volume in the glass. It helps if careful observation is maintained.

If the current state of 4 ounces in an 8 ounce glass is less than the last observed state, then the glass is in the process of being emptied and is half-way to that goal.

If the current state of 4 ounces in an 8 ounce glass is more than the last observed state, then the glass is in the process of being filled and is half-way to that goal.

Pedants will argue about the stated/assumed goal of the process altering the value of the observed states, but nobody ever listens to them. They're the ones full of piss.


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