What-If 0062: "Falling With Helium"

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Re: What-If 0062: "Falling With Helium"

Postby Zassounotsukushi » Wed Sep 11, 2013 8:18 pm UTC

davidstarlingm wrote:I mean, if movies and TV are to be believed, a blast wave is more than capable of flinging you across a room without much more than ringing in your ears. I'm guessing this is theoretically possible. One could then conceive of a Rapid Deceleration Gun that you pointed toward the ground while falling, then fired; it would have to measure the distance to the ground and the current airspeed, then fire a charge that would explode with the correct force to completely cushion your landing.


This is mostly pointless. Exploding stuff doesn't buy you anything physically. You still have the same distance over which to decrease your velocity. Now I know, you don't ever need a reason to blow things up, but I'm trying to use equations.

The most practical explosion would be just plain retro-rockets. If you limited the use to breaking your fall, it would actually be fairly practical although I don't know if it would be lighter than a normal parachute. It would certainly be more dangerous.

But that's ignoring the advantages. Just think about an entrance to a fight. If your opponent comes down from the sky in a parachute, you'll be like "alright, he looks pretty tough". But if the guy drops straight down, no deceleration, until the exact last moment when he fires rockets, then stands there and stares you in the eye, you're thinking "I'm going to die now". That's the kind of stuff that happens in Soviet Russia.

Actually, in Soviet Russia, not only do they drop in to a fight with retrorockets, they drop in with retrorockets on a tank.

http://www.youtube.com/watch?v=4uGfOppQD_g

You can easily calculate the time/distance needed to stop from terminal velocity. If you allow for pulling 7 g-s, then it only takes (200 mph)/(7*(9.8 m/s^2) = 1.3 seconds. But the distance taken is about 58 meters.

That's the real problem with the airbag approach. In order to prevent from dying, it would have to either be really really big, or you would have to sustain really really huge forces. I guess you could do a combination of both. That's the approach I would favor - in addition to some reduction of velocity due to the area of the balloon itself. You want to increase your drag dramatically, but with minimal equipment, then you could use the self-inflating balloon to take care of the rest.

Obviously retro-rockets are still cooler.

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Re: What-If 0062: "Falling With Helium"

Postby Klear » Wed Sep 11, 2013 8:39 pm UTC

Zassounotsukushi wrote:But now - what if you could climb on top of the balloon right before you hit the ground? That seems like it would be awfully unstable, but you would get the balloon to cushion your fall. In fact, it seems like it would be a better idea to do this with air. Don't bother slowing down much. Just inflate a makeshift bouncy pad.


I heard that most people falling from extreme heights don't get killed by the first impact, but rather by the one after bouncing of earth once... If that's true, I don't see a bouncy pad as an advantage.

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Re: What-If 0062: "Falling With Helium"

Postby gmalivuk » Wed Sep 11, 2013 9:41 pm UTC

Can you give any plausible reason why it would be true in the first place, though?
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Re: What-If 0062: "Falling With Helium"

Postby speising » Wed Sep 11, 2013 11:09 pm UTC


that looks a lot like what they did with Curiosity.

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Re: What-If 0062: "Falling With Helium"

Postby Mikeski » Wed Sep 11, 2013 11:47 pm UTC

davidstarlingm wrote:Is it possible to time a blast wave such that it will slow you down to a not-particularly-dangerous impact speed without causing significant injury?

Unlikely. A short blast wave that decelerates you enough that your impact with the ground is negligible is mathematically equivalent to just hitting the ground. You need a "softer" blast wave that lasts for a suitably long time, which is really not likely to be described as "a blast wave" anymore. "Updraft" would be a better term.

I suppose a super-scale bladeless fan, pointing upwards, would work. (As would a normal fan, but falling slowly through a bladed fan doesn't sound much better than falling quickly into solid ground.)

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Re: What-If 0062: "Falling With Helium"

Postby Nnelg » Thu Sep 12, 2013 1:23 am UTC

rmsgrey wrote:That was the envelope rather than the hydrogen igniting that caused the problem...

Actually, that's a myth.

Sure, the paint was flammable -but just barely. It burnt slowly and only at high temperatures (in fact, near the end of the disaster parts of the skin which where on fire died out by themselves). So it might have played a role in starting the fire, but it's absolutely certain that >90% of the fireball was fueled by the Hydrogen.
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Re: What-If 0062: "Falling With Helium"

Postby Klear » Thu Sep 12, 2013 8:18 am UTC

gmalivuk wrote:Can you give any plausible reason why it would be true in the first place, though?


OK, I dug up some references, though it seems likely I misremembered the "most people".

Here are some links reporting people bouncing after impact: [1] [2] [3]

And here, though admittedly not exactly reliable, confirmation that what I said happens to "some people" [4].

The reasoning is that while you can prepare for the first impact at least a little (aim for soft ground, land feet-first to absorb the impact etc), during the second impact you are likely to be unconscious.

I heard about this from someone on an airfield. She might have been a diving instructor, or someone simply spending a lot of time at the airfield because of her profession, I don't remember clearly. She probably said something that it happens "often" or "in come cases".

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Re: What-If 0062: "Falling With Helium"

Postby peewee_RotA » Thu Sep 12, 2013 2:46 pm UTC

Something has been bothering me about this, and a handful of other what ifs. The obvious is pointed out as if it were important to the conversation. OF COURSE PARACHUTES ARE MORE EFFECTIVE. We know that because we use them. If rapidly filling up a balloon were reasonable, we'd be doing it. We better go back and edit previous entries to insert the obvious. Baseball pitchers throw better while standing on their legs instead of their hands. Machineguns were invented to shoot at people. Oobleck does not make suitable drinking water.

And it's not even used as a benchmark in this one. There's no "if we can achieve this then it's viable" statement. It was straight up meaningless filler. Lame :P
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Re: What-If 0062: "Falling With Helium"

Postby Zassounotsukushi » Thu Sep 12, 2013 3:16 pm UTC

Mikeski wrote:
davidstarlingm wrote:Is it possible to time a blast wave such that it will slow you down to a not-particularly-dangerous impact speed without causing significant injury?

Unlikely. A short blast wave that decelerates you enough that your impact with the ground is negligible is mathematically equivalent to just hitting the ground. You need a "softer" blast wave that lasts for a suitably long time, which is really not likely to be described as "a blast wave" anymore. "Updraft" would be a better term.

I suppose a super-scale bladeless fan, pointing upwards, would work. (As would a normal fan, but falling slowly through a bladed fan doesn't sound much better than falling quickly into solid ground.)


The bladeless fan is probably unnecessary. Just build a 50 meter mesh person-catcher with an intelligent pulley system.

Just think about how much more efficient air travel would be if you could get off at any point along the way. No landing strip needed, just 1 or 2 acres of catching infrastructure. We just have to have all passengers trained in skydiving to control their landing point.

This is my more modest version of the Commutapult.

http://www.youtube.com/watch?v=S099fdRuRi0

This could deliver you straight to downtown. You can put the mesh catcher on the top of skyscrapers, because let's face it, if you miss you're going to die anyway. Then walk a few blocks and you're at your conference. Think about it. Only 15 minutes ago you were 1,000s of feet up. No you're just a little wind blown and shell-shocked.

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Re: What-If 0062: "Falling With Helium"

Postby Klear » Thu Sep 12, 2013 3:28 pm UTC

And the luggage gets thrown after you?

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Re: What-If 0062: "Falling With Helium"

Postby Copper Bezel » Thu Sep 12, 2013 3:30 pm UTC

peewee_RotA wrote:Something has been bothering me about this, and a handful of other what ifs. The obvious is pointed out as if it were important to the conversation. OF COURSE PARACHUTES ARE MORE EFFECTIVE. We know that because we use them. If rapidly filling up a balloon were reasonable, we'd be doing it. We better go back and edit previous entries to insert the obvious. Baseball pitchers throw better while standing on their legs instead of their hands. Machineguns were invented to shoot at people. Oobleck does not make suitable drinking water.

And it's not even used as a benchmark in this one. There's no "if we can achieve this then it's viable" statement. It was straight up meaningless filler. Lame :P

He says that you might as well be using a parachute if you're filling the balloon with ambient air. In other words, using ambient air is something that might come to mind as a solution to the problem, but is clearly cheating.
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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Re: What-If 0062: "Falling With Helium"

Postby Zassounotsukushi » Thu Sep 12, 2013 5:30 pm UTC

Klear wrote:And the luggage gets thrown after you?


Well you have a problem with landing rate in general. I'm sure there will be popular/common destinations. In one city you may need to drop off 5 people. But how many catcher pads will you have? The luggage (even if you could steer it) would increase the number of catchers necessary. At terminal velocity it could probably rip your head off so I wouldn't suggest letting it land directly next to you.

What's more, if you tried to tape luggage under your shirt or something you'll increase your falling speed, making it even more deadly. So I would suggest not doing this.

Travel light? That seems to be the only option.

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Re: What-If 0062: "Falling With Helium"

Postby billy joule » Thu Sep 12, 2013 11:36 pm UTC

Zassounotsukushi wrote:
What's more, if you tried to tape luggage under your shirt or something you'll increase your falling speed, making it even more deadly. So I would suggest not doing this.


Having carried a few suitcases with human bodies in them in my time, I can tell you most peoples luggage is much less dense than the human body.
I would guess the decrease in overall density when strapping a suitcase to your back would outweigh any decrease in Cd so it'd lower your terminal velocity.
Unless of course you're a lead salesman or gold smuggler..

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Re: What-If 0062: "Falling With Helium"

Postby gmalivuk » Fri Sep 13, 2013 12:06 am UTC

Yeah, people are about as dense as water (since most of us can choose to float or sink in a pool based on the amount of air in our lungs), which is less dense than books (seeing as paper floats). So unless your typical suitcase is a lot heavier than if it were completely filled with books (chosen as one of the denser things people might be familiar with carrying in large quantities in luggage), holding onto it would make your average density quite a lot lower.

If you want to be more scientific about it, pack a normal suitcase, put it in a sealed plastic bag, and throw the whole thing into your bathtub or pool. The fraction of it that floats above the surface is the fraction less dense than a human body it is.
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Re: What-If 0062: "Falling With Helium"

Postby Kethryes » Fri Sep 13, 2013 9:15 am UTC

There is some guy who is reading too much XKCD :D

http://www.theguardian.com/world/2013/sep/12/helium-balloons-jonathan-trappe-up-transatlantic
Helium balloons lift aviator Jonathan Trappe Up for transatlantic trip

An American aviator has begun the first attempt to cross the Atlantic suspended by hundreds of coloured balloons. Jonathan Trappe took off from Caribou, Maine, on Thursday morning as his capsule was lifted by 370 helium-filled balloons in heavy fog and he headed east from the US.
...
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Re: What-If 0062: "Falling With Helium"

Postby Zassounotsukushi » Fri Sep 13, 2013 12:59 pm UTC

billy joule wrote:
Zassounotsukushi wrote:
What's more, if you tried to tape luggage under your shirt or something you'll increase your falling speed, making it even more deadly. So I would suggest not doing this.


Having carried a few suitcases with human bodies in them in my time, I can tell you most peoples luggage is much less dense than the human body.
I would guess the decrease in overall density when strapping a suitcase to your back would outweigh any decrease in Cd so it'd lower your terminal velocity.
Unless of course you're a lead salesman or gold smuggler..


It's not density that matters. Take the drag equation, assume you know the density, then find the acceleration due to drag. This is important because gravity is constant acceleration. The coefficients in this expression are what really matter.

Image

As you can see, if density is held constant, it's really the ratio of A/V that matters.

Does strapping the suitcase to your stomach increase your area? Only very marginally. However, it increases your volume/mass a good deal. The fact of the matter is that a human has poor aerodynamics. By strapping junk to yourself you make yourself more spherical - and that's the real hazard. You would also go faster curling up into a ball. This is the opposite of what you want.

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Re: What-If 0062: "Falling With Helium"

Postby rmsgrey » Fri Sep 13, 2013 7:12 pm UTC

Zassounotsukushi wrote:
billy joule wrote:
Zassounotsukushi wrote:
What's more, if you tried to tape luggage under your shirt or something you'll increase your falling speed, making it even more deadly. So I would suggest not doing this.


Having carried a few suitcases with human bodies in them in my time, I can tell you most peoples luggage is much less dense than the human body.
I would guess the decrease in overall density when strapping a suitcase to your back would outweigh any decrease in Cd so it'd lower your terminal velocity.
Unless of course you're a lead salesman or gold smuggler..


It's not density that matters. Take the drag equation, assume you know the density, then find the acceleration due to drag. This is important because gravity is constant acceleration. The coefficients in this expression are what really matter.

Image

As you can see, if density is held constant, it's really the ratio of A/V that matters.

Does strapping the suitcase to your stomach increase your area? Only very marginally. However, it increases your volume/mass a good deal. The fact of the matter is that a human has poor aerodynamics. By strapping junk to yourself you make yourself more spherical - and that's the real hazard. You would also go faster curling up into a ball. This is the opposite of what you want.


So what you're saying is you should carry it in your hand?

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Re: What-If 0062: "Falling With Helium"

Postby Zassounotsukushi » Fri Sep 13, 2013 8:30 pm UTC

rmsgrey wrote:So what you're saying is you should carry it in your hand?


Actually, yes.

The way you orient and arrange things makes a HUGE difference in how fast your to speed is. The larger problem, however, is holding it out by your side, and not letting it fall on top of you, and then crush your skull when the mat slows you down at accelerations > 7g.

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Re: What-If 0062: "Falling With Helium"

Postby billy joule » Sat Sep 14, 2013 2:49 am UTC

Zassounotsukushi wrote:It's not density that matters. Take the drag equation, assume you know the density, then find the acceleration due to drag. This is important because gravity is constant acceleration. The coefficients in this expression are what really matter.

Image

As you can see, if density is held constant, it's really the ratio of A/V that matters.


Your math is wack....In the final step you canceled fluid density from the drag eqn (air in this case) with the object density (man with/without suitcase) :? .

Your a = F/m eqn (2nd line) shows if object density is decreased (by adding the suitcase) then a will decrease.

Does strapping the suitcase to your stomach increase your area? Only very marginally.

Well that depends on your orientation, If you're falling head or feet first your projected area could easily double.

The fact of the matter is that a human has poor aerodynamics. By strapping junk to yourself you make yourself more spherical.


The terminal velocity for a skydiver is about 200km/hr = 120m/hr = 55.6 m/s
if you strap junk to yourself until you become totally spherical (ie you jump inside a Zorb) the terminal velocity eqn gives a Vt of 28.5 m/s.

Vt decreases as the decrease in Cd of a sphere vs a human is swamped by the increase in projected area and the decrease in overall density.

...It's not until the object weighs about 600kg that Vt reaches 55.6 m/s, which brings me back to my original statement:

billy joule wrote:I would guess the decrease in overall density when strapping a suitcase to your back would outweigh any decrease in Cd so it'd lower your terminal velocity.
Unless of course you're a lead salesman or gold smuggler..

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Re: What-If 0062: "Falling With Helium"

Postby learsfool » Sat Sep 14, 2013 3:49 am UTC

peewee_RotA wrote:The obvious is pointed out as if it were important to the conversation. OF COURSE PARACHUTES ARE MORE EFFECTIVE. We know that because we use them. If rapidly filling up a balloon were reasonable, we'd be doing it.

Exactly!

A parachute is a balloon that's designed to fill itself up while you're falling!

Also, not technically a balloon.

:)

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Re: What-If 0062: "Falling With Helium"

Postby Zassounotsukushi » Sat Sep 14, 2013 3:09 pm UTC

billy joule wrote: Your math is wack....In the final step you canceled fluid density from the drag eqn (air in this case) with the object density (man with/without suitcase) :? .


That's true. That was careless. The equation should retain a term (rho_air / rho_body).

billy joule wrote:Your a = F/m eqn (2nd line) shows if object density is decreased (by adding the suitcase) then a will decrease.


IF you keep the (A/V) ratio the same! That's nonsense, because that is trivially not constant. The ratio (A/V) comes from the shape alone. You will change the shape with the suitcase strapping, and not for the better.

billy joule wrote:
Does strapping the suitcase to your stomach increase your area? Only very marginally.

Well that depends on your orientation, If you're falling head or feet first your projected area could easily double.


Of course it depends. Heck, if your suitcase has a parachute attached to it, then...

billy joule wrote:
The fact of the matter is that a human has poor aerodynamics. By strapping junk to yourself you make yourself more spherical.


The terminal velocity for a skydiver is about 200km/hr = 120m/hr = 55.6 m/s
if you strap junk to yourself until you become totally spherical (ie you jump inside a Zorb) the terminal velocity eqn gives a Vt of 28.5 m/s.


Yeah, we'll assume that we add volume without adding mass! That's what the Zorb is. The Zorb-human combination has an extremely low average density. Do the calculation, the Zorb has a specific gravity of something like 0.02. A human is along the lines of 1.0.

A suitcase will be less dense, obviously. But we're talking about specific gravity more like 0.25. A suitcase isn't a big balloon.

billy joule wrote:Vt decreases as the decrease in Cd of a sphere vs a human is swamped by the increase in projected area and the decrease in overall density.

...It's not until the object weighs about 600kg that Vt reaches 55.6 m/s, which brings me back to my original statement:


Only with wildly unrealistic assumptions.

Strapping something to your stomach is very 'efficient', in the terms of minimizing effective drag area. That's what I'm talking about.

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Re: What-If 0062: "Falling With Helium"

Postby billy joule » Sun Sep 15, 2013 9:58 am UTC

Zassounotsukushi wrote:
billy joule wrote:Your a = F/m eqn (2nd line) shows if object density is decreased (by adding the suitcase) then a will decrease.


IF you keep the (A/V) ratio the same! That's nonsense, because that is trivially not constant. The ratio (A/V) comes from the shape alone. You will change the shape with the suitcase strapping, and not for the better.

Any increase in projected area is an improvement as far as lowering Vt is concerned.
the effect of the shape change on the Cd is speculation, I agree it'll probably decrease but not by much..


billy joule wrote:
Does strapping the suitcase to your stomach increase your area? Only very marginally.

Well that depends on your orientation, If you're falling head or feet first your projected area could easily double.


Of course it depends. Heck, if your suitcase has a parachute attached to it, then...

So not "very marginal" then?


billy joule wrote:
The fact of the matter is that a human has poor aerodynamics. By strapping junk to yourself you make yourself more spherical.


The terminal velocity for a skydiver is about 200km/hr = 120m/hr = 55.6 m/s
if you strap junk to yourself until you become totally spherical (ie you jump inside a Zorb) the terminal velocity eqn gives a Vt of 28.5 m/s.


Yeah, we'll assume that we add volume without adding mass! That's what the Zorb is. The Zorb-human combination has an extremely low average density. Do the calculation, the Zorb has a specific gravity of something like 0.02. A human is along the lines of 1.0.

A suitcase will be less dense, obviously. But we're talking about specific gravity more like 0.25. A suitcase isn't a big balloon.
[/quote]
It does illustrate my point though:
An increase in projected area and decrease in density can outweigh a decrease in Cd and lead to a decrease in Vt.
the zorb and the suitcase both fit this scenario.

It's clear the suitcase decreases density and increases projected area.
The question is, do these two changes outweigh the (speculated) decrease of the Cd.

Wiki says the Cd of a human is 1.0-1.3.

A suitcase is rectangular.
My fluids textbook says a cube has a Cd of 1.05. rectangular rod varies between 1.3 - 2.5 depending on L/D.
I doubt combining these two shapes is likely to lead to much decrease in Cd..

It seems intuitive to me: a suitcase falls slower than a person, strapping them together will most likely lead to a terminal velocity between that of the suitcase alone and the person alone.

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Re: What-If 0062: "Falling With Helium"

Postby Copper Bezel » Sun Sep 15, 2013 8:03 pm UTC

Two suitcases have the same Vt. Strapping them together (vertically) increases their mass, leaves cross-sectional surface area unchanged, and decreases Cd. Vt goes up. Human + suitcase should intuitively have a similar result.
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Re: What-If 0062: "Falling With Helium"

Postby billy joule » Sun Sep 15, 2013 10:55 pm UTC

Copper Bezel wrote:Two suitcases have the same Vt. Strapping them together (vertically) increases their mass, leaves cross-sectional surface area unchanged, and decreases Cd. Vt goes up. Human + suitcase should intuitively have a similar result.


Except we're talking about strapping a suitcase to your stomach which will increase CSA in all orientations (unless you're really fat i guess)
I guess there are arrangements that will increase Vt but that's not what's being debated..

A more fitting analogy:
Two suitcases have the different density's and therefore Vt . Strapping them together (side by side), increases CSA, decreases density (relative to the heavy case) and (for arguments sake) decreases Cd. Vt is now between that of the heavy and light case....
This is going round is circles, I give up :D

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Re: What-If 0062: "Falling With Helium"

Postby Eternal Density » Mon Sep 16, 2013 6:13 am UTC

Klear wrote:
Quicksilver wrote:and A bit from my RSS feed that's not on the official article for some reason:
The lesson: Don't mess with medical examiners. They're apparently pretty hardcore.


It is there - note 5.

Also, this reminded me how Click and Drag was more awesome than Time...
Does not compute.
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Re: What-If 0062: "Falling With Helium"

Postby davidstarlingm » Mon Sep 16, 2013 2:59 pm UTC

Zassounotsukushi wrote:
davidstarlingm wrote:I mean, if movies and TV are to be believed, a blast wave is more than capable of flinging you across a room without much more than ringing in your ears. I'm guessing this is theoretically possible. One could then conceive of a Rapid Deceleration Gun that you pointed toward the ground while falling, then fired; it would have to measure the distance to the ground and the current airspeed, then fire a charge that would explode with the correct force to completely cushion your landing.

This is mostly pointless. Exploding stuff doesn't buy you anything physically. You still have the same distance over which to decrease your velocity. Now I know, you don't ever need a reason to blow things up, but I'm trying to use equations.

No, you'd have much more distance over which to decrease your velocity....specifically, the radius of the explosion wave instead of the < 2 meters that your body will take to stop you.

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Re: What-If 0062: "Falling With Helium"

Postby Inglonias » Mon Sep 16, 2013 4:02 pm UTC

All I really want to know is whether or not he was actually unbanned.

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Re: What-If 0062: "Falling With Helium"

Postby dalcde » Wed Sep 18, 2013 12:00 pm UTC

Inglonias wrote:All I really want to know is whether or not he was actually unbanned.

All I really want to know is whether or not he was actually banned.

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Re: What-If 0062: "Falling With Helium"

Postby Zassounotsukushi » Thu Sep 19, 2013 4:39 pm UTC

has no one posted this yet?

WolframAlpha Blog
A Response to “Falling with Helium”
by Jason Martinez
http://blog.wolframalpha.com/2013/09/19 ... -helium-2/


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