0669: "Experiment"

[myhill.bob]

I have to ask, why won't pressurised bodies with a relatively thin skin (like a plant/human) explode in a vacuum? Our bodies are really only fit for a pressure of 1atm (101.325 kPa). I was sure this pressure differential across the surface of our body would be sufficient to cause capillary and cell explosion . Never mind the air in our lungs; I think this would only be a problem in explosive decompression (although if depressurisation was gradual, surely you'd suffocate well before the pressure difference got anywhere near as high as a perfect vacuum).

Just wondering, is there some data relating to this?

[NorthLondon]

It's probably just me, but panels 1 - 4 reminded me of that scene quite early in the first Matrix movie where Neo has been captured by the agents, who are interrogating him. Neo demands a phone call and then his mouth seals over itself.

I thought the "FWOOSH" was his mouth sealing over.

Then I got to the end, and I wished I had studied physics beyond the age of 16. (For the record, reading xkcd comics, and wondering how aeroplanes stay in the air, are the only times I wish this).

[Kanonfutter]

I am due to teach interchanges between potential and kinetic energy in a high school physics class later today. This one goes to show for sure.

Thanks.

[GildedBear]

I have to ask, why won't pressurised bodies with a relatively thin skin (like a plant/human) explode in a vacuum? Our bodies are really only fit for a pressure of 1atm (101.325 kPa). I was sure this pressure differential across the surface of our body would be sufficient to cause capillary and cell explosion . Never mind the air in our lungs; I think this would only be a problem in explosive decompression (although if depressurisation was gradual, surely you'd suffocate well before the pressure difference got anywhere near as high as a perfect vacuum).

Just wondering, is there some data relating to this?

Well, I don't have any hard data, but I can provide a general answer and an illustrative example. The general, and pretty useless, answer is: because the bonds between our cells, and within our cell walls, are able to withstand a sudden, one atmosphere, pressure drop. The strength of those bonds could be calculated if one was so inclined, though I am both, uniclined and do not know the needed formulas. The illustrative example is: we routinely drag up deep sea creatures without tanks pressurized to their native levels, which can reach over 1,100 atmospheres at the deepest depths, and they do not explode in gore at the surface. This doesn't really prove anything about a vacuum, but it does do a nice job of illustrating how it's possible for us to not explode. Of course... that doesn't mean that you wouldn't get a full body space hickey... but, the uniform stretching of all of the tissue might prevent that too. I honestly don't know.

Holding your breath is a very bad idea, since your esophagus is not strong enough to hold back the pressure differential and the entire volume of your lungs will come screaming out of a passage that isn't large enough to deal with the volume of air trying to occupy it all at the same time.

[vish]

fffffffffffthis has always bothered me.

"Disregarding friction"
"Disregarding air resistance"
"A frictionless pulley of a negligible weight"

THAT'S NOT HOW THE WORLD WORKS

you are getting it wrong, physics isn't about "how the world works" it is about "is this simple model CLOSE ENOUGH to how the world works, to be useful?"

and usually, it is,
you can calculate the time a ball would take to drop from the tower of Piza, and be more accurate than your ability to measure the time on your stopwatch (though not if it was a ball of wool)

you can use newtonian physics (no relativity) for getting to the moon,(though not for calculating the orbit of mercury)

hell, you can even assume that the electrons are ORBITING a nucleus to get a very good estimate of not only the atom size but also of the wavelength of an electron. (but you would need QM for the two slit experiment)

---
the major mental leap needed in maths is the understanding that the formalism and the exactness are an end in their own right

the major mental leap in physics is to know just when they are not.
(you are worried about the friction of air ? how about you start worrying about the cubed term in the motion of a pendulum, "Harmonic Motion" my A** ) (and yet it is harmonic. for small enough amplitudes....)

[Quicksilver]

I never took physics, but wouldn't a frictonless floor mean that the stick figure would slide across the floor indefinitely?

[dangerous]

I never took physics, but wouldn't a frictonless floor mean that the stick figure would slide across the floor indefinitely?

only with an external force (i.e. pushing on a wall or something colliding) otherwise the body would stay at rest (Newton's law of conservation of energy)

Also <3 BHG

[davidhbrown]

Excellent comic in the best xkcd tradition!

[Max2009]

I like how the guy wakes up, not knowing where he is or what happened.
He probably started out here.

[halcyon1234]

I think I may print this out and show it to my physics prof. I know for a fact that he has a sense of humor.

Is it perfectly spherical?

[DVC]

Randall stole my joke! I've been making puns about working with frictionless environments leading to slip-ups for years.

Of course, Randall has never met me, so I guess we came up with it independantly.

[tesseraktik]

As a student of Engineering Physics, I approve of this comic!

However, before I considered this, my first thought was that the physicist could use the laptop to get towards the door. Just grab the laptop and fling it directly away from the door ... Of course, you wouldn't be able to grasp the laptop without friction ...

He's probably a theoretical physicist. I'd suspect applied physicists would respond faster.

In that case, the experiment would be faulty; theoretical physicists don't function well in a gravitational field unless their work is confined to a small region of spacetime.

[Faranya]

fffffffffffthis has always bothered me.

"Disregarding friction"
"Disregarding air resistance"
"A frictionless pulley of a negligible weight"

THAT'S NOT HOW THE WORLD WORKS

Close enough.

Of course, you wouldn't be able to grasp the laptop without friction

Not really true. You only really need friction to hold onto things that have a force acting in any way parallel to your hand. If you had a frictionless can, you couldn't hold it from the side, but you could hold onto it by putting your hand underneath it (and holding it perfectly upright)

If course, friction makes everything much easier. Except math.

[Comic JK]

I never took physics, but wouldn't a frictonless floor mean that the stick figure would slide across the floor indefinitely?

only with an external force (i.e. pushing on a wall or something colliding) otherwise the body would stay at rest (Newton's law of conservation of energy)

That's conservation of momentum. Since the guy's body is full of chemical potential, he can get all the kinetic energy he wants; but he can't push the floor in the opposite direction, so he's stuck.

This is why someone suggested he should throw the laptop away from the door.

[neoliminal]

I never took physics, but wouldn't a frictonless floor mean that the stick figure would slide across the floor indefinitely?

only with an external force (i.e. pushing on a wall or something colliding) otherwise the body would stay at rest (Newton's law of conservation of energy)

Also <3 BHG

But he yells. He should be going backwards now.

[dangerous]

I never took physics, but wouldn't a frictonless floor mean that the stick figure would slide across the floor indefinitely?

only with an external force (i.e. pushing on a wall or something colliding) otherwise the body would stay at rest (Newton's law of conservation of energy)

Also <3 BHG

But he yells. He should be going backwards now.

I like the concept that brings up... Barry Scott powered rockets in frictionless and no-gravity space
"BANG AND I AM GONE!"

That's conservation of momentum. Since the guy's body is full of chemical potential, he can get all the kinetic energy he wants; but he can't push the floor in the opposite direction, so he's stuck.

ah that's the law I was thinking of... I have been looking into thermodynamics far too much.

[Faranya]

I never took physics, but wouldn't a frictonless floor mean that the stick figure would slide across the floor indefinitely?

only with an external force (i.e. pushing on a wall or something colliding) otherwise the body would stay at rest (Newton's law of conservation of energy)

Also <3 BHG

But he yells. He should be going backwards now.

He is. Very, very slowly. Why else do you think he suddenly lost that balance he was so precariously maintaining?

[Whirling Dervish]

Great comic. The way my physics problems were worded I always wondered how far off the answers would be if friction and air resistance weren't disregarded.

[TheHand]

As this comic illustrates, friction isn't all bad. I use it frequently for pleasure!

[score_under]

As this comic illustrates, friction isn't all bad. I use it frequently for pleasure!

Nice fitting name you have there

[cypher]

"huh, looks like physics professors don't like working in frictionless vacuums after all." "Except in bed."

[SW15243]

"huh, looks like physics professors don't like working in frictionless vacuums after all." "Except in bed."

Tell that to female physics professors.

[Strikeferret]

I haven't laughed this hard at an engineering joke in a long time. And it was probably XKCD that did it last time, too.

Re: Exploding people

People are mostly solids and liquids. Both are reasonably incompressible. Thus, a sudden decompression would cause the gasses to expand, but not the solids or liquids. This is why the dissolved gasses in your blood/tissues would boil and the gasses in your lungs would cause massive damage, but not explosions. Shy the lungs, most gasses are in small amounts anyway. It would cause local trauma, but not explosive decompression.

Re: Estimations

Honestly, the reason we use approximations is for speed. The margin of safety is there to include such things as those masses. Then again, with modern FEA and similar systems, you can get pretty close, but I've had real-world designs that survive for tens of thousands of cycles catastrophically fail in FEA. I'm sure that's a symptom of my bad data entry or a bad assumption, but it's still something to be aware of.

Besides, if your design fails completely with the estimation, imagine how much time you saved without doing the hard-core number crunching!

Pulleys are relatively low in moment of inertia in any case, and friction can be assumed to be negligible for back of the envelope proof of concept work.

It's like not knowing what spin state an electron is in during calculation of a gas-turbine cycle. It doesn't matter to the scale you're working at. Like me changing the rotation of the Earth by walking.

So, I'm all for you learning everything you want, that's the best attitude in the world, the biggest part of engineering is knowing what to ignore, and how to simplify systems.

It's nice to be able to talk about this. It's been years.

[Walter.Horvath]

What is the line for in the second-to-last panel?

[tim314]

What is the line for in the second-to-last panel?

He's in some kind of glass dome or something, with Black Hat Man watching from outside. The slanted line is the roof of the dome.

[nirvana_grace]

How does one milk a spherical cow?

[phillipsjk]

I have to ask, why won't pressurised bodies with a relatively thin skin (like a plant/human) explode in a vacuum? Our bodies are really only fit for a pressure of 1atm (101.325 kPa). I was sure this pressure differential across the surface of our body would be sufficient to cause capillary and cell explosion .

Simple: the vapor pressure of water at 37C (47.1mmHg, assuming an "ideal" blood-stream is lower than your blood pressure (64-112mmHg, assuming you are "normal").

[PaleBlueDot]

I find this comic funny even though I don't really understand physics at all.

[Robstickle]

How does one milk a spherical cow?

Spear it!

[Voltaire II]

As little physics as I know/remember, this one made me laugh for about 10 minutes, so I have to say, kudos to XKCD.

I find this comic funny even though I don't really understand physics at all.

Exactly.

[The Scyphozoa]

If the floor is frictionless, wouldn't he have kept sliding across the floor when he fell? Or did he fall EXACTLY vertically??

[linear approximation]

I read physics and I don't get it

[keyanf]

I'm not a physics expert, but shouldn't the guy go sliding around (surely he didn't fall straight down), not just fall flat with no friction?

[MSTK]

I'm not a physics expert, but shouldn't the guy go sliding around (surely he didn't fall straight down), not just fall flat with no friction?

Only an outside force can cause acceleration/change in velocity of a center of mass of a system. If there's no force applied to him, the physicist's center of mass cannot move (or rather, cannot change velocity).

If he had chopped off his arm and thrown that across the stadium, the center of mass must remain stationary (there are no outside forces), so then while his arm moves one direction, his body will move the other.

If the physicist was holding his laptop and threw the laptop, the center of mass of the physicist-laptop system must remain constant in the same manner. Nothing can budge this without an outside force.

So, what would make the guy go slide around? If you think about it...what would cause him to move? Would his feet push him in a direction? (They couldn't -- there is no friction). Is there a wind? (no air). Maybe when he yelled, some of the air left and was sent the other direction, so his body must have moved to keep he center of mass constant. But that wouldn't be by any significant matter (air is pretty non-heavy)

[sithwalrus]

Well, i have physics tomorrow.
if things get boring, i guess i will bring this out and show it to the teacher, might be worth a laugh.

[ijuin]

For all who are confused:

He was supposed to WORK on the laptop, that's what it's for; but he didn't, and that is what the BHG is commenting on.

Work equals force multiplied by the distance through which it is exerted. However, with no friction, he can not reach the laptop in order to exert a force upon it.

[methmath]

mmmm. Is it supposed to be "The other two are lost on the infinite plane of uniform charge density"....? Sorry... I mean.... it's just that an infinite plane of uniform density isn't very useful to anyone... whereas an infinite plane of uniform charge density has an easy-to-calculate electric field.

[lemmings]

But that wouldn't be by any significant matter (air is pretty non-heavy)

The mass of a human male is ~70kg.
That male will have a lung capacity of ~5L
Worlfram says that the mass of that volume of oxygen is ~7g

The ratio of a person's mass to the mass of oxygen in their lungs is therefore 10,000:1


The person is about 3 meters from the door, he has at best 60 seconds to get there before his brain starts shutting down.

The total momentum that the he needs to obtain is therefore 70 * (1/20) NS = 3.5 NS

The mass of the gas in his lungs also need this momentum so 0.007 * S = 3.5

S = 500 m/s

That is faster than the speed of sound at 1ATM... it's not looking good unless he can get to that laptop.

[Red Hal]

But that wouldn't be by any significant matter (air is pretty non-heavy)

The mass of a human male is ~70kg.
That male will have a lung capacity of ~5L
Worlfram says that the mass of that volume of oxygen is ~7g

The ratio of a person's mass to the mass of oxygen in their lungs is therefore 10,000:1


The person is about 3 meters from the door, he has at best 60 seconds to get there before his brain starts shutting down.

The total momentum that the he needs to obtain is therefore 70 * (1/20) NS = 3.5 NS

The mass of the gas in his lungs also need this momentum so 0.007 * S = 3.5

S = 500 m/s

That is faster than the speed of sound at 1ATM... it's not looking good unless he can get to that laptop.

1 Minute is probably an over-optimistic assessment. I'd say about 15 seconds. I've had this site in my "cool" bookmarks for years; I knew it would come in handy one day: http://www.geoffreylandis.com/vacuum.html

[mathbard]

All right, I've defended some of the others, but this one is going to be 100% lost on anyone who hasn't taken Physics I.

Not true, I've never taken a Physics course. Dif/Eq, Baby Reals (AKA Adv. Calc), non-Euclidean geometry, Group and Ring Theory, yes to all those. No Physics ever.