## 0697: "Tensile vs. Shear Strength"

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wagner
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### Re: "Tensile vs. Shear Strength" Discussion

tacvek wrote:One would almost certainly use a cable rather than a ribbon.

You use a ribbon because you need sufficient surface area for the climber to grab on to, while keeping cross sectional area (and mass) to a bare minimum.

mobiusmind
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### Re: "Tensile vs. Shear Strength" Discussion

The Burma Shave gag is of course intentional... but I couldn't help but read this comic to the tune of "We Didn't Start the Fire"

mirni
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### Re: "Tensile vs. Shear Strength" Discussion

Geostationary physics aside, I would connect the cable/ribbon/tube to the earth for the simplest of reasons: wind. An ultralight thread gone with the wind does not sound very prudent.

A Space Elevator is my wildest dream.

-m-

phillipsjk
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### Re: "Tensile vs. Shear Strength" Discussion

I found this funny. I think it's because I realized that repair is much easier if you have another one to fall back on...
Did you get the number on that truck?

Red Hal
Magically Delicious
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### Re: "Tensile vs. Shear Strength" Discussion

Even my late father was too young really to remember Burma-Shave signs.
Lost Greatest Silent Baby X Y Z. "There is no one who loves pain itself, who seeks after it and wants to have it, simply because it is pain..."

jjb123455
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### Re: "Tensile vs. Shear Strength" Discussion

Arancaytar wrote:After countless
Engineers
Spend trillions over
Fifty years
A modern Babel
Disappears
Because some fuck brought
Pruning shears

BURMA SHAVE.

So who else thought it?

He who drives

Half asleep

Now is buried

Six feet deep

...First one of those I ever saw and it immediately popped in my head! Love it!

Arancaytar
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### Re: "Tensile vs. Shear Strength" Discussion

Never fear though; even though the elevator is gone, we will still get our manned mars mission:

Free! Free!
A trip
To Mars
For 900
Empty jars

Burma-Shave

So get shaving, NASA.
"You cannot dual-wield the sharks. One is enough." -Our DM.

JeromeWest
Posts: 46
Joined: Wed Oct 21, 2009 9:47 am UTC

### Re: "Tensile vs. Shear Strength" Discussion

For some reason, I started reading this from the bottom upwards (maybe because my eye was drawn to the action at the bottom of the picture?) It made perfect sense that way around too, until the last (top) line.

po2141
Posts: 174
Joined: Wed Aug 01, 2007 3:55 pm UTC

### Re: "Tensile vs. Shear Strength" Discussion

@tacvek:

No, tensile strength does not scale up like that, a ribbon or cable would be *significantly* weaker than simply multiplying out the cross-section of one nanotube.

Carbon nanotubes are still just made of carbon-carbon bonds, just like alot of things - Nylon for example.

Its still pretty strong stuff, but theres a reason why no-one has been able to make a cable/ribbon out of it yet. And when they do it will not be a magical wonder-material. Certainly theres a chance that we can manufacture something extremely strong from carbon nanotubes, possibly even build a space elevator one day, but its the imperfections you see? All the tiny mis-alignments, chemical impurities, crystalline flaws etc etc will significantly reduce its properties.

Note that this is not unusual.

@hollowclown

The energy to send your cargo flying into space still has to come from somewhere, the angular momentum of something in geosynchronous orbit is pretty spectacular. As you hoist something up, there will be a "draggy" effect as in order to keep station with the cable, your cargo has to be accelerated perpendicular to the cable, in the direction of the Earth's rotation.

Imagine the ice-skater who is doing one of those spins, as she brings her arms and legs closer to herself, her rotation speeds up, but her angular momentum is constant (sorta). Hoisting cargo up an elevator will be the opposite, the rotation of your cargo around the earth's centre of mass will want to slow, you have to boost it along its orbital track for it to gain altitude and stay over the same point on earth and attached to the cable.

This seems counter-intuitive and is one reason a space elevator isn't quite as magical as it sounds (*gasp!* did I just say that!).

And if you want to "fling" a cargo into space, your cable will have to be of such alength that the velocity of the cable w.r.t earth is at or above escape velocity or your cargo will just be put into an elliptical orbit when you let go of it. This means a *very* long cable and this means that its weight will not be balanced and your cable WILL have to be VERY anchored to the earths surface.

All-in-all, nothing is free.
I washed my mug, and now my coffe tastes like soap.

Mr. Freeman
Posts: 96
Joined: Sat Mar 17, 2007 7:09 am UTC

### Re: "Tensile vs. Shear Strength" Discussion

We just covered normal and shear stress in my mechanics of materials class. This makes two comics now that align perfectly with my education. Brilliant.

kirrus
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### Re: "Tensile vs. Shear Strength" Discussion

Had a discussion yesterday with a friend (comic writer, nascent sci-fi writer) who thought that steel would be sufficient for a standard space elevator..

Is there an article I can link him to about space elevators, which includes math, but is sufficient for (UK) A-level maths understanding?

tastelikecoke
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### Re: "Tensile vs. Shear Strength" Discussion

long time in xkcd, the first time I learned what is this space elevators randall's talking about.

wonder if some of the engineers acquired cancer from building it

hordriss
Posts: 25
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### Re: "Tensile vs. Shear Strength" Discussion

A couple of thoughts about the space fountain idea. What would it do to the Earth's magnetic field and things like the Van Allen belts and the deflection of solar particles in the region. And how noisy would it be? Is there anywhere you could put it without permanently wrecking the place?

TheGrammarBolshevik
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### Re: "Tensile vs. Shear Strength" Discussion

fabiocbinbutter wrote:
wisnij wrote:
fabiocbinbutter wrote:My theory is that a space elevator is plausible, but the material itself would not need to support the weight of the structure, it would merely be a channel to carry high-energy ions that do all the load bearing, both of the payload and the structure.

My theory is that you don't know what any of those words actually mean.

I'll play the part of the philosopher and say that all meaning is semantic anyway. Words mean what we want them to mean and yet mean nothing at the same time. But seriously, there aren't any mysterious words in my post.

No, no, no - wisnij didn't say that your words are mysterious, just that the way that you use them doesn't make sense. To give another example, "all meaning is semantic" demonstrates complete ignorance of what "semantic" means.
Nothing rhymes with orange,
Not even sporange.

po2141
Posts: 174
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### Re: "Tensile vs. Shear Strength" Discussion

Easy now, although some overly-flowery language has been used, its obvious he just means that he thinks a "space fountain" might be more practical than a "space elevator".

Although IMO, we are talking about technology s far in advance that there aint much to choose between them. If we had the ability to build either, its likely we would also have the ability to solve a great deal more than how to get stuff into space easily.

Perhaps a space elevator or fountain would end up being a gimmicky extravagance, a bit like Vegas.

I washed my mug, and now my coffe tastes like soap.

Nintendon't
Posts: 61
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### Re: "Tensile vs. Shear Strength" Discussion

Wait, did nobody else notice the 'grand opening' of the elevator?

mirni
Posts: 14
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### Re: "Tensile vs. Shear Strength" Discussion

po2141 wrote:Perhaps a space elevator or fountain would end up being a gimmicky extravagance, a bit like Vegas.

hehe

Excellent analogy. No wonder I'm so fond of Space Elevators then. I find the Fabulous Las Vegas, Nevada close to irresistible!

-m-

dean.menezes
Posts: 135
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### Re: "Tensile vs. Shear Strength" Discussion

Arancaytar wrote:After countless
Engineers
Spend trillions over
Fifty years
A modern Babel
Disappears
Because some fuck brought
Pruning shears

BURMA SHAVE.

So who else thought it?

I did.

reevey
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### Re: "Tensile vs. Shear Strength" Discussion

lrossouw wrote:Book by Arthur C Clarke:

I think the Space Elevator was a favourite of Clarke's - I've only read The Last Theorem but the concept features in there quite heavily. Not a great book, incidentally, I was left distinctly underwhelmed given his reputation. I've certainly preferred Iain M Banks' sci-fi novels but then again I've never met a book by him I didn't like.

On the Burma Shave thing, being English that flew way over my head but thanks for explaining it. I take it those signs are still up? I drove down part of Route 66 when I was visitng Vegas last year but I never saw any of those ads!
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ritvax
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### Re: "Tensile vs. Shear Strength" Discussion

Arancaytar wrote:BURMA SHAVE.

So who else thought it?

I thought it! Of course, I'm way younger than the "Burma Shave" generation, I was hoping against hope that the rollover text was going to be "~BURMA-SHAVE"

Alas, it was not. An opportunity for Creative Anachronism lost.

-ritvax
I wish I was running VMS v.6.5

Balesk Baj, Timeburner
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### Re: "Tensile vs. Shear Strength" Discussion

We just studied tensile and shear strength in my class today.
The comic just blew me away.

Does Randall have any telekenetic powers or something?
Magnanimous wrote:I'd have a giant I Tetris piece shoved down my throat, after which my neck will flash and disappear.

mmxbass
Posts: 5
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### Re: "Tensile vs. Shear Strength" Discussion

Arancaytar wrote:BURMA SHAVE.

So who else thought it?
GUILTY AS CHARGED.

project2051
Posts: 178
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### Re: "Tensile vs. Shear Strength" Discussion

Pierrot wrote:For some reason I am not shocked that the elevator was cut. What I'm worried about is that who is going to clean up the 80km of cable that is connected to the counterweight moving at rotational speed. Which we all know it's going to inevitably cause tremendous damage.

Kim Stanley Robinson's Mar trilogy made good use of space elevators, including what happens when you let one fall on the planet.

navigatr85
Posts: 60
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### Re: "Tensile vs. Shear Strength" Discussion

Everyone's been saying that the use of the word "shears" in this comic is a pun. Maybe I'm being a little too nit-picky here, but I wouldn't call it a pun. The word "shears" is directly etymologically related to the concept of "shear strength" in physics. If I understand correctly, the concept of "shear strength" in physics was actually named after the cutting tools known as "shears". The term "shearing" in physics is simply a detailed description, at a microscopic level, of what shears and other cutting tools do.

If two words are so closely etymologically related, I wouldn't call their usage a pun. For example, if I say:

"I want to drink something, so let's go get some drinks."

....then that's not a pun. I'm using two different definition of the word "drink," but those two definitions are very closely related. If I say:

"A dog's bark is worse than its bite. An apple tree's bark tastes worse than its fruit."

...then that IS a pun, because the two definitions of "bark" are unrelated.

Dauric
Posts: 3989
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### Re: "Tensile vs. Shear Strength" Discussion

mmxbass wrote:
Arancaytar wrote:BURMA SHAVE.

So who else thought it?
GUILTY AS CHARGED.

He lit a match

To check the tank

and now they call him

skinless frank.

BURMA SHAVE

Yeah, first thing that popped in my head with that format as well, and yeah I sooo wished the rollover text would have been "Burma Shave", but perhaps that would have been too obvious.
We're in the traffic-chopper over the XKCD boards where there's been a thread-derailment. A Liquified Godwin spill has evacuated threads in a fourty-post radius of the accident, Lolcats and TVTropes have broken free of their containers. It is believed that the Point has perished.

Coffee
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### Re: "Tensile vs. Shear Strength" Discussion

Car in ditch

driver in tree

the moon was full

and so was he

-Burma Shave-
Far away boys, far away boys, away from you now.
I'm lying with my sweetheart, in her arms I'll be found.

Posts: 934
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### Re: "Tensile vs. Shear Strength" Discussion

reevey wrote:On the Burma Shave thing, being English that flew way over my head but thanks for explaining it. I take it those signs are still up? I drove down part of Route 66 when I was visitng Vegas last year but I never saw any of those ads!

There's a few recreations on Route 66 in Arizona, but it's been nearly fifty years since Burma-Shave stopped putting those ads up.
Compiling.. wrote:xkcd... where EVERYONE loves EVERYONE...

atchius
Posts: 1
Joined: Wed Feb 03, 2010 3:51 pm UTC

### Re: "Tensile vs. Shear Strength" Discussion

navigatr85 wrote:Everyone's been saying that the use of the word "shears" in this comic is a pun. Maybe I'm being a little too nit-picky here, but I wouldn't call it a pun. The word "shears" is directly etymologically related to the concept of "shear strength" in physics. If I understand correctly, the concept of "shear strength" in physics was actually named after the cutting tools known as "shears". The term "shearing" in physics is simply a detailed description, at a microscopic level, of what shears and other cutting tools do.

If two words are so closely etymologically related, I wouldn't call their usage a pun. For example, if I say:

"I want to drink something, so let's go get some drinks."

....then that's not a pun. I'm using two different definition of the word "drink," but those two definitions are very closely related. If I say:

"A dog's bark is worse than its bite. An apple tree's bark tastes worse than its fruit."

...then that IS a pun, because the two definitions of "bark" are unrelated.

Thank you! That's what I was thinking.

BeagleFury
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### Re: "Tensile vs. Shear Strength" Discussion

navigatr85 wrote:Everyone's been saying that the use of the word "shears" in this comic is a pun. {lots of fine points about puns and definitions and such...

Shear Inanity? Sheer inanity!

jasonkoller
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### Re: "Tensile vs. Shear Strength" Discussion

It would be the guy with the black hat.

rabidchicken
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### Re: "Tensile vs. Shear Strength" Discussion

I am sure they will have a crew on standby at all times to resecure it...
really, BHG would be hard pressed to cause any lasting damage.

gnutrino
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### Re: "Tensile vs. Shear Strength" Discussion

kirrus wrote:Had a discussion yesterday with a friend (comic writer, nascent sci-fi writer) who thought that steel would be sufficient for a standard space elevator..

Is there an article I can link him to about space elevators, which includes math, but is sufficient for (UK) A-level maths understanding?

Not really had a lot of experience with jsMath so this apologies if this doesn't work:

Consider a thin slice of cable with cross-section [imath]\sigma[/imath] and thickness [imath]\delta r[/imath], then (as this is still part of the cable) there will be tension force on either side of the slice. Let the tension pulling on the bottom side be [imath]T[/imath] and the tension on the top side be [imath]T + \delta T[/imath]. Then by balancing the forces on the slice (assuming no transverse forces), remembering that we are working in a non-inertial frame and so will have a centrifugal force, and noting that the mass of the slice is [imath]\rho \sigma \delta r[/imath] where [imath]\rho[/imath] is the density of the cable, one can see that
$\delta T =(\frac{\rho \sigma M_{E} G}{r^2} - \rho \sigma r \omega^2)\delta r$
where [imath]M_E[/imath] is the mass of the earth and [imath]\omega[/imath] is the angular velocity of the whole thing (fixed by the fact that the elevator is geostationary to be [imath]2\pi[/imath] rad/day = [imath]7.27 \times 10^{-5}[/imath] rad/s).

Dividing through by [imath]\delta r[/imath] and taking the limit gives the differential equation for T (you still do calculus at A-level right?):
$\frac{dT}{dr} =\rho \sigma (\frac{M_{E} G}{r^2} - r \omega^2)$

Now the tensile stress of a cable is [imath]\frac{T}{\sigma}[/imath] and there is some maximum value of this beyond which the cable snaps which depends only on the material of the cable (and as such is constant along the length), which is (one definition of) the tensile strength, call it [imath]\epsilon[/imath]. Then, if we assume that the cable is at the point of breaking all along its length (not a good situation for the cable to be in but it will give us a lower bound on the maximum cross section for the cable), [imath]T = \epsilon \sigma[/imath] and [imath]\frac{dT}{dr} = \epsilon \frac{d\sigma}{dr}[/imath].
We now have a differential equation for [imath]\sigma[/imath]:
$\frac{d\sigma}{dr} =\frac{\rho \sigma}{\epsilon} (\frac{M_{E} G}{r^2} - r \omega^2)$
depending only on [imath]\sigma[/imath], [imath]r[/imath] and constants which can therefore be solved (left as an exercise for the reader). If the cable has cross-section [imath]\sigma_0[/imath] at the surface of the earth than the
solution to this is (I hope):
$\ln(\frac{\sigma}{\sigma_0}) = \frac{\rho}{\epsilon}(M_E G (\frac{1}{R_E} - \frac{1}{r}) - \frac{\omega^2} {2}(r^2 - R_E^2))$
putting in [imath]r = R_G[/imath] where [imath]R_G[/imath] is the radius of geostationary orbits (also the radius at which [imath]\sigma[/imath] has a maximum as can be verified by putting [imath]\frac{d\sigma}{dr} = 0[/imath] in the above differential equation) and plugging in some numbers I get (you might want to check this yourself):
$\sigma_{max} = \sigma_0 \exp(\frac{\rho}{\epsilon} \times 6 \times 10^7)$
in S.I. units where [imath]\sigma_{max}[/imath] is the cross-sectional area of the cable at geosynchronous orbit (and as mentioned also the maximum cross-section for the cable - note that this is actually a lower bound on the maximum cross-section assuming that the cable is on the point of failure, in practice some tolerance would have to be built in). Putting in values for [imath]\rho[/imath] and [imath]\epsilon[/imath] for steel (which I got off wikipedia) I get somewere of the order of [imath]\sigma_{max} = 10^{460} m^2[/imath] which is clearly ridiculous and hence steel can't be used for a space elevator.

The real problem here is that [imath]\sigma_{max}[/imath] goes exponentially in [imath]\frac{\rho}{\epsilon}[/imath] so it is still possible for a sufficiently strong, light material to be used. However at present carbon nanotubes are the only thing that manages to strong and light enough and unfortunately we haven't managed to make any longer than about a few millimeters (as of 2008 according to wikipedia).

Sorry that turned out rather longer than I thought it would. Hope it helps (and that it's correct but I'm sure someone will point out if it isn't) though.

EDIT: Ninja edit to fix some markup
Last edited by gnutrino on Wed Feb 03, 2010 9:44 pm UTC, edited 2 times in total.

Prometheus
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### Re: "Tensile vs. Shear Strength" Discussion

Great, now I have something new to worry about.

In regards to the space fountain; I find the idea intriguing, but the engineering is hellishly complex, even compared to the space elevator. Not to say that they won't have their place in the eventual scheme of things. They are perfect for places like Mercury where the rotation is negligible and there's no convenient Lagrange point. Why mention Lagrange points? Because you can still use a space elevator on the moon if the center of gravity is at L1 since it is tidally locked. Would have to be (according to some crude math) about 38,000 miles, or 75% longer, but would be under considerably less tension (I'd supply that too, but I never did learn calculus.)
Well I thought it was funny.

mythopoeia
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### Re: "Tensile vs. Shear Strength" Discussion

BURMA SHAVE.

So who else thought it?

I did!

I'd love to see a few more of these "modern technology meets Burma Shave" comics. I think the gag would still be funny the next few times around, especially if it's infrequent.

Xlythe
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### Re: "Tensile vs. Shear Strength" Discussion

"After countless
Engineers
Spend trillions over
Fifty years
A modern Babel
Disappears
Because some fuck brought
Pruning shears"

It has nothing to do with the scene in the comic, but if really about shaving... [I'm perverted]

squall_line
Posts: 170
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### Re: "Tensile vs. Shear Strength" Discussion

It took about 35 posts for the word to show up, 34 posts before it showed up again, and now another 3 posts before someone (this guy, right here) points out that

Spoiler:
There is NO SUCH THING as CENTRIFUGAL FORCE

And, to think, I hadn't logged in to post anything in the last year or so, since I weighed in on something in the freeform discussion threads a while back.

Oh, and it was a decent comic, but I agree, there's no pun involved in the word "shear". Hat Guy should have been wearing a Burma-Shave t-shirt, though.

Carry on...

fencer42
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### Re: "Tensile vs. Shear Strength" Discussion

If you read only to the right of the elevator it reads like a prophecy.
Engineers
Fifty Years
Disappears
Pruning Shears

TimeSpaceMage
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### Re: "Tensile vs. Shear Strength" Discussion

Bah, communist internet... can't seem to find the Rocky & Bullwinkle version anywhere >_>

tpow
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### Re: "Tensile vs. Shear Strength" Discussion

squall_line wrote:It took about 35 posts for the word to show up, 34 posts before it showed up again, and now another 3 posts before someone (this guy, right here) points out that

Spoiler:
There is NO SUCH THING as CENTRIFUGAL FORCE

Thank you.

http://www.xkcd.com/123/
"We should take harpoons!"
"And dynamite!"
"Don't you think that is going a bit over-board..."
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JamesGecko
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### Re: "Tensile vs. Shear Strength" Discussion

Arancaytar wrote:Never fear though; even though the elevator is gone, we will still get our manned mars mission:

Free! Free!
A trip
To Mars
For 900
Empty jars

Burma-Shave

So get shaving, NASA.

If a trip
to Mars you earn,
remember, friend,
there's no return

(Turns out Mars is in Germany somewhere.)