0473: "Still Raw"

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Mane
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Re: "Still Raw" Discussion

Postby Mane » Mon Sep 08, 2008 6:16 pm UTC

rwald wrote:
Mane wrote:What I don't understand about the whole pluto as a planet or not thing is, we talk about Jupiter and Neptune as planets, but we also call them Gas giants, why is it that dwarf planets have to be separate from the normal planets?

Or to better make my point, we already classed planets into Gas Giants and Rocky ones, why can't we create another classification for planets like pluto, without removing the label of 'planet' from them? I don't think many people would seriously argue that Saturn isn't a planet...


We already have a term for "random bodies floating in the Solar System"...OK, maybe we don't, but when people think of "planet" they're not thinking of, for example, random asteroids. They're thinking of "big things." If dwarf planets were considered to also be "real" planets, the category of "planet" would include some decidedly "small" things...as well as having tens, if not hundreds of members. If the word "planet" is to have any utility in the sense of "the most gravitationally significant bodies in the solar system," it can't include dwarf planets.


See, but the problem is that we're using our solar system as a sort of paragon for all other solar systems, even though we know, for example, that Brown Dwarfs exist.

Which was more or less my point, why do we talk about Jupiter as a planet, and not as if it was a failed star? We don't even know if it really does have, for example, a core of any sort, it could be nothing more then a giant ball of gas. Lets say that in a few hundred years we discover that Jupiter and the other gas giants are really just dead stars that never got their reactions going, will we remove them as well?

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Re: "Still Raw" Discussion

Postby wolfticket » Mon Sep 08, 2008 6:45 pm UTC

If the wheel bearings are frictionless, then the plane takes off.
If there is any friction at all and the speed of the treadmill always matches the speed of the wheels, the speed of the treadmill will increase very rapidly to the point that the friction matches the forward trust.
In "real life" :roll: there would be friction but the treadmill would not be able to react quick enough or reach a high enough speed and the aeroplane would take of easily.

And Pluto should never have been a Planet.

However, I am not actually real, I am from the internet.

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Re: "Still Raw" Discussion

Postby radtea » Mon Sep 08, 2008 6:48 pm UTC

H2SO4 wrote:
That make sense?


Who knows? Draw a free body diagram so we know what you're talking about. Until you do that, you haven't said anything about the physics of the problem in the standard, unambiguous language of the subject, so it's pretty much impossible for anyone to determine if you've made sense or not. Free body diagrams are, or should be, taught in high-school physics, so it's not asking too much to say that people who want to talk about an elementary physics problem should use them.

The problem isn't exactly hard to diagram. For frictionless wheels it looks like this (ignoring lift and weight):

d<---.--->T

where d is aerodynamic drag and T is the thrust of the engines, and I've assumed the plane is pointed to the right. d depends on the airspeed, T depends primarily on throttle setting. Weight is constant and lift depends on airspeed, and the plane will take off when lift exceeds weight. Note the absence of terms that depend on ground speed. Ergo, if you assume frictionless wheels the treadmill speed drops out of the problem, and the diagram communicates this clearly and unambiguously.

If the wheel friction is non-zero, the diagram looks like this:

d<---.--->T
f<---

The wheel friction adds to the drag force, and will depend on ground speed. Now, if you tell me the coefficient of friction of the wheel bearings as a function of ground speed, and the plane's take-off speed and weight, and the maximum thrust of the engine AND assuming the treadmill can provide any amount of force at any speed, I'll tell you if the plane can take off at any given treadmill speed.

It would be curious to know if there is a simpler question that one could ask that would generate as much confusion as this one, or if this question has the maximum possible confusion/simplicity ratio. I think it gets extra value from the incoherent "speed of the treadmill matches the speed of the wheels", which is one of those things that sounds like it makes sense, but doesn't.
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Re: "Still Raw" Discussion

Postby atimholt » Mon Sep 08, 2008 7:28 pm UTC

A better confusion to length ratio? How about "Do you walk to work, or do you carry your lunch?" Or "Do you like it better here than you do in the summer?"
Granted the confusion on these two questions is only momentary, low, but we're talking about ratios. They are much shorter than the airplane treadmill thing, so the ratio might be higher. Anyone here willing to do the math?
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Re: "Still Raw" Discussion

Postby cnoocy » Mon Sep 08, 2008 7:37 pm UTC

radtea wrote:It would be curious to know if there is a simpler question that one could ask that would generate as much confusion as this one, or if this question has the maximum possible confusion/simplicity ratio. I think it gets extra value from the incoherent "speed of the treadmill matches the speed of the wheels", which is one of those things that sounds like it makes sense, but doesn't.


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Re: "Still Raw" Discussion

Postby Death by Kidney » Mon Sep 08, 2008 7:42 pm UTC

You've broken my heart. :(

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Re: "Still Raw" Discussion

Postby Twist9 » Mon Sep 08, 2008 7:47 pm UTC

Ok, so in response to the plain on a treadmill discussion, in a real-life situation where the "treadmill" has enough potential power to match the speed of the plain taking off the plain would NOT take off for the following reason:

the landing gear would fail before the plain gained enough speed to take off, as the tires on the landing gear would spin twice as fast as they are intended to because of the speed of the treadmill beneath them, causing the wheel axises to overheat, contort, and subsequently break, suddenly introducing the full friction of the solid metal support legs onto the treadmill, flipping the plain and creating a fiery ball of destruction moving rapidly in the opposite direction from takeoff.

if, however, the landing hear were somehow reinforced to take the brunt of a double-speed takeoff, and the pilot managed to keep the wheels perfectly on course (twice the speed=double the amplification of steering errors, try driving 100 mph vs. 50 mph and you'll know what I"m talking about) then the plain would take off with out a hitch as the force for takeoff comes not from the wheels in contact with the treadmill, but rather the engines on the wings.

In a third scenario the treadmill moves rapidly enough to create enough friction in the wheels to equally oppose the power of the plain's engines, and (if the wheels do not fail) causing the plain to move too slowly to take off, but that would take one hell of a treadmill.

SO, there you have it.

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Re: "Still Raw" Discussion

Postby Steve the Pocket » Mon Sep 08, 2008 7:51 pm UTC

Sounds to me like this whole airplane-on-a-treadmill thing isn't really a thought experiment; it's a trick question, carefully conceived to trick people into forgetting what they already know about how airplanes work (i.e. that their propulsion is caused by the propeller/jets, not the wheels), and then get them to give a scientific sounding answer that ends up being wrong.

I like the comparison atimholt just made. It does to engineering knowledge what "Is it hotter down south than it is in the summer?" does to linguistics, except that it has a real answer.

And to those demanding a free-body diagram... here's the only free-body diagram this question really needs:
Image

;)
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Re: "Still Raw" Discussion

Postby osmigos » Mon Sep 08, 2008 8:13 pm UTC

One day a professor passes two students during lunch who are arguing about the plane/treadmill problem. He stops and interrupts them "I'm sorry to bother you but I wonder if you two would be able to help me with a simple problem. What do you get when you add and multiply three 2s together?". The first student answers quickly "Well that's obvious. 2 + 2 x 2 is 6". the second student looks at the first and says "where did you learn math? (2 + 2) x 2 is 8!".

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Re: "Still Raw" Discussion

Postby william » Mon Sep 08, 2008 8:16 pm UTC

SimonBob wrote:ps. switch doors every time

A more interesting variant of Monty Hall: Because Monty Hall is a jackass(or, as an economist would put it, rational), he will always give you the choice to switch when you have chosen the car but he will not always give you the choice to switch if you did not choose the car on your original pick. He wants the probability for you to be right when you switch to be one-half. With what probability should he give you the option?
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Re: "Still Raw" Discussion

Postby Elvish Pillager » Mon Sep 08, 2008 8:17 pm UTC

Southwest wrote:I predict that, due to this comic (and perhaps this post), this website explaining the airplane/treadmill thought experiment will see much more traffic.


It's wrong about a thing or two.

For instance, if I control an ideal treadmill and want to prevent the plane from taking off, I can. I need simply to run the treadmill so fast that the centripetal force required to keep the wheels from flying into pieces exceeds their structural strength. The entire aeroplane could be destroyed in the time it takes for its topmost part to fall to the ground, if not less. :twisted:

The only thing is, in the original question, the conveyor isn't controlled by a malicious personage like me - it was set up to exactly match the speed of the plane, in the opposite direction. Well, guess what: In that case, the conveyor belt would not do anything until the plane was already in motion.
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Re: "Still Raw" Discussion

Postby jspenguin » Mon Sep 08, 2008 8:20 pm UTC

pyroman wrote:
rwald wrote:I wouldn't go so far as to say that Pluto never should have been a planet; when it was first discovered, it was thought to be larger than we now know it to be, and the Kuiper Belt was entirely unknown. But now that we have more correct information, there's no logical justification for keeping Pluto a planet. And "because I learned it that way in elementary school" is not a logical justification, to answer that objection.



of course as an EE what pisses me off about this is if they can do that for Pluto why couldn't they do that with electricity when Franklin messed up with the labeling of charges.


Can we rename the speed of light too? It is more accurately described as the "speed of information". The actual speed of light can be made significantly slower travelling through a medium.

Calling it the "speed of light" is like calling your Internet bandwith the "speed of porn".

Code: Select all

from __future__ import skynet

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Re: "Still Raw" Discussion

Postby osmigos » Mon Sep 08, 2008 8:21 pm UTC

Elvish Pillager wrote: original question

Can you provide a source? I have yet to see anyone able to point to anywhere where the question was first asked.

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Re: "Still Raw" Discussion

Postby william » Mon Sep 08, 2008 8:24 pm UTC

Elvish Pillager wrote:For instance, if I control an ideal treadmill and want to prevent the plane from taking off, I can. I need simply to run the treadmill so fast that the centripetal force required to keep the wheels from flying into pieces exceeds their structural strength. The entire aeroplane could be destroyed in the time it takes for its topmost part to fall to the ground, if not less. :twisted:

Depends on the structural strength of the treadmill methinks.
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Re: "Still Raw" Discussion

Postby Atre » Mon Sep 08, 2008 8:28 pm UTC

I'm convinced this is a private joke by Randall to induce nuclear war in the forum thread...

Talking of which, I think post 13# is one of the subtlest pieces of "baiting" trolling I've ever seen

I'm convinced that everyone (fine, 99%, I'm sure some that have no idea what's going on say it too) in the 'it will fly' camp is simply trolling


Either that or *censors ad hominem rant peaking with *further censoring of details**

And as to both these arguments... :roll:

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Re: "Still Raw" Discussion

Postby GodShapedBullet » Mon Sep 08, 2008 9:13 pm UTC

The best Monty Hall strategy is to get you to commit to switching to the unopened door and then open the door with the prize.

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Re: "Still Raw" Discussion

Postby Vonriel » Mon Sep 08, 2008 9:18 pm UTC

william wrote:Depends on the structural strength of the treadmill methinks.

About time I saw this. Everyone talks about the strength of the wheels on their respective axes, but what about the bearings on the treadmill? If you simply handwave them away as being unaffected by friction, why can't the wheels on the plane be the same way?

And don't even try to tell me it's a treadmill like the one the MythBusters used, because it can't be - there is only a finite amount of matter in the universe, therefore, the plane would eventually reach the end of the belt, touch ground, and either explode because of failure in the wheels (eta: I like explosions :P ) or take off as it finally was capable of gaining the lift required.

If you're going to further handwave this by saying "well, this is theoretical, practical things like the finite amount of matter in the universe don't apply," I say, "well, this is theoretical, my plane doesn't have wheels, it flies by a complicated system of unobtanium that allows it to hover a few inches off of any surface I try to take off of." I can be as irrational as you can, suckers!

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Re: "Still Raw" Discussion

Postby av4rice » Mon Sep 08, 2008 9:41 pm UTC

jspenguin wrote:Can we rename the speed of light too? It is more accurately described as the "speed of information". The actual speed of light can be made significantly slower travelling through a medium.

I thought "speed of light" was just short for "speed of light through vacuum" so it inherently refers to a standard medium. Then we get other labels ("speed of light through ______") for other mediums because of course the number changes with those.

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Re: "Still Raw" Discussion

Postby MotorToad » Mon Sep 08, 2008 9:53 pm UTC

So, let me get this straight... if an asteroid is on a treadmill we call it a "dwarf planet?" :mrgreen:

Great comic, though. It cracks me up that people get bent out of shape over how we classify a (relatively) tiny rock that's as far away as it is.

I like the idea above that anything orbiting the Sun can be called a planet. I bet they get tired of making up names before they're 0.0001% through the Kuiper belt. :) While we're at it, we could name all of Saturn's "moons!"
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Re: "Still Raw" Discussion

Postby Elvish Pillager » Mon Sep 08, 2008 10:03 pm UTC

osmigos wrote:
Elvish Pillager wrote: original question

Can you provide a source? I have yet to see anyone able to point to anywhere where the question was first asked.

...by which I refer to the original question in the article whose reference I quoted.
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Re: "Still Raw" Discussion

Postby 3.14159265... » Mon Sep 08, 2008 10:08 pm UTC

Randall Munroe Fact # 180 - I love you.
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Re: "Still Raw" Discussion

Postby GMontag » Mon Sep 08, 2008 10:38 pm UTC

Random832 wrote:
pyroman wrote:Assuming the force of friction (of the wheel bearings not the tires on the tread mill) is 0 then it doesn't matter how fast the treadmill is moving.


What you're not getting - which is really annoying because it makes the people saying it wouldn't take off go "ah-HAH! But the bearing isn't ever really frictionless, and that changes the problem" - is that you don't have to ignore friction: The force of friction is _constant_ - it does not go up as the wheels spin faster. If the plane can take off at all, it can take off with a tailwindtreadmill.


As I pointed out earlier, even ignoring friction (which is not really constant, but it's close enough for low speed approximations), there is the force imparted by the wheels rotational inertia. This force is proportional to the acceleration of the treadmill. The faster the treadmill accelerates, the more force the treadmill imparts on the wheel axles.

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Re: "Still Raw" Discussion

Postby mrbaggins » Mon Sep 08, 2008 11:23 pm UTC

The only problem with the plane taking off is either friction, or the wheels spinning so fast that their structural integrity makes them asplode. (Google "Discs on die-grinders" for examples)

Of course, given that the treadmill matches the speed of the plane, then the wheels are only going to be going as fast as they would in a normal takeoff, or slightly faster than a normal landing. (The landing is the important one. Wheels go from 0m/s to large numbers nigh instantly. Takeoff is gradual)

And I think that if you ignore the fact that the treadmill's bearings are just as flawed as the planes bearings, it's perfectly fair to give both nil friction.
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Re: "Still Raw" Discussion

Postby likk9922 » Mon Sep 08, 2008 11:31 pm UTC

Now let's say, theoretically, you could build a treadmill large enough to put Pluto on... :twisted:

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Re: "Still Raw" Discussion

Postby FireCrack » Mon Sep 08, 2008 11:35 pm UTC

Ahh.. very interesting discussion here, but can anyone tell me:


If an aeroplane is positioned on a runway on Pluto, facing "west" and Pluto is rotating to the "east" at a velocity equal to the plane's groundspeed, then will the plane take off? (assume the plane has rocket engines replacing it's jet engines)


Secondly, more importantly, does the ability of a plane to take off from Pluto make it a planet?

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Re: "Still Raw" Discussion

Postby Grogs » Mon Sep 08, 2008 11:50 pm UTC

it would be way cooler to have 13 planets than 8

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Re: "Still Raw" Discussion

Postby 43r0sp4c3 » Tue Sep 09, 2008 12:31 am UTC

Ahh.. very interesting discussion here, but can anyone tell me:


If an aeroplane is positioned on a runway on Pluto, facing "west" and Pluto is rotating to the "east" at a velocity equal to the plane's groundspeed, then will the plane take off? (assume the plane has rocket engines replacing it's jet engines)


Secondly, more importantly, does the ability of a plane to take off from Pluto make it a planet?


Well, to the first, Pluto does not have sufficient atmosphere for the plane to generate lift over the wings. While the plane will take off due to the rocket engines, it will cease to be an airplane, and then be considered a spacecraft.

To the second, if the plane COULD take off, then Pluto would presumably need to be massive enough to maintain an atmosphere, and would probably also be massive enough to meet the standards of the "I am a planet" club.

For the airplane/treadmill question, I don't want to get into the semantics of the question (http://xkcd.com/169/), but I will point something out that doesn't seem to have been mentioned much. Due to the no-slip condition, the air velocity at the treadmill surface will be the same as the velocity of the treadmill. Due to viscosity, this will generate a boundary layer that, if the treadmill is sufficiently long and wide, will encompass the aircraft. If the pilot and treadmill operator design their speeds properly, they should (I hate to use that word in a discussion like this, but the internet isn't worth a transient analysis) be able to get the plane to have a velocity relative to the Earth of 0. This basically means the aerodynamic drag on the airplane is the significant retarding force, rather than any friction generated in the wheels. However, because this situation involves the aircraft obtaining a non-zero airspeed velocity, there is the possibility for the airplane to still take off, despite having zero velocity relative to the surface of the Earth.

I'm sure some of you will have reasons why you think I'm wrong, most of which wrong (the rest of which I'll discard as being a result of the assumptions from my simplified analysis, or some similarly flimsy excuse :P ), but I think it's certainly an interesting thing to consider.

If any of you are convinced and wondering why they don't do this on aircraft carriers or some such, I'll point you to the phrase "if the treadmill is sufficiently long and wide". An aircraft carrier that is twice as large as a large municipal airport isn't too practical.

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Re: "Still Raw" Discussion

Postby MotorToad » Tue Sep 09, 2008 1:31 am UTC

43r0sp4c3 wrote:To the second, if the plane COULD take off, then Pluto would presumably need to be massive enough to maintain an atmosphere, and would probably also be massive enough to meet the standards of the "I am a planet" club.

Forgive me if I'm wrong, and too lazy to check (the internet is so far away... :) ), but I think that Pluto has a tiny bit of an atmosphere when its orbit is closest to the Sun. I guess it's not maintained since it refreezes, but maybe we could have a seasonal Pluto Aero Club. :)

It cracks me the hell up that people are still trying to argue the plane/treadmill thing. I think it'd be a more constructive thought experiment to wonder whether, if we threw all the treadmills in the ocean, would boats still float?
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Re: "Still Raw" Discussion

Postby fiveofoh » Tue Sep 09, 2008 1:46 am UTC

rwald wrote:
pyroman wrote:
rwald wrote:I wouldn't go so far as to say that Pluto never should have been a planet; when it was first discovered, it was thought to be larger than we now know it to be, and the Kuiper Belt was entirely unknown. But now that we have more correct information, there's no logical justification for keeping Pluto a planet. And "because I learned it that way in elementary school" is not a logical justification, to answer that objection.



of course as an EE what pisses me off about this is if they can do that for Pluto why couldn't they do that with electricity when Franklin messed up with the labeling of charges.


You guys also call the imaginary constant "j" because it's too hard to differentiate lower-case and upper-case I. So I wouldn't be too smug.

But yea, a third example would be pi: by all rights it should equal the ratio of the circumference to the radius, but because it's actually circumference:diameter we have 2*pi everywhere. I guess after things have been mislabeled for too long, it's hard to fix them; that's why Pluto needed to be corrected before its 100-year mark.

@pyroman: agreed. I've always hated that...I usually just give up and ask the person next to me which way the current is going :P
(seriously, though, I do know it...during the year...stupid tests and such)

@rwald: There are a few factors going into that...partially that sometimes we use lower-case i, but mostly that engineers are, by definition, lazy*, and usually have terrible handwriting. Both of which necessitate using j.

*By which, of course, I mean efficient. We build bridges because we get tired of going around or finding a boat. Also, to those of you who would accuse us of shoddy work because of our laziness, it's kind of pain to rebuild them if they collapse, not to mention all those pesky lawyers and stuff...it's just easier to build it right the first time. Laziness, but a far-sighted kind of laziness.

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Re: "Still Raw" Discussion

Postby 43r0sp4c3 » Tue Sep 09, 2008 2:00 am UTC

MotorToad wrote:
43r0sp4c3 wrote:To the second, if the plane COULD take off, then Pluto would presumably need to be massive enough to maintain an atmosphere, and would probably also be massive enough to meet the standards of the "I am a planet" club.

Forgive me if I'm wrong, and too lazy to check (the internet is so far away... :) ), but I think that Pluto has a tiny bit of an atmosphere when its orbit is closest to the Sun. I guess it's not maintained since it refreezes, but maybe we could have a seasonal Pluto Aero Club. :)

It cracks me the hell up that people are still trying to argue the plane/treadmill thing. I think it'd be a more constructive thought experiment to wonder whether, if we threw all the treadmills in the ocean, would boats still float?


I should have known someone would call me on that one. According to wikipedia (which as we all know is the be-all and end-all of fact checking... AKA I don't care enough to read the cited sources), Pluto's atmosphere is around 0.3 Pa at 43 K . Designing a plane to fly in those conditions would be quite a feat of engineering. That said, I'm up for joining your seasonal Pluto Aero Club... what season is Pluto currently in and how long till it reaches summer?

Since you're probably right about your more constructive thought experiment, I figure I ought to take a stab at it. But I'm going to need to know... are we throwing the treadmills into the ocean through all the boats, or in some other way to cause damage to the boats?

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Re: "Still Raw" Discussion

Postby pyroman » Tue Sep 09, 2008 2:05 am UTC

I would like to start off by pointing out the fact that Randall nerd sniped us all quite badly. ( I myself almost got hit by a bus while composing arguments earlier as i will often walk around campus while on my laptop) Apparently this forum has been very active while i was in class so i am going to combine this into one post.

SolkaTruesilver wrote:
linguistic wrote:
pyroman wrote:of course as an EE what pisses me off about this is if they can do that for Pluto why couldn't they do that with electricity when Franklin messed up with the labeling of charges.


Yes. Yesyesyesyesyesyes.

Marry me.

Whenever I bring this concept up, people look at me funny and back away.


Well, they should. And at the same time, the USA should get into the 21st century and finally convert to the metric system.


@linguistic I'm glad to see my comments made you happy and as for marrying you if you happen to be one of them attrictive lady folks then its a possibilty otherwise i would have to decline on that one. Nothing personal of course.

@solka I have no problem with addopting the metric system. I use it all the time anyways and its much easier to work with. There are people that complain about having to learn the metric system in addition to the English one but could you imagine having to do the reverse?

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applebranch wrote:
Quixotess wrote:People only care about five because of base ten. In reality, five's kind of an awkward number. Much more important to be able to divide things into clean thirds than into clean fifths.


I really think you only consider five to be an 'awkward number' because you live in a base 12 system. In a base 10 system, five is half. And is therefore inherently useful. It's our (read: the rest of the world's) equivalent of your six. I just think 10 is a really nice number because it fits with base ten logarithims, and the base 10 scientific notation! And its more imporant to divide things into clean fifths rather than cleans sixths (or clean 12ths) so metric wins there, according to your logic.

Nonsense. I live in a base-10 system. As in, I write ten as 10. If I lived in a base-12 system, I would write twelve as 10. I think days should be 20, or twenty-four (twozen?) hours long.

I am being playful here, but still. The aesthetics of base-10 repel me.

I am willing to accept either the point about calculators making mental math obsolete or the point about base 10 being easier to count on your fingers.



@quixotess I Personally would be in favor of going to binary. you say its convenient to use base 10 because you have 10 fingers so its easy to count. How ever with binary you can count up to 1023 things or up to 31 on one hand. Seriously its awesome i use it all the time. Plus it often makes math easier as you can easily add a power of 2 to the total with next to zero effort. The only problem with it is i find i tend to stop at 4 or 5 a lot which tends to get me in trouble = / The other great thing about binary is there would now be 11000 hours in a day! This means i might actually have time to get all my work done :lol:
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety. - Benjamin Franklin

robot256
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Re: "Still Raw" Discussion

Postby robot256 » Tue Sep 09, 2008 2:09 am UTC

Actually, the real reason we use "j" for sqrt(-1) is because we use upper case I for constant or average currents and lowercase i for instantaneous currents. Since we use currents more often than we use i, It would be stupider to use j for current than to use j for i. Don't ask me why we started using I for current in the first place, though. Maybe because "c" was already taken.

EDIT: Which mean's it's the physicists' fault anyways. But I'm one of those, too, so I can't talk.

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Re: "Still Raw" Discussion

Postby PeteSF » Tue Sep 09, 2008 2:18 am UTC

GMontag wrote:As I pointed out earlier, even ignoring friction (which is not really constant, but it's close enough for low speed approximations), there is the force imparted by the wheels rotational inertia. This force is proportional to the acceleration of the treadmill. The faster the treadmill accelerates, the more force the treadmill imparts on the wheel axles.

Bah.
I refuse to consider anything other than a perfect (massless and frictionless) treadmill, and perfect (also massless and frictionless) wheels!

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Unforgiven
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Re: "Still Raw" Discussion

Postby Unforgiven » Tue Sep 09, 2008 3:11 am UTC

Here's another one: if an airplane were to land on a treadmill going in the opposite direction, would this reduce the length of the landing roll?

(the answer I guess would be "yes it would" because airplanes do use wheel braking so the treadmill can affect it then)
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Rippy
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Re: "Still Raw" Discussion

Postby Rippy » Tue Sep 09, 2008 3:25 am UTC

wolfticket wrote:If the wheel bearings are frictionless, then the plane takes off.
If there is any friction at all and the speed of the treadmill always matches the speed of the wheels, the speed of the treadmill will increase very rapidly to the point that the friction matches the forward trust.
In "real life" :roll: there would be friction but the treadmill would not be able to react quick enough or reach a high enough speed and the aeroplane would take of easily.

And Pluto should never have been a Planet.

However, I am not actually real, I am from the internet.

[/thread]? (and on the first post, no less)

From what I can tell, there are 3 correct answers, which one it is depends on your initial assumption:
1. No friction (plane takes off)
2. Friction is too much for the landing gear (landing gear fails, plane explodes in a giant fireball)
3. Plane takes off before friction becomes too great, OR treadmill is not ideal. (plane takes off)

Did I miss anything? The whole issue is just confusion over the question being asked imo. I'm pretty sure this covers the different viewpoints.

Unforgiven wrote:Here's another one: if an airplane were to land on a treadmill going in the opposite direction, would this reduce the length of the landing roll?

(the answer I guess would be "yes it would" because airplanes do use wheel braking so the treadmill can affect it then)

Does it count if the treadmill is going fast enough to break/melt the landing gear? I mean, the plane's going to stop way faster if the entire thing is sliding down the runway. :)

von Eisenstein
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Re: "Still Raw" Discussion

Postby von Eisenstein » Wed Sep 10, 2008 1:30 am UTC

Well, if we suppose that it is possible to construct a treadmill that is large and structurally sound enough to accommodate an airplane, surely we can presume that all rotating parts are near-frictionless...? If someone has enough money to build such a treadmill, I think it's safe to say that he/she has enough money to obtain parts that rotate with insignificant friction - magnetic bearings housed in a vacuum? (That statement sounds unclear and ambiguous but it makes sense in my mind =D)

But speaking in general about debating about theoretical things, the following tops the list:
~ Proving/disproving the statement: "Anything can kill you." or, as my slightly psychopathic classmate suggested. "You can kill someone with anything." It became a huge argument that was repeatedly debated on long bus rides. I concurred with that statement - even if it is merely an atom, you can still kill someone with it by placing it in an accelerator (a la CERN) and shooting it at someone. Highly impractical, but it...would work. Especially if speeds go high enough and it becomes antimatter, as Dan Brown put forth in Angels & Demons.

Runners up for minor arguments that nearly bloomed into fistfights:
~ Whether or not Taiwan is a part of China. (It is NOT, thank you. In fact, the international label "Republic of China" is illegitimate, since there is no legal process by which Chiang Kai-shek could have constructed a government-in-exile on another island, and then TAKEN OVER THAT ISLAND at the same time. Taiwan was not part of China since China tossed it away as a useless piece of land back in the 15th century, and it was picked up by Japan, and then later after WWII by the US in the Treaty of Shimonoseki and/or Treaty of San Francisco. Japan and America are the only two MODERN powers that have a legitimate claim to Taiwan.)
~ Whether or not it is possible to have a "vacuum implosion device," which is a Tier III superweapon in Red Alert 3. My position is still open on this one, though I'm leaning toward "no."
~ Whether or not fossils have a definite beginning age to qualify as a fossil. (I.E. A fossil is only a fossil only if it is 10,000 years old? My bio teacher said no, since fossilization is an ongoing process and therefore the only requirement is that it has completely been replaced by minerals. I contested that view for 20 minutes during class, holding that by that argument, we could then technically create "fossils" through chemical means by putting a bone in a lab overnight in an elaborate bath of chemicals, and he pulled me aside after the bell to tell me to please not make him "lose face" in front of his students.)

By the way feel free to fact-check me on anything I mentioned in this post. I <3 criticism.
To a certain degree. >.>;;

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Re: "Still Raw" Discussion

Postby Random832 » Wed Sep 10, 2008 2:48 am UTC

von Eisenstein wrote:I concurred with that statement - even if it is merely an atom, you can still kill someone with it by placing it in an accelerator (a la CERN) and shooting it at someone. Highly impractical, but it...would work. Especially if speeds go high enough and it becomes antimatter, as Dan Brown put forth in Angels & Demons.


An atom of anti-lead, going at the speed of light, has 15.5 nanojoules of kinetic energy and will annihilate the equivalent mass of one lead atom releasing an energy of 62 nanojoules. That's, quite simply, not going to kill anyone. It may be going really fast, but it's just an atom. And an atom of antimatter is still just an atom. It's just too damn small to do anything

~ Whether or not Taiwan is a part of China. (It is NOT, thank you. In fact, the international label "Republic of China" is illegitimate, since there is no legal process by which Chiang Kai-shek could have constructed a government-in-exile on another island, and then TAKEN OVER THAT ISLAND at the same time. Taiwan was not part of China since China tossed it away as a useless piece of land back in the 15th century, and it was picked up by Japan, and then later after WWII by the US in the Treaty of Shimonoseki and/or Treaty of San Francisco. Japan and America are the only two MODERN powers that have a legitimate claim to Taiwan.)


First of all, Kai-shek took over Taiwan in 1945 and didn't lose control of the mainland until 1949, hardly "at the same time", so it's not clear what your point is.

Imperial china (Qing dynasty) actually ruled Taiwan from 1683 until 1895 (the Treaty of Shimonoseki was the instrument which ended this, and has nothing to do with the end of WWII) - not the 15th century by a long shot. China hadn't even discovered Taiwan by the Fifteenth century, let alone "tossed it away".

he treaty of San Francisco came into effect in 1952. Neither the treaty of Shimonoseki nor the treaty of San Francisco, as far as I know, included any claim on the island by the United States, and the treaty of San Francisco formally renounced any Japanese claim (and thus, saying Japan has a legitimate claim is incorrect)

By the way feel free to fact-check me on anything I mentioned in this post. I <3 criticism.
To a certain degree. >.>;;

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Re: "Still Raw" Discussion

Postby Sudo-Fu » Wed Sep 10, 2008 3:08 am UTC

I hate to say it, but I just got nerd snipe'd by the airplane vs. treadmill problem.

I don't believe Mythbusters attempt proved anything, but yet I do believe the plane would take off... Perhaps it has to do with the treadmill equaling the instantaneous velocity of the wheels? Then it would seem to me that both would just accelerate until one or both exploded.
Wait..... what??

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Re: "Still Raw" Discussion

Postby phlip » Wed Sep 10, 2008 3:13 am UTC

Random832 wrote:An atom of anti-lead, going at the speed of light, has 15.5 nanojoules of kinetic energy and will annihilate the equivalent mass of one lead atom releasing an energy of 62 nanojoules. That's, quite simply, not going to kill anyone. It may be going really fast, but it's just an atom. And an atom of antimatter is still just an atom. It's just too damn small to do anything

Ah, but you're forgetting about mass dilation at relativistic speeds... A lead atom travelling at 0.9999999999c will have just over 2mJ of kinetic energy. Getting closer to the speed of light will make it go even higher (growing to infinity as you approach c). Yes, you'd need a really good particle accelerator to get those speeds, but still...

How did you think the LHC was going to get its single protons with 7TeV of kinetic energy?

Code: Select all

enum ಠ_ಠ {°□°╰=1, °Д°╰, ಠ益ಠ╰};
void ┻━┻︵​╰(ಠ_ಠ ⚠) {exit((int)⚠);}
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taelus.calimshan
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Re: "Still Raw" Discussion

Postby taelus.calimshan » Wed Sep 10, 2008 3:22 am UTC

I had to join the forums just to get in on this one. It's too much fun and I have too much spare time apparently.

Alright, to deal with the Pluto part, it's a planet because it was first called one and anyone who says otherwise is not my friend. That should be sufficient for that, so moving on...

So many assumptions it's amazing, but here we go. In the grand scheme of things, let's just assuming that no part of the plane or treadmill will break under any circumstances. They're all made of Unobtainium (no, not the kind referenced in "The Core" but the kind we physicists use to make our problems easier to solve) of varying mass and density as is appropriate for airplane and/or treadmill parts. Let's also assume that all bearings are frictionless and that the plane will never run out of fuel or the like. Basically, the whole bloody thing can just keep on moving for as long as it likes (at any speed it likes whether or not we violate the "speed limit"), or until something happens to give us a result. Also the treadmill can accelerate at any rate, to any speed. Now, let's get into it:

The plane itself as it applies thrust, regardless of the minimal nature of the mass of the wheels compared to the mass of the jet, can be held in place by the treadmill accelerating at a rather impressive rate. The force applied to the wheels by the treadmill will hold the airplane in check. This pattern can be continued indefinitely or until a result is achieved, assuming no limit on the speed of the treadmill. As the treadmill moves and the engines generate thrust, the air in the area will be moved as well. It's reasonable to assume that the rate at which the air is pushed through the engines of the jet is greater than or equal to the ground speed needed for the jet to take off regularly (if this weren't true, no jet could ever achieve lift). Assuming no other impediments to the airflow itself, both the engines and the track on the treadmill will move air. As the treadmill increases speed (and it has to do so constantly as the engines provide constant thrust), it would actually begin to generate airflow itself. After all, by definition, the track on the treadmill has a maximized friction coefficient (again, if not, the airplane couldn't be held in place as the tires would slip) and so air would be moved at increasing speeds. While not noticeable at lower speeds (say, anything less than 10 kilometers a second or the like), as the treadmill approaches otherwise impossible speeds, the air would be forced into motion to account for the force on the particles in contact with track itself. The combination would, at some point, translate to airflow sufficient to generate lift. I'd rather not do the calculations to find out at what stupefying track speed this would occur, but it would indeed happen.

So, assuming a treadmill build using available materials and technology, the plane cannot be kept from forward motion and either violates the conditions of the problem making it irrelevant or it gains ground speed until achieving lift. If the problem uses impossible tools and materials to allow for us to examine it as the treadmill's track speed approaches infinity, then the plane still achieves lift eventually. Albeit, the plane also reaches a point where it likely crashes back down as the speed of the air around it decreases rapidly with distance from the ground, but that's neither here nor there, the question is a matter of lift-off, not sustained flight.

There's your dissertation folks, so enjoy, and thanks for letting me partake in the discussion.


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