Let's have an argument - Argh it keeps recurring!
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Diluted Dante wrote:Yossarian wrote:Just watched the Mythbusters stuff.Spoiler:Spoiler:
The wheels are free spinning, you can pull the conveyor belt at any speed and the plane would stay in place with very little pressure exerted on it. It doesn’t take much from the plane to start moving forward. -
In fairness to not not not not not not not not not not not not not not gurt, always read the question is a fair lesson."Plus he wore shorts like a total cunt" - Bob
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It absolutely would not stay still. They might be free-spinning but they're not frictionless, and they have a sizeable mass pushing them into the ground.
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GurtTractor wrote:Keep in mind that I never said the plane was being held stationary by anything, that was an assumption most of you lept to.
I don't think anyone made this assumption -
If I was really evil, I could have posted this version of this question -
Which is rather problematic as detailed here -
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Not sure about the Mythbusters explanation.
It perhaps works with what they show as a small plane but the question says "a plane".
Would a VC10 full of gold still be able to take off?
Define plane. -
Diluted Dante wrote:
I don't think anyone made this assumptionGurtTractor wrote:Keep in mind that I never said the plane was being held stationary by anything, that was an assumption most of you lept to.
Well to say it another way people think that because of the backwards-moving conveyor the plane will stay stationary thus meaning no airflow over the wings and no lift. That's the initial intuition, it was certainly mine when watching this episode earlier. -
Diluted Dante wrote:
.GurtTractor wrote:Keep in mind that I never said the plane was being held stationary by anything, that was an assumption most of you lept to.
I don't think anyone made this assumption
It's just about the only assumption I made."Plus he wore shorts like a total cunt" - Bob -
For what it's worth, thread delivers on title. Garbage OP though.
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Unlikely wrote:It absolutely would not stay still. They might be free-spinning but they're not frictionless, and they have a sizeable mass pushing them into the ground.
It would take some amount of pressure from a fixed point or forward thrust to keep it in place, but it would be a fraction of the energy necessary to be moving the conveyor belt, and so the plane will be able to overcome it. -
Well this was stupid. -
However you square it with my explanation or Mythbusters the answer is it depends on the plane.
In fact it is roughly the same. If you frame it as using the exact same amount of thrust as normal then the answer is no it won't take off as it still has to overcome the all be it very small amount of additional friction. -
Probably any plane could manage it, unless you chose something that had so little power or was so overladen to the point that it could only just take off in the best of circumstances, in which case it's possible that the tiny amount of extra friction from the wheels on the conveyor might be enough to prevent it from taking off.
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Well my intital thought was Yes. Then everyone was saying no apart from Gurt. At which point i knew my initial thought was correct.
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Thats more wording though innit.
"Imagine a plane" well I imagined a VC10 full of gold.
The 747 framing can give an exact answer but you still need to know what the belt is made of and what is in the 747.
This is why I hate this shit, it always comes down to specifics not provided.
"Imagine a man has just eaten chicken nuggets. Can he run 100m in under 10 seconds".
Well what man? -
I mean I think even this question wording falls under 3. After reading your edit the sencond half contradicts the first.
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Fucking hell. Knew it would be some bullshit like this. Where’s that Unlikely post from the last page? -
A cursory look around the interwebs reveals just how contraversial this thing is, the Mythbusters talked about how vitriolic the discussion became on their fansite and yeah they aren't kidding. If the question isn't worded properly you get people talking past each other -
https://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/So, people who go with interpretation #3 notice immediately that the plane cannot move and keep trying to condescendingly explain to the #2 crowd that nothing they say changes the basic facts of the problem. The #2 crowd is busy explaining to the #3 crowd that planes aren’t driven by their wheels. Of course, this being the internet, there’s also a #4 crowd loudly arguing that even if the plane was able to move, it couldn’t have been what hit the Pentagon.
And you see a lot of pilots, engineers, physics students etc getting caught in one or more logical traps, either due to their hasty reasoning or because the question was flawed. I fucked it up a little there initially and had to add in the clarifying statements to make it practically realisable, thanks Unlikely.
What I think is really wonderful about this question is how it gets you to think about your presuppositions, your intuition leads you one way and you chuck in some learned science into the prediction in your head and you come out being quite sure of what the result will be. When you realise that you've led yourself into a fallacy this way I think it's quite liberating, how many other firm understandings that we have of our world could be so flawed or incorrect? -
Diluted Dante wrote:I mean I think even this question wording falls under 3. After reading your edit the sencond half contradicts the first.
Could you explain? If I've fucked it up I should learn how to write this stuff better.
Like I've said though, this was in good faith and not a trick question, it's a fairly simple description of a physical scenario that could be engineered. What you imagine (a plane (any plane) on a conveyor belt runway) is what this is about. You can faff about with varying speeds and plane types and weights and durabilities of things but that's all particular detail that doesn't actually have much weight in this instance.
You can take a wide variety of planes, have a wide range of conveyor belt speeds forward or backwards, assume that the thing won't fall apart, and the plane will be able to take off. The specifics aren't actually terribly important here. -
I’m not sure the question the way it was initially worded in this thread avoided the phrasing trap. To my mind, a “conveyor [...] programmed to move backwards at exactly the same speed as the plane moves forward relative to the ground on the sides of the conveyor” would be moving at a sufficient speed so as to prevent the plane from moving at all, but maybe I’m missing something.
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Feynman Sprinkler beats this without all the wordplay bullshit."Plus he wore shorts like a total cunt" - Bob
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Yossarian wrote:I’m not sure the question the way it was initially worded in this thread avoided the phrasing trap. To my mind, a “conveyor [...] programmed to move backwards at exactly the same speed as the plane moves forward relative to the ground on the sides of the conveyor” would be moving at a sufficient speed so as to prevent the plane from moving at all, but maybe I’m missing something.
Yes I think that's me fucking it up there, that would cause a paradox -
http://c-aviation.net/plane-conveyor-belt-explained-debunked/This is the problem! The wording of this quiz is wrong and makes it physically impossible.
Divide this situation to several steps. At first, everything is just the same as in my explanation above. We apply thrust and we run the conveyor belt in the opposite direction. The wheel starts turning. As the plane moves forward – the conveyor belt accelerates. Keep in mind the preassumption “conveyor belt is designed to exactly match the speed of the wheels”.
When the aircraft moves forward then its wheels have to travel further than the conveyor belt has moved back. But this is impossible in this situation. The conveyor belt is designed to match the speed of the wheel – so it will increase the speed. But the plane still moves forward – there is again a difference between the speed of the wheel and the speed of the conveyor belt. But we do not allow such situation! So we increase the speed of the conveyor belt!
Actually, this is not a process that happens in steps – all of this happens simultaneously – the plane tries to accelerate and the conveyor accelerates to keep up with the wheels. Adding to the speed of wheels. And the wheels accelerate even more. So the conveyor belt…
It lasts until wheels and conveyor belt speed reaches infinity. Or until they reach the speed of light. If you wish to know what happens then – consult the screenwriters of Start Treck or Star Wars.
The wording of this quiz is wrong! This case is impossible. It is impossible right from the very beginning where the preassumption is that the speeds will always match. If we know (and we know!) the forces applied on the plane we know that during the takeoff there is a huge imbalance of forces. So quoting the Newtonian law:
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
The object will not stay motionless because we have unbalanced forces. So we can not design the conveyor belt to move at the same speed as wheels. -
This has made me angry. A debacle from start to finish. Any respect for Gurt gone in the blink of an eye. -
I think any slight variation of wording is beside the point. I read it, I thought the point of the question was ‘if a plane is kept stationary, would it take off’. The question is really ‘would this keep the plane stationary?’ but I’d have still read it the first way most of the time, because that just seems like the whole point of bothering with thinking about it in the first place.
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I don't mind it. We all misread the question and the answer was in plain sight in retrospect."Plus he wore shorts like a total cunt" - Bob
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monkey wrote:I think any slight variation of wording is beside the point. I read it, I thought the point of the question was ‘if a plane is kept stationary, would it take off’. The question is really ‘would this keep the plane stationary?’ but I’d have still read it the first way most of the time, because that just seems like the whole point of bothering with thinking about it in the first place.
Hmm, well that is very odd to me. I guess it's another wonderful thing about this question that people can go in with such completely different assumptions about what is being asked. As I said I never specified that it was supposed to be held stationary by this conveyor, that's something you've brought to it.
I'm not responsible for any disgruntlement over this, well mostly, I did balls up the wording by being a bit too vague. Sorry about that. But I'll maintain that the spirit of the question was quite simple, a bit of fun. -
@SpaceGazelle If that Feynman Sprinkler thing is worth a discussion/argument over you should bring it up. Sounds interesting.
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Speed relative to the ground next to the belt isnt the same as the speed the plane is going.
The distance the plane is moving is whatever the belt moves in that time plus whatever distance the plane moves forwards from its start point.
The belt speed and therefore distance moved is equal to the distance the plane moves.
Plane total speed is 2x belt speed assuming absolute wheel friction.
For the plane to move forward 1 metre it needs to counter the 1 metre the belt would move as well (again assuming absolute friction).
The Mythbusters thing states the friction isnt absolute. Lets say it is 99% frictionless. That means the plane has to move 1.01m to move forward 1m.
Relative to the ground the plane will take off and that caveat is crucial to it working.
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