MetaFilter's Own Physics Quandary

January 30, 2008 2:55 PM

Tonight a debate that has pitted brother against brother, friend against friend, MeFite against MeFite, will be finally laid to rest. The long awaited Mythbusters episode about airplane vs. conveyor belt airs tonight at 9 o'clock. For real this time. No really, it's airing, Adam Savage said so.

I have a scoop, guys! I think that Adam Savage is secretly a MeFite. Check out this user... I think it's Adam Savage! Imagine that! Adam Savage on MetaFilter. I guess we'll have to start thinking of him as one of our own, now.

posted by Kattullus to MetaFilter gatherings at 2:55 PM (120 comments total)

Shirley you must be joking!
posted by Krrrlson at 3:10 PM on January 30

I was just trying to describe to a co-worker the bitter arguments that may - or may not - be put to rest by tonight's episode. She was all You hang out on a website where they have bitter fights about physics questions?

Yeah.

BUT, I said, we also sometimes haz cheeseburgers, and other light-hearted fare!

I love Wednesdays: Project Runway and Mythbusters!
posted by rtha at 3:11 PM on January 30 [1 favorite]

A meetup huh?
posted by puke & cry at 3:11 PM on January 30

At the meetup in SF, Adam told us how it ends. I won't spoil it here, but it was the opposite of what I thought would happen.
posted by mathowie at 3:19 PM on January 30

Is that 9 eastern? Because if so I'll be on a plane. I expect a traditional take-off.
posted by Pants! at 3:19 PM on January 30

"I won't spoil it here, but it was the opposite of what I thought would happen."

The airplane remains on the runway and the conveyor belt soars majestically into the skies.
posted by mr_crash_davis at 3:21 PM on January 30 [42 favorites]

I stupidly read this post out loud, and now there is a "bitter argument" going on between my husband and myself. Tonight will not come soon enough, and then I never want to hear "airplane" and "conveyor belt" used in the same sentence again.
posted by Orb at 3:23 PM on January 30 [3 favorites]

This won't settle any arguments regardless of the outcome since many of us believe the question is not even framed properly enough to be testable. That is, as it stands, any empirical test relies on making unstated assumptions.
posted by vacapinta at 3:23 PM on January 30

will be finally laid to rest

The only thing that's going to get laid to rest is hatchets. In skulls.
posted by cortex at 3:26 PM on January 30

Oh, dear. SOMEONE is going to smash their television tonight. Anyone want to start a pool?
posted by exlotuseater at 3:27 PM on January 30

I tend to agree with vacapinta -- much like the Monty Hall problem, the airplane and conveyer belt problem seems to be less a question of what the answer is and more a question of what the question is.

Half the world will howl triumphantly when this is aired. The other half of the world call foul on the whole thing.
posted by jacquilynne at 3:29 PM on January 30

"I won't spoil it here, but it was the opposite of what I thought would happen."

Adam and Jaime make out passionately in a pile of ballistics gel using Buster as a visual aid, Grant looks insanely jealous and constructs a robotic pnumatic cannon that shoots giant rubber dongs, Tory looks sick, Kari gets violently ill all over Grant, the plane takes off, world peace ensues and everyone gets a free puppy and an ice cream cone.
posted by loquacious at 3:33 PM on January 30 [6 favorites]

And a few of us will have strangled our spouses with our bare hands long before the show even airs ... if they don't SHUT UP about airplanes and conveyor belts.

Why, oh why, did I have to tell him it was going to be on in a few hours!
posted by Orb at 3:33 PM on January 30

Truly, this is a Savage republic.
posted by googly at 3:38 PM on January 30 [1 favorite]

Some friends and I are getting together to watch it, some will be cheering for the airplane, some for the conveyor belt. We thought of making t-shirts but then we were too lazy.
posted by Kattullus at 3:38 PM on January 30

In your dreams, loquacious.
posted by Dave Faris at 3:40 PM on January 30

Grr. I hate, haaaaaaate the delay between the US and Canadian Discovery channels. I'm so torn between the knowing-but-spoiling of leaving this thread in my Recent Activity and the ignorance-but-tension of removing it. Grr, dammit!

On the upside, Last One Standing is on here tonight.
posted by CKmtl at 3:44 PM on January 30

"I won't spoil it here, but it was the opposite of what I thought would happen."

Adam and Jaime make out passionately in a pile of ballistics gel using Buster as a visual aid, Grant looks insanely jealous and constructs a robotic pnumatic cannon that shoots giant rubber dongs, Tory looks sick, Kari gets violently ill all over Grant, the plane takes off, world peace ensues and everyone gets a free puppy and an ice cream cone.
posted by loquacious at 5:33 PM on January 30 [+] [!]

loquacious totally made up the ice cream part, but I swear the rest is true.
posted by Pants! at 3:45 PM on January 30

Tha suckas gonna fly gonna fly gonna fly!!!!!!

*does potty dance*
posted by snsranch at 3:45 PM on January 30

I'll be cheering for the airplane. Because if it doesn't take off som irate New Yorker or something will walk up behind it and scream "right side is for standing, left side is for walking" and then the plane will get all confused and quietly sob "but I am walking, really I am" and the New Yorker will go "you ain't walkin', you ain't movin'" and then the plane will feel all worthless and no puppy dogs or ice cream will help, only time can heal those wounds. :(
posted by soundofsuburbia at 3:47 PM on January 30 [1 favorite]

loquacious, you're referring to show business special episode and that myth was The Aristocrats. It was "Plausible".
posted by Free word order! at 3:51 PM on January 30 [1 favorite]

What loquacious said. Because asavage was sitting right across from me at the meetup where he spilled the beans (on a plate, even!) and that's totally what I remember him saying happened. I think. There was a lot of beer that night. And frjtz. Mostly beer, though.
posted by rtha at 3:53 PM on January 30

Will someone post a spoiler after this airs as I don't plan on watching it?
posted by davidstandaford at 3:57 PM on January 30

As a refresher, here's how StraightDope answers the question:

- Original answer

- Revisiting the answer

(Summary: Cecil says in both answers that the plane takes off, and the wheels simply spin twice as fast)
posted by churl at 3:58 PM on January 30

No matter what the outcome, the other side will call bullshit. If the plane takes off, the grounders will claim it wasn't moving fast enough. If the plane doesn't take off, the flyers will claim that the wrong type of plane was used, the runway was sub-optimal, etc.

This will not settle anything.
posted by chrisamiller at 4:00 PM on January 30

(PS - Definitely not trying to reopen the debate here, just thought the StraightDope "theoretical" answer might be fun context for seeing the "actual" experiment)
posted by churl at 4:01 PM on January 30

Hmmm? People are still interested in this? Oh my, that's right. Some of you poor chaps didn't go to the SF meet-up and have Adam tell you how they managed to construct a quarter mile long conveyor belt. How utterly provincial!
posted by team lowkey at 4:02 PM on January 30

I hate, haaaaaaate the delay between the US and Canadian Discovery channels.

Dude, download it off the internets tonight after it airs, I'm sure it'll be on every bittorrent tracker known to man.
posted by mathowie at 4:15 PM on January 30

Must keep doing Math, must not go to the nerd side... wait math is the nerd side. Never mind
posted by wheelieman at 4:19 PM on January 30

For those of us who don't live in the US and thus: a) couldn't get to SF, and b) can't watch the show live, can I ask that any discussion of the results are kept to this thread until we've had a chance to torrent the episode and watch it ourselves? That'd be totally awesome of you all, really...

I'll see you back in this thread sometime tomorrow! :-)
posted by benzo8 at 4:19 PM on January 30

Adam was at a SF meetup? I so have to move.

[not-stalkerist]
posted by desjardins at 4:21 PM on January 30

And here paulsc seems to have made quitting Metafilter his New Year's Resolution.
posted by TheOnlyCoolTim at 4:31 PM on January 30

WOOHOOO
posted by dirtynumbangelboy at 5:05 PM on January 30

If the plain doesn't take off, it's because Adam and Jamie fucked things up.

SO THERE.
posted by delmoi at 5:17 PM on January 30

Dude, download it off the internets tonight after it airs, I'm sure it'll be on every bittorrent tracker known to man.

Not, of course, that you'd condone such activity, but were merely (perhaps ironically?) pointing out one possible, entirely hypothetical, outcome to the quandary in question?
posted by Brockles at 5:24 PM on January 30

If the plain doesn't take off...

It's because large, flat, areas of land don't fly.
posted by Brockles at 5:25 PM on January 30 [14 favorites]

I've gotta wonder about the logistics of even doing this test. Even if you used a small airplane You'd still need like 1000 ft of runway to even take off. How do you make a conveyor belt that big? I'm sure there's a simple solution, but thats going to make people on one side of the myth howl.
posted by sanka at 5:33 PM on January 30

Well would you look at that.
posted by sanka at 5:39 PM on January 30

Do not leave leave your TV on the Discovery Channel after the episode of Mythbusters is over.
Smash Lab - the horror, the horror.
posted by Tenuki at 5:49 PM on January 30

Yeah, thats show's "science" is disturbingly amateur.
posted by sanka at 5:55 PM on January 30

Jason Kottke is live-blogging this episode, fwiw.
posted by gen at 5:56 PM on January 30

I mentioned this to my professional physicist father over Christmas and the discussion ended in a broken bottle, upturned chair, and a sobbing mother.

So I'm hoping that the episode comes down on my side tonight. He keeps sending me carpet cleaning bills and threating me with small claims court.
posted by robocop is bleeding at 5:57 PM on January 30 [3 favorites]

Brockles : It's because large, flat, areas of land don't fly.

One of us is doing it wrong, because I frequently find the opposite to be true.

cortex : The only thing that's going to get laid to rest is hatchets. In skulls.

It's about god damned time.

*gets hatchet*

*waits expectantly*
posted by quin at 6:03 PM on January 30

Robocop the Rabbit settles in to watch Mythbusters.

He doesn't think the plane will go anywhere. But then again, he's just a bunny and doors confuse him. I mean, how can Treat Guy just walk through that wall!??!
posted by robocop is bleeding at 6:17 PM on January 30 [1 favorite]

He keeps sending me carpet cleaning bills

Which explains the spray bottles of carpet cleaner in front of your tv.
posted by horsemuth at 6:23 PM on January 30

I'm in SF, I skipped that meetup, & I don't have cable...

The composition of this whinge was interrupted by a call from a friend, inviting me over to watch blaxploitation flicks & that just makes me happy.

You all go on & get down with your nasty selves, though.
posted by Pronoiac at 6:28 PM on January 30

oof! Make that your bad selves.
posted by Pronoiac at 6:39 PM on January 30

"Jason Kottke is live-blogging this episode"

from a treadmill.
posted by mr_crash_davis at 6:53 PM on January 30 [1 favorite]

Woo-hoo! Look at that cute little airplane go!
posted by maudlin at 6:58 PM on January 30 [4 favorites]

Maybe I misunderstood, because wasn't the thing to have it sitting stationary and see if it took off then? They should have gotten a real conveyor belt. Eh, whatev.
posted by cashman at 6:59 PM on January 30

If the anti-flight hypo had worked, the plane would have remained stationary. Instead, the propeller pulled it forward and gave it lift. Er, I think.
posted by Atreides at 7:02 PM on January 30

BUSTED
posted by Demogorgon at 7:08 PM on January 30 [1 favorite]

Maybe I misunderstood, because wasn't the thing to have it sitting stationary and see if it took off then?

Oh god. Thus totally missing the point that the conveyor can't make the plane stay stationary as it acts only on the wheels.

Proving that zero airspeed won't allow the plane to take off is a given. But the question relies on trying to manipulate ground speed to overcome airspeed. Which doesn't work. Hence why the plane is not....touching....the....floor....
posted by Brockles at 7:10 PM on January 30

CLOSE THE THREAD NOW!
posted by nowonmai at 7:11 PM on January 30

Maybe I misunderstood, because wasn't the thing to have it sitting stationary and see if it took off then?

Oh god. Thus totally missing the point...

Cashman's not the only one. For the longest time, I was totally missing the point, too. "Of course, it's not going to take off -- it has zero airspeed! What's that you say? Ohhh ... oh yeah, it would totally take off. Nevermind."
posted by Cool Papa Bell at 7:15 PM on January 30

The above comments are right about Smash Lab, btw. That show is a joke.
posted by puke & cry at 7:15 PM on January 30

I totally knew that cockroaches weren't as tough as flour beetles.

I CALLED IT!

Wait, what were we talking about?
posted by quin at 7:19 PM on January 30

Jason Kottke cracks me up.
posted by danb at 7:20 PM on January 30

I've always thought that there are two people that consider this problem and so end up on two sides:

Team A: (the plane will take off...duh!) - These are the people that don't understand how other people can't understand how planes fly.

Team B: (It doesn't take off) - These are the people that don't understand that they don't understand how planes fly.
posted by Brockles at 7:22 PM on January 30 [2 favorites]

TYPES of people, Godammit!
posted by Brockles at 7:23 PM on January 30

AWESOME.

*wiggles cheekily with smug satisfaction*
posted by loquacious at 7:26 PM on January 30

kottke:Back to the roach thing. More recapping and a little bit more setup. I don't see how people can watch this show...it's sooooo slooooow.

That's a bit unfair, the roach bit is probably the worst bit I've ever seen on here. Its uninteresting, the cast cant get into it, and theyre just killing insects like children. Normally, this show is pretty engaging, well paced, and well edited.
posted by damn dirty ape at 7:28 PM on January 30

theyre just killing insects like children.

Really? You kill children by irradiating them?
(I'm not criticizing, I'm just curious.)
posted by Horace Rumpole at 7:37 PM on January 30 [6 favorites]

I couldn't watch it on TV, but Kottke's liveblog of this episode caps off this whole months-long argument so magnificently with this gif.
posted by churl at 7:39 PM on January 30 [1 favorite]

No, what Im saying is that this seems like such a lowbrow and stupid thing for these otherwise talented people to being doing. Its a shame this will be the first impression a lot of people get for this show.


Yes, I have killed insects with radiation. Magnifying glass + sunlight = beam of ant death. Light is radiation, yo.
posted by damn dirty ape at 7:40 PM on January 30

this settles nothing. the faith based people will still laugh at the reality based people smug in their belief that the world is only 6,000 years old and evolution is just a theory, a wrong one the plane could never take off in a proper test. ;)
posted by caddis at 7:44 PM on January 30

Team B: (It doesn't take off) - These are the people that don't understand that they don't understand how planes fly.
posted by Brockles 20 minutes ago

No. I'm on neither team as stated above. Also Team B might be people who understand the problem to be one of matching forces which means the plane will not fly. Even Cecil admits this in his analysis of the problem.

As I said above, the problem is under-specified. Which is why any true experimental physicist would not touch it.
posted by vacapinta at 7:45 PM on January 30 [1 favorite]

Nothing's settled, guys. Do you really think a little thing like empirical evidence is going to sate the no vuela crowd? The partisans I know wouldn't be adverse to traveling back in time to the beginning of the universe and kicking around the protomatter until the fundamental forces cleaved their way.
posted by Iridic at 7:49 PM on January 30

Also Team B might be people who understand the problem to be one of matching forces which means the plane will not fly.

That's as maybe, but while the world relies on real, provable means of applying those forces, then the plane will take off, every single time. I believe a recent television show proved just that.

As soon as you take this question to the realms of fantasy, whereby a theoretical infinite acceleration can overcome bearings that are not (in this mythical world) also frictionless, then your plane still...takes....off. Applying fictional, yet also dependent on being selective, parameters does not make anyone right. It is either conducted purely in the real world or purely in the mathematical realm of physics. In both examples, the plane will always take off. The matching of the forces example (that Cecil alludes to as solely being the source of confusion as to the real result) would also have to make the wheel bearings frictionless if you can come up with a frictionless, infinitely accelerating conveyor belt. So trying to argue that without friction on the belt it may fly is crap, as without friction there is still no means to apply force to the plane from the belt (through the bearings) and the plane still flies. So your 'forces' argument is moot.

Or, as my normal friends call it: "Bollocks". :)
posted by Brockles at 7:59 PM on January 30

so where's the youtube?
posted by Pants! at 7:59 PM on January 30

that without friction on the belt it won't fly.

Arses. My mirth at your riposte affected my typing.
posted by Brockles at 8:01 PM on January 30

Perhaps re-posting old comments of one's own is gauche, and so too is gloating, but I'm going to indulge myself:

When the plane takes off, we will have a situation similar to what occurred when the first photographs of the earth from space were publicized. From Wikipedia:

In 1956, the Flat Earth Society found itself under scrutiny. There were various photographs of the earth from space and later the moon. Member Samuel Shenton remarked: "It's easy to see how a photograph like that could fool the untrained eye."
posted by Tube at 9:25 PM on December 11
posted by Tube at 8:13 PM on January 30

Tube, for shame. Surely you know to reign re: planes falls mainly to the vain.
posted by hydrophonic at 8:25 PM on January 30 [2 favorites]

I'm getting email as we speak that the test was unfair. Plane was too light. Tarp was pulled too slowly. Etc.

God, You'd think Kottke's readers would be smarter than this.
posted by Mikey-San at 9:43 PM on January 30

Why don't they just take the wheels off the plane set it on a platform and full throttle the engine?
posted by Jeremy at 10:41 PM on January 30

I'm watching the late-night rebroadcast right now.

loquacious writes: Adam and Jaime make out passionately in a pile of ballistics gel using Buster as a visual aid, Grant looks insanely jealous and constructs a robotic pnumatic cannon that shoots giant rubber dongs, Tory looks sick, Kari gets violently ill all over Grant, the plane takes off, world peace ensues and everyone gets a free puppy and an ice cream cone.

I already have two puppies, but I'll take the ice cream cone.
posted by amyms at 10:42 PM on January 30

Of course the plane takes off, it has wings.
Dummies.
posted by Alvy Ampersand at 10:43 PM on January 30

Flying planes off conveyor belts! That's where I'm a Viking!
posted by louche mustachio at 10:46 PM on January 30

Are the people who thought it wouldn't fly somehow under the impression that the thrust comes from the wheels rather than the prop? Because I cannot otherwise fathom how intelligent people could think that the conveyor belt does anything to prevent the plane from moving/taking off, if the prop is providing the thrust.

Also Team B might be people who understand the problem to be one of matching forces which means the plane will not fly.

What matching forces? Lateral force the wheel (with an ideal bearing) exerts on the plane = 0
posted by jpdoane at 10:49 PM on January 30

Are the people who thought it wouldn't fly somehow under the impression that the thrust comes from the wheels rather than the prop?

They come in two varieties, actually:

1) People who don't understand enough about physics to really understand what's being asked, or recognize the flawed assumption in the question as it is usually posed.

and

2) People who understand all that, but who didn't think it through and answered it wrong the first time, making it absolutely vital to their continued existence on this planet that the come up with some bizarre hypothetical interpretation of the problem such that their original answer was "right".
posted by tkolar at 11:25 PM on January 30 [5 favorites]

No, I think the people who dont think it can fly are seeing it as a logic problem.

Premise: It is stated that the conveyor belt will always match the speed of the airplanes wheels.

This means the plane cannot move forward since if it does, then the premise is violated. QED!

That is, the conveyor belt is always providing enough force (yes, applied through the wheels and friction) to keep the plane from moving forward relative to the wind. No lift - no flying plane.

i understand enough physics, thank you, to know that the entire problem is not properly stated. I'm not saying that the plane does or does not take off but that an intelligent person will quickly realize that both answers are equally valid depending on unstated assumptions. This is only a controversial problem because it is so ambiguous. Stated properly, its a high school physics problem.
posted by vacapinta at 11:35 PM on January 30

The same thing i am saying here was said only a few comments into the original mefi thread, by Grimgrin.

Of course that did nothing to stem the hundreds of comments that followed.
posted by vacapinta at 11:38 PM on January 30

Their setup was not how the problem was originally described (at least to me). It looked more like the old whisk-the-tablecloth-from-under-the-dishes trick than a conveyor belt.

As vacapinta noted, the question is so poorly formed that it's untestable. WTF does "the aircraft is moving at takeoff speed" mean? If it's at takeoff speed, it's going to take off -- that's the definition of takeoff speed.

OTOH, it's trivially easy to imagine a setup in which the airplane will not take off. Consider a belt that exactly matches the speed and acceleration of the aircraft. As the airplane moves forward on the belt, the belt moves backward such that the airplane remains directly above the same spot on the ground. In any given interval of time, the airplane has not moved forward relative to the ground.

Some people (Uncle Cecil among them) invoke the principle of summation of forces to prove that the airplane will still move forward and take off. There's thrust acting in one direction and no matching force acting in the other. That's true. A summation of forces results in a net force F = ma. Let's inspect that "a" a little closer. Acceleration (a) is the second derivative of distance with respect to time. That distance (let's call it x) is measured from the origin of a reference frame (let's call it J1). In this case, the origin of J1 can be defined as the point on the conveyor belt where the airplane started. Thus, the net force F is pushing the airplane mass m down the conveyor belt with an acceleration a in the reference frame J1.

Now we get to the sneaky part. We've defined the part of the conveyor belt that the airplane sits on as J1. However, we're standing in another reference frame fixed to the ground that we'll call J2. As the airplane is moving happily through J1, the conveyor belt is busy moving the entire reference frame J1 in the opposite direction within J2. As the airplane moves a given distance x in J1 in a given time, the belt moves J1 the same distance in the opposite direction in J2 in that same given time. Although the airplane has moved a distance x down the belt in J1, that section of the belt has been moved the other way in J2 so the airplane is still where it started relative to the ground.

When people apply the summation of forces principle, a lot of them forget that an assumption of the principle is that the reference frame is fixed or that a moving reference frame is accounted for. They perform the summation in one reference frame (J1), then they apply the result in a different reference frame (J2). In this situation, the reference frame J1 is fixed and summation of forces holds true: there's a net force that produces an acceleration in that frame. The belt moves J1 within J2 in such a way that the distance the airplane travels in J2 is zero. Therefore the acceleration in J2 is zero and the net force on the airplane in J2 is zero. No net force on the airplane means it won't move relative to the ground and therefore won't get the lift it needs to take off. (BTW, the rope argument Cecil puts forward changes the conditions of the question since it prevents the belt from moving the reference frame of the object on the belt.)
posted by forrest at 11:39 PM on January 30

forrest - you're overthinking it. The plane doesn't have to move relative to the ground. The plane has to move relative to the air. You blow a fan at the plane with enough power to produce wind equal to take off speed, and the plane will lift up. It will do this even though the plane didn't move at all relative to the ground. What's more, the moving treadmill has NO effect on the plane's motion (disregarding friction) because the wheels spin independent of the plane's motion. The plane moves forward at 50 MPH. The treadmill moves backwards at 50 MPH. The wheels move at 100 MPH (the 50 forward speed of the plane + the 50 backward speed of the "ground")
posted by willnot at 11:54 PM on January 30

There's no way the plane could have taken off if the conveyor belt was indeed moving in the opposite direction at the same speed as the airplane's forward thrust. If the plane moved forward at all then the experiment was a failure, because clearly the conveyor belt wasn't matching the forward velocity of the plane, which was the whole premise to begin with.

That, or the question was something completely different than what I originally thought.
posted by Civil_Disobedient at 12:09 AM on January 31

Or maybe not. Crap. I guess I should just watch the video, huh? Where's that youtube link, anyway?
posted by Civil_Disobedient at 12:12 AM on January 31

forrest, the force F is pushing the plane in the J2 reference frame, not J1. It is the force of the props against the air that is still with respect to the ground.
posted by zsazsa at 12:15 AM on January 31

This thread is worthless without Ethereal Bligh and paulsc.
posted by Rumple at 12:33 AM on January 31

Here's 40 seconds of cameraphone footage with the plane taking off.
posted by Pronoiac at 12:57 AM on January 31

Maybe this is easier to grok if you think of the wheels as the things that keep the fuselage off the tarmac. That's all they do. You could easily replace them with strong magnets which would suspend the aircraft above magnets of the opposite pole, like a maglev train. Move that lower surface as fast as you want to, and the plane isn't going to care--or move (well, there would be a miniscle amount of friction from the air caught between the moving belt and the stationary aircraft, so it might eventually start moving a bit in the direction the belt is going).

Or coat the conveyor belt with glycerin and make the wheels out of teflon. With sufficiently friction-free bearings (say, electromagnetic wheel bearings, or even very very good mechanical ones), you could start the conveyor up with the plane sitting there with its brakes off and it might not even move, due to its inertia.

The only way the conveyor prevents the plane from taking off is if the added friction load is greater than the power of the engine, which is not going to happen unless there is something horribly wrong--such a plane wouldn't be able to take off anyway; or if the plane is counting on power transmitted to the wheels directly by a drive mechanism like a chain or a driveshaft to get to take off speed.
posted by maxwelton at 1:09 AM on January 31

That video is awesome. Especially Adam's maniacal, triumphant laughter as the plane takes off.
posted by PercussivePaul at 1:39 AM on January 31 [1 favorite]

If the plane moved forward at all then the experiment was a failure, because clearly the conveyor belt wasn't matching the forward velocity of the plane, which was the whole premise to begin with.

The central point to understand is that it is physically impossible for the moving surface to exert sufficient force on the *plane* to counteract the thrust of the plane's propellers.

The confusion comes, in some people, when attempting to analogize this with wheeled ground vehicles on treadmills. Ground vehicles apply their motive thrust TO the surface, and thus the surface moving away from the drive wheels NEGATES the motive power of the wheel and SIMULATES a zero-friction surface to the wheel.

If you place a 7000lb plane on a sufficiently large treadmill and accelerate the treadmill back and forth you will see how little the movement of the surface affects the momentum of the plane, assuming the wheel brakes are off.
posted by panamax at 1:46 AM on January 31

that section of the belt has been moved the other way in J2 so the airplane is still where it started relative to the ground

Smart enough to use algebra and "reference frame" correctly, yet completely forgets to take the handbrake off.
posted by cillit bang at 2:01 AM on January 31 [1 favorite]

I'm sure glad that Mythbusters has finally laid this to rest and that there is no disagreement whatsoever on the matter any longer.
posted by splice at 3:01 AM on January 31

I guess Barcalounger pilots aren't so stupid, after all.

Premise: It is stated that the conveyor belt will always match the speed of the airplanes wheels.

It depends on what you mean by speed. If you mean rotational velocity, then yes, the question is impossible. If you mean 'the rate at which the wheel's center of mass moves forward', then it's perfectly possible. That's a much more reasonable definition, since the other interpretation clearly can't happen at any speed other than zero. As soon as you apply power with the engine and start the conveyor, their speeds will never again match. Since the thought experiment presumably includes engine power and a moving conveyor, it's not a reasonable interpretation.

I'm sad that Ethereal Bligh is gone. He and I went back and forth and back and forth with paulsc on this one. Ethereal, fellow Barcalounger pilot, I'll set a beer on this end of the belt for you. Grab it as goes by.
posted by Malor at 3:17 AM on January 31 [1 favorite]

If anyone still disagrees, here's another way of thinking about the problem: can a ship sail upstream?
posted by Malor at 3:19 AM on January 31

Huh, I didn't realize the plane would move forward, I thought it would be standing still as it moved on the conveyor belt.
posted by Brandon Blatcher at 4:01 AM on January 31 [1 favorite]

It depends on what you mean by speed. If you mean rotational velocity, then yes, the question is impossible.

See, this is the twisted logic that has been referred to up there ^. There isn't really any ambiguity of any sort in teh question, as both 'assumptions' will get the same result.

The plane moves at Speed 1, the conveyor belt moves the other way at speed 1. What on earth makes the wheels do anything other than Speed1 x 2? What can the conveyor possibly do to stop the plane moving?

Nothing.

If you think otherwise, you are more wrong than Mad John McWrong of Wrongtown.

The question states the conveyor matches the plane speed. It doesn't even mention the wheel speed. Even if it did, you'd need a conveyor belt of infinite speed to assume the conveyor belt matches the wheel speed. Even if it did, to do that, you have to discount any effects of friction (or the belt couldn't work) in which case the wheels also have no friction and so the conveyor belt STILL can't stop the plane moving forward as it can't react the only force moving the plane.

Yes, it'd be impossible to move the wheels fast enough in that example. But WAIT A MINUTE!!!!! You'd already got an impossible conveyor belt! You're already in the impossible, so just get over it that the wheel DOES go fast enough, or accept that the belt will, inevitably,. fail to match the wheel's speed as soon as they diverge from 0. And then read the question again and realise it has jack shit to do with the wheelspeed anyway. It's just about the plane relative to the (Not moving) ground.

Wrong, wrong wrong. Keep kicking, but if you believe even the question is flawed, and don't stop grabbing at straws by twisting it around, you are clearly in the wrong. The only change you can make to make the plane not take off instantly makes the entire system impossible and (this is the best bit) still not prevent the plane from moving unless you apply those impossibilities entirely selectively.
posted by Brockles at 5:04 AM on January 31 [1 favorite]

I'm hoping the next episode will involve putting Slinkys on escalators, and whether radioactive cockroaches can fly if they're placed on a conveyor belt.

Also, why has Kari Byron been carrying a parasol? If Kari Byron stands with her parasol on a conveyor belt, can she take off?
posted by fandango_matt at 5:24 AM on January 31

That the outcome was ever in doubt frightens me.
posted by aramaic at 6:08 AM on January 31

Premise: It is stated that the conveyor belt will always match the speed of the airplanes wheels.

Ok...

That is, the conveyor belt is always providing enough force (yes, applied through the wheels and friction) to keep the plane from moving forward relative to the wind. No lift - no flying plane.

Stop. Speed is not the same thing as force. Not even remotely.

The original problem told you the speed of the conveyor. It did not tell you the force. Why you assume the force to be the same as that provided by the propellers is not clear. It didn't come from the problem statement, that's for sure.
posted by yath at 6:25 AM on January 31 [2 favorites]

This is the only standardized test that matters. If you didn't get enough Physics to correctly answer this question by the end of high school, SORRY, NO DIPLOMA FOR YOU.
posted by sdodd at 6:34 AM on January 31 [1 favorite]

The models on the treadmill were good, but I think it would have been helpful to show us another control, where they just yanked the tarpaulin backwards while the ultralight was turned off, or simply idling. Then we could have seen the "tablecloth" trick, where the plane's wheels turned while the plane itself remained (relatively) stationary. Then it would have been clearer that what makes the plane go forward on a moving conveyor belt is the engine pulling the plane through the air, regardless of the how fast the belt is turning the wheels. Oh, well.

They were probably worried about smashing the guy's ultralight without a takeoff attempt.
posted by steef at 6:36 AM on January 31

The roaches were way cooler.
posted by cashman at 6:48 AM on January 31

forrest, you ignorant slut.

I went to bed last night thinking I had finally provided a definitive answer to something on this site. About an hour later, I woke up knowing that something was Horribly Wrong. That something was the fact that, if my proposed system worked, it applied to any type of object on a conveyor belt which means that an airplane and a car had to follow the same rules. That's something I had already discarded in previous cogitations.

The design of a propellor is such that it produces movement in a fluid that in turn produces thrust in an action/reaction pair. In the airplane's case, the fluid is air and that air is outside the J1 reference frame I defined. In essence, the airplane is pulling on a rope and I've already stated that the rope prevents the conveyor belt from translating the J1 reference frame to counteract the movement of the airplane within the frame.

So then I considered a rocket motor in a vacuum, which wouldn't require air. In that scenario, the action/reaction pair is like a pole that is pushing against one wall of the universe -- something that is also out of the J1 reference frame I defined.

What I pretty much did in my previous post is waste everyone's time with an explanation of why you can run four miles on a treadmill without even leaving the gym. Sigh, the only thing worse than having your pants pulled down in front of the entire Internet is doing it yourself to show off and then realizing that you don't have the largest penis in the world after all.
posted by forrest at 7:17 AM on January 31 [9 favorites]

lol forrest - A1 climbdown, would mock again +++
posted by benzo8 at 7:32 AM on January 31

You're right that you were incorrect yesterday -- congrats for figuring it out, and that's one of the funnier images I've run into. Are we supposed to laugh and point now? :)
posted by Malor at 7:36 AM on January 31

What I pretty much did in my previous post is waste everyone's time with an explanation of why you can run four miles on a treadmill without even leaving the gym.

But your handsome retraction and brilliant explanation of how you fooled yourself was well worth it. You win at MetaTalk!
posted by languagehat at 7:54 AM on January 31

forrest: See, I saw all those references to 'J reference planes' and was stumped as to how your logic went completely skewy. Why J? What is the significance of 'J'? It confused me for ages.

Then I figured that it was a superbly dry and sarcastic mock of the 'plane not flying' logic and the 'J' referred to 'Joke'. And it was actually a really sly answer showing it didn't work.

You'd got away with it, you fool! You were home clear! Why did you drive the bus off the mountain path???!!!!
posted by Brockles at 7:57 AM on January 31

I went to bed last night thinking I had finally provided a definitive answer to something on this site. About an hour later, I woke up knowing that something was Horribly Wrong.

This is why I pretty much keep my mouth shut in AskMe.
posted by cortex at 8:00 AM on January 31

The definitive airplane on a treadmill answer. If you think the airplane will not take off, read it very carefully. It explains why the airplane must take off if the pilot desires to take off.
posted by five fresh fish at 8:41 PM on January 31 [1 favorite]

TYPES of people, Godammit!

It's the curse of people who point out the spelling mistakes of others...

i understand enough physics, thank you, to know that the entire problem is not properly stated. I'm not saying that the plane does or does not take off but that an intelligent person will quickly realize that both answers are equally valid depending on unstated assumptions. This is only a controversial problem because it is so ambiguous. Stated properly, its a high school physics problem.

Um, no. The plane always takes off in every circumstance except for those where the treadmill is going so fast it destroys the plane entirely. The problem wasn't misstated at all.

OTOH, it's trivially easy to imagine a setup in which the airplane will not take off. Consider a belt that exactly matches the speed and acceleration of the aircraft.

NO, FAIL, THE PLANE STILL TAKES OFF.

If the plane moved forward at all then the experiment was a failure, because clearly the conveyor belt wasn't matching the forward velocity of the plane, which was the whole premise to begin with.

WRONG.

That was truly a great day for humanity.
posted by delmoi at 11:11 PM on February 2

It's the curse of people who point out the spelling mistakes of others...

Er. What? I missed a word out of my own post. I haven't corrected anyone's spelling.
posted by Brockles at 10:00 AM on February 3

“This thread is worthless without Ethereal Bligh and paulsc.”

I wonder where paulsc is?

I'm not sure why vacapinta is acting as the apologist for the no-fly crowd. Anyone who can get the right answer should get the right answer. It's stupid to attempt to answer half a physics question while ignoring the impossibility of the other half upon which it depends...and it's stupid to answer a physics question as if it were something other than a physics question. There is no convincing argument for a purported reasonableness of the no-fly crowd.

There is always ambiguity of some sort or another in almost all questions; learning how to interpret a question correctly is the greater part of being able to answer it correctly.
posted by Dances with Werewolves at 5:26 AM on February 17

The blue-eyed planes take off when the treadmill's speed equals n-1. Duh.
posted by signal at 7:07 AM on February 18

Finally saw the plane on a conveyor belt show from Mythbusters last night. Model plane took off on a conveyor belt. Got a guy in a plane who insisted that the plane would take off. He got in his plane, they ran the belt, the plane took off. Boy was he surprised. And . . .they explained why it worked just like it had been explained in this thread.
posted by Ironmouth at 7:40 AM on April 17

Huh. Meetup thread.
posted by cortex at 7:58 AM on April 17

I can't for the life of me remember why I put this into the "MetaFilter gatherings" category. Probably I meant to make it "MetaFilter related" but clicked the wrong option.

Other than that...

...I call dibs on the stale cheetos! Yum! Two month old goodness!
posted by Kattullus at 10:08 AM on April 17

Next up: Can a helicopter take off from a counter-rotating turntable?
posted by quin at 10:26 AM on April 17 [1 favorite]

The answer, quin, to that conundrum, I believe, is: your mom.

try the Fresca, surprisingly unmoldy.
posted by Kattullus at 10:30 AM on April 17