Bernoulli where are you when I need you?
March 25, 2006 7:00 AM   Subscribe

Can a plane take off?

Assume the plane is sat on a runway that can move - IE a conveyor belt - the plane moves in one direction, the belt in the opposite direction.

So as the plane speeds up so does the conveyor to be exactly the same speed but in the opposite direction.

Can the plane take off?
posted by DrtyBlvd to Science & Nature (66 answers total) 1 user marked this as a favorite
 
Nope. What matters for takeoff is airspeed, not groundspeed. If there's sufficient wind a plane can get airborne without even moving.
posted by Dipsomaniac at 7:03 AM on March 25, 2006




No. The plane will not be moving forward, no airflow will caused across the wings and therefore no change in pressure difference between upper and lower surfaces of the wings. There can be no lift or flight.
posted by brautigan at 7:05 AM on March 25, 2006


Thank you. I think. :)
posted by DrtyBlvd at 7:16 AM on March 25, 2006


Please read unknowncommand's link before answering. Yes the plane takes off. The wheels on a plane serve a different function from wheels on a car: a plane's wheels are (more or less) freely-spinning, and forward thrust is provided by air pushing through the engines. (I'm really oversimplifying here, I know.) Contrast this with a car: a car's engine turns the wheels, which provides the forward momentum. As long as the engines are functioning, the plane will still move forward (relative to the space around it) even if the wheels are on a conveyor belt; the wheels will just be spinning much faster than on "solid" ground. The plan takes off.
posted by arco at 7:16 AM on March 25, 2006




Yes, the plane takes off.

Try these related thought experiments: If the wheels on the airplane were replaced by skis, would it still take off? Sure it would -- we have seaplanes that do exactly this. What about if the runway were greased so that the airplane's wheels just slid along and didn't turn? It would take off just fine, since we would have effectively converted the wheels into skis. Finally, what if we greased up the runway and also attached little motors to the wheels that made them turn backwards? Well, the wheels motion or lack thereof is just as irrelevant as before, so the plane would take off in this case too.

That kottke thread was infuriating to read. So much ignorance, so loudly expressed.
posted by gleuschk at 7:30 AM on March 25, 2006


To people that say yes: do you believe everything you read on the internet?

How about a link to a video or something?...

Dipsomaniac said it right... Perhaps with toy planes, the propeller will generate enough localize air currents to provide the necessary lift; for real planes, the propeller makes the plane move forward, and the motion through the (mostly) stationary air create the Bernouilli lift required for the plane to fly.
posted by aroberge at 7:32 AM on March 25, 2006


Actually, to re-visit now that I've had coffee, the plane *may* lift off. With an ideal plane (no friction in the wheels, etc), it could. In real life, I doubt it. My first answer was not thought through enough. As said above, a ski plane might have more luck, although the friction on those things is a big problem.

That best answer I got is probably not one I deserve.
posted by Dipsomaniac at 7:37 AM on March 25, 2006


read martinrebas's link.

It's easier for me to think about the plane or car on frictionless ice. The car's thrust goes through its wheels, which spin on the ice. The plane pushes itself against the air and its wheels slide across the ice, not rotating.
posted by thrako at 7:38 AM on March 25, 2006


Then the plane takes off.
posted by thrako at 7:40 AM on March 25, 2006


Yes, the plane takes off. This has been covered over, and over, and over again in so many places it's not even funny any more.
posted by dmd at 7:41 AM on March 25, 2006


The straightdope link relies on some pretty esoteric definitions of "speed." Esoteric in that it isn't the kind of speed one would casually talk about in conversations where this question might be asked.

If the question is properly framed, it can be answered, as explained in the straightdope article. But, as it's casually asked here, the plane would not take off.
posted by odinsdream at 8:06 AM on March 25, 2006


there's confusion because the question does not make sense. you cannot keep an aeroplane stationary by moving a conveyor belt in the opposite direction. as soon as the plane's engine's start up, it will move forwards. increasing the speed of the conveyor belt will have no effect except spin the aeroplane's wheels faster and faster.

that's a slight idealisation, assuming there's no friction in the wheel bearings, etc etc.

the answer is completely different to a car, where the car is pushed forwards by the wheels. the wheels on an aeroplane do nothing except let the aeroplane move (at whatever speed it wants, relative to the surrounding air). the wheels in a car go round at an rpm governed by the engine.

so the question makes an incorrect assumption.
posted by andrew cooke at 8:38 AM on March 25, 2006


Maybe someone should ask the MythBusters. They accept fans' ideas for show topics.
posted by fvox13 at 9:07 AM on March 25, 2006


Paging asavage. You guys want to take on this one?
posted by arco at 9:20 AM on March 25, 2006


so the question makes an incorrect assumption.

Actually, there's nothing incorrect about the question's assumption (that the conveyer belt will move at a velocity exactly opposite of the plane's). The question at no point says that the plane will remain stationary. The problem occurs when a person reads the question, mistakenly thinks first of a wheel-propelled vehicle, and assumes that the plane will remain stationary.

The question is of course set up for the reader to make that incorrect assumption—that's the trick—but the question doesn't make that assumption itself.
posted by DrJohnEvans at 9:22 AM on March 25, 2006


Try these related thought experiments: If the wheels on the airplane were replaced by skis, would it still take off?

My favourite was the rollerblades-on-a-treadmill one from the kottke thread.
posted by DrJohnEvans at 9:24 AM on March 25, 2006


The confusing part is this:
as the plane speeds up so does the conveyor to be exactly the same speed but in the opposite direction.

Note that the plane is speeding up. It does not say that the plane stays motionless with respect to the Earth's surface, or to the conveyor, or to anything else. Suppose that the plane reaches 50 mph. The conveyor is also going 50 mph, in the opposite direction. That doesn't mean the plane stops; it just means the wheels are turning at 100 mph. the plane is 50 mph closer to takeoff speed than when at rest.

The mistake is in assuming that the conveyor's speed cancels the plane's speed. It doesn't.

Unless the conveyor's speed is so great that friction in the landing gear slows the plane below takeoff speed, the conveyor can't keep the plane from flying.
posted by Kirth Gerson at 9:25 AM on March 25, 2006


I'm with andrew cooke. The incorrect assumption is that the conveyor belt will move fast enough to keep the plane stationary with respect to the ground. This will not happen; the jets or propellers will create thrust which will move the plane forward regardless of the conveyor. Assuming zero wind, the plane will create its own air speed, which will equal the ground speed (not the conveyor speed) and the plane will take off. The wheels will just be spinning twice as fast until takeoff. If the conveyor were reversed, the plane could take off without the wheels moving at all, or with the wheels moving backwards.
posted by weapons-grade pandemonium at 9:26 AM on March 25, 2006


I get the sense this is one of those non sequitur riddles, like the classic "which one weighs more, a ton of feathers or a ton of concrete?" question. The conveyor belt doesn't matter because all it acts on is the wheels of the plane. The landing gear wheels are there only as a low-friction support to facilitate takeoff; replace them with square blocks and the plane could still take off (give enough power), even if those blocks never rotate.

So what would happen is the plane would take off as normal, but its wheels would spin much faster than usual because of the two forces acting on it, the conveyor belt and the thrust of the plane.

Also, I don't think the question is making any false assumptions; the reader is, hence the mindfuck quotient.
posted by chrominance at 9:28 AM on March 25, 2006


bleagh. ok, the question doesn't explicitly make the assumption. i thought it was a genuine question, not a game. but yes, if you think this is a "trick question" rather than an honest misunderstanding then the question is fine and the inconsistency is in the user's head.

[of course, it still won't take off because you assumed there was an atmosphere and really there's not... god how i hate "games" like this.]
posted by andrew cooke at 9:44 AM on March 25, 2006


Also, at no point does the question state that the plane has wings. Or wheels. So yeah, I've always just assumed this was a meme written by some physics snot somewhere who loves seeing people get tricked.

Although I would like to point out that the level of discussion of this thread is many orders of magnitude higher than that of the kottke thread.
posted by DrJohnEvans at 9:49 AM on March 25, 2006


After reading more carefully the statement, I realise that I made a possibly unwarranted assumption. The original statement says:

So as the plane speeds up so does the conveyor to be exactly the same speed but in the opposite direction.

This is not possible for a freely moving conveyor; for this to occur, the conveyor belt must be moved by a motor to keep up with the plane. I took it for granted that it was a motorised conveyor belt which was talked about ... but this does not appear to be the case ... and the conveyor belt can not be moving at exactly the same speed but in the opposite direction (unless it is an idealised frictionless belt).

If there is a external motor that keeps the belt synchronised, then the plane does not take off. If there isn't, then the plane will move forward. To see this, replace the "propeller pushing against the air" by as "external rope pulling the plane" as the original source of energy. Will the plane move forward? The answer is yes in this case ... (unless a *motorized* belt moves in the opposite direction and, through friction with the wheels, provides an equal but opposite force to that of the rope, keeping the plane stationary.)
posted by aroberge at 9:51 AM on March 25, 2006


aroberge, the belt is irrelevant. "the plane speeds up . . ." The belt speeds up, too, but it does not slow the plane. If it did, the plane would not "speed up". When the plane speeds up to takeoff speed, it flies.
posted by Kirth Gerson at 10:06 AM on March 25, 2006


Ditto, aroberge. Doesn't matter that the conveyor is motorized. If the plane needs an air speed of 200 mph to take off, the top of the conveyor belt will move at 200 mph in the opposite direction with respect to the earth, and the plane will move away from a point on the surface of the belt at 400 mph. The underside of the belt will match the plane's speed and direction.
posted by weapons-grade pandemonium at 10:19 AM on March 25, 2006


aroberge,

Read above. It doesn't matter what the conveyor is doing, whether it's motorized, stationary etc.

The plane's jet engine acts on the air around it, and not the wheels. The powerful thrust from the engine will drive the plane forward through air (increasing its groundspeed as opposed to conveyor speed) while the wheels will merely spin faster to compensate for the conveyor's opposite direction of travel. The only assumption made which affects anything is that the wheels' coefficient of friction needs to be very low - that they don't create sufficient drag as to impede the plane's forward motion.
posted by jimmythefish at 10:20 AM on March 25, 2006


ac is correct, as usual. The whole idea of groundspeed is completely irrelevant to an airplane wing having lift or not. In the context of the question, the convey belt will simply make the wheels go around faster than usual.

Planes can and do lift off with 0 ground speed. You don't leave a small Cessna parked on the runway in a gale force wind: it can flip over.
posted by bonehead at 10:21 AM on March 25, 2006


Well, if you want to get *really* picky, the conveyor belt will create a moving airstream going in the same direction as the belt, but at a lower speed. So conceivably if the belt moves fast enough the plane actually could get airborne, even if it was actually moving backward WRT the earth - as long as the airstream is moving fast enough over the lifting surfaces.

I suspect that the belt would at least have to break the sound barrier, though.
posted by Dipsomaniac at 10:22 AM on March 25, 2006


Here's the thing... if the conveyor moves backwards at the plane's airspeed (assuming no wind), or at the plane's speed with respect to the surface of the Earth, then the wheels turn twice as fast as they normally would, the plane moves normally relative to the air, and the plane takes off, assuming it doesn't blow a tire. The conveyor belt is completely irrelevant. This is the obvious, sane definition and the one that was probably intended when this dadgum defective riddle was invented.

If the conveyor belt is defined to move backwards at a speed equal to the plane's speed relative to the conveyor, or the plane's "wheel speed" (a rather tortured definition not really appropriate to the situation, but whatever... we'll run with it) then the plane will maintain zero airspeed and will not take off. The thrust of the engines will go to counter the backwards force provided by wheel friction. Since this force is very small compared to the thrust of the engines, the conveyor belt will move at hundreds and hundreds of miles per hour to balance the thrust, perhaps even at supersonic speeds, and will do so as soon as the plane releases its brakes. (Obviously, we're talking about an ideal conveyor here, not one that could actually be built.) The plane's tires are not designed to withstand anything close to supersonic groundspeeds, and will burst. (This would surely occur significantly before Dipso's moving airstream, which would probably be too turbulent for takeoff, came into play.) The plane will then hit the conveyor and Bad Things will happen. Cecil claims this definition of the conveyor speed represents a paradox and is completely invalid, which is true if and only if you ignore wheel friction, as Cecil's questioner specified.

So, as usual, Cecil's right on this one.
posted by musicinmybrain at 10:46 AM on March 25, 2006


In the previously mentioned (painful) discussions about this, the best analogy I saw was: What if you are wearing roller skates and standing on a treadmill? Not only can you stand still, you can use your arms (a separate force) to pull yourself forward.

Just replace the skates with the plane wheels, and your arms with the prop (or jets). The plane moves forward, and takes off fine. The friction just slowed it down a bit and the wheels spun faster than normal.
posted by SpookyFish at 11:52 AM on March 25, 2006


I'm assuming the brakes are on because what sort of idiot would go to the trouble of building a runway sized conveyor and then set the plane up so that the conveyor is unable to exert any force on it?

The plane won't take off because it won't move forward and there wouldn't be enough air moving over its wings.
posted by cillit bang at 12:10 PM on March 25, 2006


cillit, what sort of idiot would try to take off in a plane with the brakes on?

I know it's been said, but some people still don't seem to get it, so I'll contribute my breakdown:

1) A runway is built as a conveyor belt. It is independently powered and motorized, and configured such that if the plane moves forward, the belt moves backwards an equal distance at an equal speed.

2) The airplane's wheels, by design, are free-spinning. They are also designed to exert as little friction as possible on the airplane.

3) The forces which move the plane forwards during take-off are the result of the propellor or engines acting on the air around the plane.

In a typical take-off procedure, the pilot will apply the brakes, apply the throttle, and release the brakes. On release of the brakes, the forces caused by the propellor pulling air towards the plane causes an equal-but-opposite force on the airplane, moving it forwards. The wheels, being free-spinning and low-friction, allow the plane to roll forwards. As the plane gains forwards speed, the air moving around the wings gives the plane lift, allowing for takeoff.

The fun part of this is that the ground is completely irrelevant. An airplane, at rest on the runway, facing into a sufficient headwind, could easily gain enough lift to take off. Airspeed matters, not ground speed. In fact, even in the air, if you are flying into a strong headwind, it is entirely possible for you to be travelling at a significant airspeed, yet be traveling backwards relative to the ground.

So now we put the pilot and his airplane on the conveyor runway. He released the brakes, and plane is pulled foward by the propellor, which is acting on the air in front of the plane. The conveyor begins moving backwards, exerting a force on the wheels. The wheels, being free-spinning and having little friction, fail to exert any significant force on the airplane.

The airplane thus continues taking off as usualy. The effect of the little friction that the wheels have may cause the need for an extra 5-10 knots, no different than taking off into a slight wind.
posted by CrayDrygu at 12:49 PM on March 25, 2006


Lets save the trick questions for April 1st or something..
posted by Chuckles at 1:17 PM on March 25, 2006


You of course know this CrayDrygu, but I'm an ass and I feel like trashing as much of your explanation as possible, just to feel better about myself. :-)

"The effect of the little friction that the wheels have may cause the need for an extra 5-10 knots" you say. Extra knots would translate to increased airspeed over the wings and as you pointed out, being on the conveyor belt would not require that. It would need more thrust and consequently more gas (too overcome the tiny amount of extra friction from the imperfect bearings), but it wouldn't need to go faster--relative to the surrounding air.

I'm such an ass, sorry, but I do feel better about myself now.
posted by JeNeSaisQuoi at 1:23 PM on March 25, 2006


Straightdope is down for me at the moment but they did a follow up to the original which goes into more detail.

In any case, you could make the plane not take off if the conveyor belt could provide an equal and opposite force on the plane than the thrust provides. This would require an accelerating conveyor belt and would be tremendously difficult to achieve, given how little friction there is in the wheel bearings.
posted by 6550 at 1:27 PM on March 25, 2006


CrayDragu, my point was you can make any set of assumptions about things that aren't mentioned in the question and come up with any answer you damn like, and then pretend you're smarter than everyone else for getting the "right" answer.
posted by cillit bang at 1:52 PM on March 25, 2006


JeNaisSaisQuoi: You're right. What I meant was, it would require more effort from the plane to achieve the same airspeed.

Also, there's something about your name, but I can't quite place it...
posted by CrayDrygu at 2:06 PM on March 25, 2006


(aside from the fact I misspelled it)
posted by CrayDrygu at 2:07 PM on March 25, 2006


I think what most people are confusing with this question is how the airplane moves the air across its wings. Unlike a glider, where, to take off at 0 wind on a flat surface it must be dragged by a vehicle, the airplane is capable of generating its own wind and therefore lift.

If this were a glider with a towing vehicle in front, the conveyor would (powered or passive), with 0 wind, cause the glider NOT to take off as no wind is being "generated" from the ground speed.

(Yes, I was confused at first, until I thought about it. I assume with ridiculously large enough engines an airplane with its brakes on could take off from a stand-still...)
posted by shepd at 2:10 PM on March 25, 2006


If you had a submarine sitting on a giant belt-sander, with magnesium strips glued to the bottom of the sub, could you roast marshmallows?
posted by Kirth Gerson at 2:10 PM on March 25, 2006


no, shepd. it's not about blowing air across wings, which is what i think you think. it's about the plane moving forwards no matter what the conveyor belt does.
posted by andrew cooke at 2:30 PM on March 25, 2006


Or consider a helicopter. It has zero translational motion. Yet when the angular velocity of its blades reach a certain value, then it will take off.

Now think what would happen if that helicopter was placed on a rotating turntable with a matching angular velocity. Then with a an equal magnitude but opposite direction angular velocity.

Considering pathological conditions where the random kinetic motion of the gas molecules (which usually indicates random motion that is far, far greater than the average drift velocity of the particles) is significantly reduced is also interesting.
posted by meehawl at 2:45 PM on March 25, 2006


If you had a submarine sitting on a giant belt-sander, with magnesium strips glued to the bottom of the sub, could you roast marshmallows?

Given than a magnesium flame is pretty hot (hotter than any blowtorch), I'd say chances are poor. Your going to get chunks of charcoal, not s'mores. Also? Long stick. No, longer than that.
posted by bonehead at 2:58 PM on March 25, 2006


(you're)
posted by bonehead at 2:59 PM on March 25, 2006


The plane would not take off because the air is not effectively moving at any crucial point, which is what causes the plane to lift off the ground. If the air itself were moving and the plane were stationary, it would, because the air is what lifts the plane (such as in a wind tunnel where the object is relartively fixed). The ground has nothing to do with it -- it's just a visual side effect, as it were, like the perceived position of the sun when riding a bicycle -- the sun doesn't propel a bicycle, per se. The propeller, such as for a biplane, whirl so fast that the air is swum thru, whereas the wings are merely rudders.
posted by vanoakenfold at 3:41 PM on March 25, 2006


I wrote that assuming that the plane was stationary and the conveyor belt was moving underneath it quick enough in a pulling-the-tablecloth-from-underneath-the-dishes maneuver. If the conveyor belt was moving the plane forward, the plane would still not take off because once the plane even has the slightest lift, it would immediately fall back to the belt because its source of propulsion is the ground, not itself, so it would slow back down/drift due to gravity and the fact that the propeller is slowing the plan down by not turning -- like a rowboat slowing down when the paddles are just held perpendicular in the water without rowing. The difference in this conveyor belt and say, one of those little hook things on aircraft carriers that hooks to the jet when it takes off is just to give the jet an escape velocity of sorts -- but the jet itself is making its own away-from-the-belt propulsion whereas the plane is propelled solely by the belt, alone.
posted by vanoakenfold at 3:49 PM on March 25, 2006


I read the question again, and those two prior answers were based on the belt itself being the driving force, rather than the plane propelling itself. If the plane's propeller, such as with a biplane, were fully functional and operating as the propulsion, I wager the belt wouldn't even move in any direction because it is not the friction upon which the plane is hindered -- the air is. The belt would have minimal (at best, perhaps more if it is bumpy) impact on the plane's ability to take off. The wheels do not drive the plane forward as an automobile does, just as air flow does not propell an automobile forward (indirectly, in some cases perhaps, such as downward pressure of the reverse-lift that generates better traction for the tires).

Say for instance there was a diver at the bottom of a pool that wanted to get up the side of a pool but there was a ladder on the side that moved downward in an escalator fashion that wouldn't let him rise to the surface any quicker if he used the ladder. Swimmers don't use ladders to travel thru water, they use the water. Once they begin using the water as the propulsive matter, the direction of the ladder's movement becomes irrelevant.
posted by vanoakenfold at 4:00 PM on March 25, 2006


Consider a jet airplane on a conveyor belt whose acceleration exactly matched that of the propulstion from the jet engine. Air pressure is inversely related to airflow. I.e. the greater the airflow, the less air molecules actually push on the surface of the wings. This is how airplane wings work. Because the top portion of the wing is curved, the air travels faster over the top portion of the wing relative to the bottom. In this manner, the air pressure on the bottom of the wing is actually greater than the air pressure on the top, pushing the wing upward. However this only works with air that is actually moving! In the conveyor belt example, the airflow over the wings remains constant, because the plane is staionary relative to the air surrounding it. Therefore, there would be no lift, and the airplane would not take off.
posted by Nquire at 4:26 PM on March 25, 2006


The only interesting thing about this question is how poorly framed it is, allowing reasonable people to disagree.

I think musicinmybrain summed it up well. But that wont stop N more responses each forcefully re-stating one of the two views.
posted by vacapinta at 4:44 PM on March 25, 2006


vanoakenfold.. i think you may have your physics confused. A row boat doesn't slow down because you stop rowing.. it slows down due to the friction of moving through the water.

The plane would take off (notwithstanding friction on the wheels) because it creates its own acceleration.

I think shepd's analogy about a glider is a good one. That's an example of something that wouldn't take off becuase it's only exerting force on the ground, not on the surrounding air like a normal aircraft does.
posted by bruceyeah at 4:47 PM on March 25, 2006


Nquire... why do you assume the plane stays stationary relative to the air around it? The question doesn't state that fact.

If the aircraft is pushing on the air will move forward - unless something is stopping it - and that's why the friction is the wheels becomes relevant.
posted by bruceyeah at 4:51 PM on March 25, 2006


The plane's jet engine acts on the air around it, and not the wheels. The powerful thrust from the engine will drive the plane forward through air (increasing its groundspeed as opposed to conveyor speed) while the wheels will merely spin faster to compensate for the conveyor's opposite direction of travel. The only assumption made which affects anything is that the wheels' coefficient of friction needs to be very low - that they don't create sufficient drag as to impede the plane's forward motion.

That's the tricky thing about this thought experiment, is the premise that... "as the plane speeds up so does the conveyor to be exactly the same speed but in the opposite direction." What this implies is that the force of the conveyor belt exactly matches the force of the jet engine, keeping the net horizontal force at zero. Because of this, the plane fails to accelerate forward through the air. It's analagous to putting a plane in front of a wall - no matter how powerful the thrust on the plane, there's an equal and opposite force on the plane from the wall.
posted by Nquire at 4:54 PM on March 25, 2006


Another way to think about this problem is backwards. Assume the airplane is flying 100 mph and lands on a belt moving backwards at 100 mph. Do you think the airplane instantly comes to a stop (as all of the passengers go flying through the windshield)? No, of course not, and that is exactly the same situation when the airplane takes off.

Fortunately for the passengers, the landing airplane continues to roll past the terminal at 100 mph with the wheels spinning at 200 mph on the belt.
posted by JackFlash at 5:06 PM on March 25, 2006


The propeller, such as for a biplane, whirl so fast that the air is swum thru ...

Actually, any pilot will tell you that the propeller is just a fan to keep the pilot cool -- because if it ever stops he really starts to sweat.
posted by JackFlash at 5:24 PM on March 25, 2006


From The Straight Dope.
"A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily."

And...

"However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way."

I stand humbly corrected, unless we agree that the conveyor belt can move at infinite speed? Uh, nevermind.
posted by Nquire at 5:32 PM on March 25, 2006


Nquire: "as the plane speeds up so does the conveyor to be exactly the same speed but in the opposite direction."What this implies is that the force of the conveyor belt exactly matches the force of the jet engine, keeping the net horizontal force at zero.

I think part of the magic of this question is that it tricks people into equating speed and force, as well as confusing ground speed with airspeed.
posted by KirkJobSluder at 5:34 PM on March 25, 2006


JackFlash: Ha!
posted by vernondalhart at 5:38 PM on March 25, 2006


Actually, the point is moot.
The wheels of the airplane and the wheels driving the conveyer belt would instantly accelerate to the speed of light, each creating a singularity that will crush the airplane, runway, and most of the surrounding country-side. The singularities will then combine, sink to the center of the planet, and slowly consume the rest of the earth.

So, umm, let's not have Mythbusters try this one, eh?
posted by Eddie Mars at 7:26 PM on March 25, 2006


So far we've had jet planes, biplanes, Cessnas, gliders, conveyor belts, rollerblades, treadmills, rowboats, water, air, pavement, and some clown threw in a submarine and a belt sander. I'm waiting for bees.
posted by Kirth Gerson at 7:28 PM on March 25, 2006 [1 favorite]


Cecil thinks you doubters are dum. [sic]
posted by sachinag at 10:08 PM on March 25, 2006


Actually, the question is flawed: it calls the belt a conveyor belt, but in fact the belt does not convey.

I could tell you many more pearls of wisdom to aid in understanding this question, but suddenly I am destroyed by a swarm of bees.
posted by ikkyu2 at 11:23 PM on March 25, 2006


I feel a little obliged to clarify some background relative to my question - firstly, thank you *all* for flexing your intellect on this - it wasn't my intention to cause such a thread, as I actually didn't know it was considered to be a standing conundrum - I was sent the question by a friend and we were arguing amongst ourselves as to 'yes' or 'no' being the correct answer.

I thought (expected!) there would be a straight forward answer - it seemed such a simple question to my non-physics oriented mind!

Despite searching on here, I actually didn't google it... lesson learned there - my apologies for inadvertently causing furstration.

I concur with DrJohnEvans regarding the quality of the answers - and thank you again for taking the time to provide such.
posted by DrtyBlvd at 4:22 AM on March 26, 2006


I don't understand why the does-flyers get so wrapped around the axle regarding the mechanics of the conveyor belt. This is a thought experiment, a word problem, its condition is a belt which precisly negates the plane's forward speed. Given that condition (even if it wouldn't be practical or even possible in the real world) no Bernoulli, no lift, no takeoff -- in other words, I agree with Nquire, and I'm surprised Cecil doesn't get it.
posted by Rash at 8:31 AM on March 26, 2006


But on preview, looks like Nquire has humbly corrected himself, whereas Cecil (in his Basic Realization #2) has returned to my conclusion (which is shared by a pilot I discussed this with).
posted by Rash at 8:47 AM on March 26, 2006


Rash, please re-read the context of Cecil's Basic Realization #2. Here's the belt specification from the question here:

So as the plane speeds up so does the conveyor to be exactly the same speed but in the opposite direction.

And here's your version of the belt spec:

its condition is a belt which precisly negates the plane's forward speed

These are two entirely different situations. If the belt is simply moving at a velocity opposite that of the plane (as in the first spec), the plane will take off, as not enough backwards force will be exerted on the plane to affect its forward motion. If the belt exerts enough force to negate the plane's forward motion (as in the second spec), then of course the plane will not move and not take off, as per Cecil's BR #2.

However, Cecil's BR #2 was not a moment of enlightenment with respect to the original question: it was in response to the question, "Is there a way to set up the conveyor so that it overcomes the thrust of the engines and the plane remains stationary and doesn't take off?" This is not the original riddle. The closest thing to the original riddle we have is from that page on The Straight Dope, which has the following belt specification:

This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

This is the same as the spec defined in this AskMe question. So under the original specifications, the plane takes off. It does not take off only if you modify the specifications to answer the question, "how do we make it not take off"?

(Also, I think you misunderstood Nquire, but I'll let him correct that.)
posted by DrJohnEvans at 10:14 AM on March 26, 2006


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