MythBusters tackles "plane on a conveyor belt problem"
Last night I spoke to Adam Savage, co-host of MythBusters. He's in Alaska, where current temperatures range between 0 to 30 degrees below zero. He just finished a test on "cabin fever" (he wouldn't say how long he lasted before murdering his cabin mates and running out into the frozen wilderness wearning nothing but longjohns and a sleeping cap) and is now working on another episode up in the 49th state.
But the real reason I talked to him was to find out about the next episode of MythBusters (airing this Wednesday). It tackles the famous "plane on a conveyor belt problem," that has pitted brother against brother and friend against friend for years.
To get to the bottom of the thought-problem, Adam and Jamie used a real plane, in this case a 400-lb ultralight, and a large conveyor belt. Did the plane take off? You'll have to wait until Wednesday to find out, but Adam said even the pilot guessed wrong. Link
"zero to thirty degrees below zero"
... Celsius or that weird Fahrenheit-thingy scale? It makes a difference, you know!
Mythbusters fucking rocks. They should repackage their shows for a high school physics curriculum, each busted myth demonstrating a specific principle.
Fahrenheit.
"So you think we could make a lead zeppelin?
Jamie FTW.
My 6 and 3 year old watch this with me, it's one of their favorite shows. I'm actually a little concerned why Smash Lab is on. It's like the execs said, "Well, we like Mythbusters, but they don't always blow stuff up. Can we just make a show where they always blow something up?"
It's kind of scary the amount of debate the initial article started. It's really not very difficult to visualize... but the mind is a terrible thang!
Of course the plane would take off. What matters is the airflow over the wings to generate lift, not the speed at which the ground is traveling. Planes obviously aren't powered through the wheels like cars. (duh!)
btw, it doesn't take a "treadmill" to prove this. A take off or landing with a tailwind also has the effect of making the ground move opposite the vector of travel. Also, the wheels (and treadmill) would simply rotate at double the rate of forward speed.
@#1
If only the summary read "zero to forty degrees below zero" I could have usurped a clever line from Futurama:
Leela: Fry, night lasts two weeks on the moon.
Moon farmer: Yep, drops down to minus-173.
Fry: Celsius or Fahrenheit?
Moon farmer: First one, then the other.
Airspeed. You either have it or you don't.
If you don't, you stall.
Is the prop/engine power/volume able to pull a large volume of _air_over_the_wings_? If so, takeoff occurs.
Questions about whether the treadmill is ideal or practical, whether it accelerates infinitely or can keep up with the wheels, whether the wheels catch fire or not - beside the point.
In short: The entire formulation misses the real point.
If you were to put a real aircraft on a real treadmill that really traveled the same speed as an aircraft taking off, only in reverse, the aircraft would take off. Period. The wheels would be rotating at twice the speed of the aircraft, but assuming the wheels/tires don't explode from the stress, you'll get airborne.
Frictional losses would not overcome the engine thrust. The MTOW and runway length required may be different, but assuming an aircraft that is not overloaded, it'll fly.
Saying that their experiment misses the point because it ignores "ideal" treadmills misses the point that magical super treadmills don't exist.
It's a good thought experiment and it gets you thinking about what forces are really in play.
I have to second the smash labs thing I had it on the DVR for one epsode and it was BAD! mythbusters is way better diffrent shows I know but man the hosts on smash labs just seem OFF some how. My whole Fam loves MythBusters its number 1 on the DVR I even watch the comercials some times
The treadmill thing means the plane isn't actually moving - just it's wheels are.
If it's not moving, then the air isn't moving accross it's wings.
No air moving accross the wings means there's no lift.
No lift = no flying.
I really don't see why this is so hard for people to understand.
More Kari Byron!
Bardfinn has it. It's all about bernoulli's principle. Lift is generated by the difference in pressure between the top and bottom of wings. The curvature of the wings makes a low pressure area over the top, and bam! we have lift. Thus, with respect to the wings, at least, there has to be moving air.
Nevermind the ideal characteristics of the treadmill, or the wheels, or whatever. Does the plane physically move forward with respect to it's original position? If no, then there has to be some other source of airspeed for the plane to take off.
In the case of a jetplane, where the jets are in the tail of the plane, or are generating thrust behind the plane as in a jumbo jet, there is no airspeed on the lift generating device, the wings. Thus, no takeoff.
The only way these jetplanes that "must" generate takeoff that also "work in space" to work is if they are pointed at the ground, generating lift. However, the question is even dumber then, so let's say that the engines are pointed parallel to the ground.
If the engines are pointed parallel to the ground, you might as well have a giant team of midgets on each wing, off the conveyor belt, pushing for dear life.
I guess, however, RL engines do generate airflow that cannot be perfectly parallel to the ground, so real engines must generate some push against the ground....
damn my lack of cable!
i would be thrilled if you posted their results here on wednesday.
Daemon, why would the treadmill keep the plane from moving?
What force is counter acting the thrust from the engines?
so, waitaminnit -- when is this "cabin fever" show, and can we really see adam (or jaime?) in longjohns and sleeping cap? i can't be the only gay man in america who wished i was that piece of tape running across jaime's mustache in last week's episode...
Oh, and while I do like the idea behind Mythbusters, and the people involved, they have a lot of problems with the setup of some of their experiments. Some of them are great, but others are really not testing what they think they are testing.
Replace the landing gear with casters, and replace the conveyor belt with a moving surface that randomly changes direction (0 - 360 degrees) and speed every second. The plane will take off.
Daemon,
You are mistaken. The plane isn't propelled by its wheels; it's propelled by its, well, propeller...or engine. What will happen is that the plane will move forward just like normal (from the perspective of a stationary observer on the ground); the only difference will be that the wheels will be spinning very fast because of the conveyor belt. It
#10, read the problem again, specifically:
The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction.
So when the aircraft is moving 10 MPH forwards, the conveyor belt is moving 10 MPH backwards.
Since the wheels on an aircraft are free-wheeling (that is they do not provide propulsion), the wheels will just spin at 20 MPH.
The common misconception is due to people thinking in automotive terms, where the wheels are providing propulsion. In an aircraft that is not the case.
I think this website has the best explanation of what will likely happen:
http://www.modernpolymath.com/journal/2008/1/28/airplane-on-a-conveyer-belt.html
Okay my 2 cents. If the plane moves with respect to an observer on earth, ie. (plane's velocity) = -(velocity of the belt) with all w.r.t earth, then the plane will have airspeed and therefore lift off. However if the plane is held stationary w.r.t to earth by increasing the speed of the treadmill (very impractical) it is still possible for the plane to take off because the treadmill will push the air too creating a sort of wind tunnel.
Put the plane on a treadmill, engines off. Turn the treadmill on. The wheels on the plane spin, but the plane stays still.
Exactly, guys.. For example, a Boeing 747-200 has 219,000 foot-pounds of thrust in its engines. All that thrust is transferred through the air - it has nothing to do with the wheel rotation speed. No wheel bearings on earth would transfer enough friction to counteract 219,000 lb-ft.
If the plane isn't moving then there's no lift. No lift means no flight.
scisco,
You're sort of on to something, but the belt wouldn't move the air like that. The only thing that matters is the plane's velocity relative to the air around it. So if you had a massive fan in front of the plane creating a wind at air speed velocity, the plane could lift off while remaining motionless relative to the ground. It's like when you see hawks gliding in place because of a strong wind.
Guysmiley is correct. Some friends and I have debated this over and over on another (aviation-related) board. The airplane will take off and its wheels will be rotating at slightly less than twice their normal speed. Its groundspeed will also be slightly slower than normal (owing to friction). Bonzo - if you put the plane on a treadmill with engine off, it will move with the treadmill because there is just enough friction for that to happen. You could literally hold the airplane still with one hand though, and in that case its wheels would spin.
Unlike a car, where the wheels push against the pavement, an airplane engines push against the air. The wheels are irrelevant to an airplane's forward motion and are present to reduce friction between the plane and the runway.
When running on an exercise treadmill we stay stationary relative to the room because the force generated by our feet pushing against the treadmill belt is not transfered to the floor. If we were to strap on a Buzz Lightyear rocket pack while running on the treadmill and light it off, it wouldn't matter how fast the treadmill was going because the rocket would be pushing against the air and away we would go.
Simply put, the wheels on an airplane act as a lubricant between the plane and the runway. If the treadmill did counteract the forward motion of an airplane than it would be impossible to take off on an icy runway.
reminiscent of "what do rockets push against in a vacuum?"
It all comes down to: does the treadmill go at a fixed speed inverse to the plane's normal land speed (resulting the situation described in #18), or do they accelerate to compensate? In the latter case, you get a feedback loop which jumps to infinity and then depending on how theoretical our setup is, maybe the plane catches fire or maybe it takes off.
the real question is, "will being on a treadmill stop a plane from moving forward?"
@RV9FACTORY.... "if you put the plane on a treadmill with engine off, it will move with the treadmill because there is just enough friction for that to happen."
That depends on how quickly the treadmill starts. If it accelerates quickly enough, you'll be able to do the whole "tablecloth out from under the dishes" trick, yes?
Pushing against air is an oversimplification. Propeller driven planes are pushing against the air. Jet engines achieve thrust because the exhaust is pushing against the engine so the presence air is not relevant unless it is required for combustion.
For those that think the plane wouldn't take off:
Start running on a treadmill.
Then get a friend, not standing on the treadmill, to give you a big shove.
Let us know what happens.
The plane takes off.
The treadmill always moves at the speed the plane is moving forward. The wheels spin at twice the speed the plane is moving, unaffecting the speed of the plane, or the flow of air over the wings.
It's the same as pushing a matchbox car over a treadmill. You, as a person, use the same amount of force to push the car forward, no matter if the treadmill is on or not. You are the "engine" in that case, completely independent of the treadmill.
Jccalhoun: Being on a treadmill will not stop a plane from moving forward. As I asked earlier: What force would be counteracting the forward thrust from the engine?
> The conveyor belt is designed to exactly match
> the speed of the wheels, moving in the opposite > direction.
>
> So when the aircraft is moving 10 MPH forwards,
> the conveyor belt is moving 10 MPH backwards.
> Since the wheels on an aircraft are free-wheeling
> (that is they do not provide propulsion), the
> wheels will just spin at 20 MPH.
Right, but read the instructions again: "The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction." That means the conveyor belt is moving at 20MPH because the wheels are. But, that means the wheels are now spinning at 30MPH, repeat, repeat, repeat.
> The common misconception is due to people
> thinking in automotive terms, where the wheels
> are providing propulsion. In an aircraft that is
> not the case.
No, you aren't understanding what's really going on in this problem. The problem is this: the question is framed ambiguously. We all know that the plane takes off due to the airspeed. The question is this: is the plane moving relative to the world? The question is ambiguous - some people say "yes", and some people say "no". If the plane is moving relative to the world, then it has an airspeed, and it will take off. If the plane is not moving relative to the world (and we ignore windspeed and the effects of the propeller pushing air under the wings), then it has no airspeed, and it will NOT take off.
The odd thing is that aircraft use propellers / jets to push themselves forward on the runway, not wheels. So, when the question says, "The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction." - the ONLY way that can happen is if the aircraft has zero airspeed. If the aircraft has any airspeed at all, this becomes nonsensical because the spinning of the wheels *cannot* be the same as the movement rate of the conveyor belt. To put it in other terms: the speed of the wheels = airspeed + conveyor belt speed. However, the question says "the speed of the wheels = conveyor belt speed", so what about the airspeed? Is it zero? If the propeller/jet is pulling the aircraft through the air, then the airspeed will not be zero and so, the speed of the wheels is NOT equal to the conveyor belt speed. People create different answers because the question can be interpreted in two very different ways. However the Mythbusters "test" this, they're going to take one or the other interpretation - and whichever interpretation they take will be "the winner".
I think most posters missed the point. The assumption in the experiment is that one can get a treadmill to behave as desired, i.e. keep the plane stationary with respect to the ground. That is a daunting task. Never mind if it requires powered movement by the belt or a freewheeling belt. Airplanes fly because of lift, which is created by relative movement of air past the wings. If no air is moving, there is no lift and no flight. If the treadmill design doesn't work and the plane moves down the runway, of course it will fly. Rockets do not rquire wings to fly, so don't include them in the discussion. That's vertical flight anyway.Bottom line, in the experiment as described, the plane will not fly. Anyone want to place a bet?
The airplane treadmill seems to generate a lot of debate b/c the problem is poorly worded and generates confusion. Depending on how you read it, that treadmill is either no problemo or an infinite impossibility.
I originally read it to mean the treadmill was somehow preventing forward motion of the plane relative to the air, so that the plane was essentially taking off vertically. That wouldn't work, and I think that's where the confusion comes in -- not that people think of planes driving like cars, but that they think air isn't moving over the wings.
As others point out, there's really no way a treadmill can pull this off with aircraft wheels. If the problem isn't constraining the movement of the plane relative to their air and the rest of the earth, the wheels and treadmill are irrelevant -- they wheels are only there to minimize the friction between the ground and the plane, and the treadmill has little effect. This seems to be how most people read the problem in the first place.
#33, that's an excellent answer.
This problem isn't just a test of logic. It seems to also test how well you can see things from other people's point of view.
The winners realise that there's an ambiguity, because it isn't specified whether the aircraft is allowed to move relative to the ground and, by extension, the air.
The losers get a coronary from shouting at each other.
Brit, no matter which interpretation you use, there needs to be a force counteracting engine thrust in order to keep the plane still. What force would that be?
Neither interpretation involves enough force to hold the plane still. The treadmill is not connected in any meaningfully solid way to the aircraft. It has no method of acting with enough force on the airplane to keep it still.
The forces I can see are:
-Thrust
-Drag (Wind Resistance) - same as usual
-Wheel bearing friction - slightly increased
The only change in forces involved is an increase in wheel bearing friction due to the higher speed of the wheels. As long as we're not jumping into lala land with theoretical instantly accelerating infinite speed capable treadmills that melt the tires, wheel bearings, landing gear, and finally the underbelly of the plane, the plane will move forward just fine. The difference in takeoff speed and travel distance will be minimal.
>>>If the aircraft has any airspeed at all, this becomes nonsensical because the spinning of the wheels *cannot* be the same as the movement rate of the conveyor belt.
As long as the wheels are in contact with the belt, their speed will be /exactly/ that of the belt. You can speed the belt up as much as you like and the negligible friction in the wheel bearings won't really slow the plane down with respect to the airmass it is moving through.
Since the conveyor belt in Pogue's original thought experiment provides an equal counterforce to the acceleration the plane's engine would normally create, the plane will remain stationary with respect to the atmosphere.
Because the plane is stationary with respect to the atmosphere, and because lift is a product of the friction of the atmosphere against the lifting surface of the wings caused by the relative motion of the plane through the atmosphere, the plane will not fly.
If you placed the same plane in a windtunnel and locked its wheels then forced air over its wings at the same velocity the plane achieves at takeoff the hypothetical pilot would be able to pull back on the stick and leave the ground. This is how hang gliders work.
>>>reminiscent of "what do rockets push against in a vacuum?"
Nothing. But that's not how rockets work (by pushing against something). It is simple Newtonian physics (mass ejected from the back of the rocket * the mass' velocity) causes an equal but opposite reaction from the rocket.
But we digress...
#33: If you read it that way, the plane STILL TAKES OFF. If the conveyor belt keeps the wheels from turning, the aircraft still has a velocity with respect to the Earth, it has "indicated airspeed" and it takes off.
Assuming you could control the treadmill speed perfectly, to an observer on the ground it would look like the aircraft is just moving along the conveyor with the wheels not moving.
There's nothing magical about it, it's simple physics.
The important thing is how is the air moving realitve to the wing's upper surface? There is no air moving over the surface of the wing if the wing is stationary. A plane whose forward momentum is being translated into the treadmill's surface aquires no lift. The only air that's moving is caused by the propeller, other than the "hug" air that is dragged along the treadmill surface due to micro vortices caused by the friction of the tread next to stationary air.
Dangerous, Pogue wasn't the originator and completely misunderstood the problem and the forces involved. There is no way for a conveyor belt to provide an "equal counterforce" to the thrust of the airplane's engines. So it will not be stationary with respect to the atmosphere.
Dangerous, the original question (as I found it) was posed thusly:
“Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?"
The belt matches the speed of the wheels. This makes sense as long as they are in contact with each other. The real question is: What do the wheels have to do with how fast the plane is moving through the air? The answer is: precisely nothing.
#5 FTW
The plane will take off.
#42: No, totally wrong.
What force is counter-acting the thrust from the engine(s)? The wheels free-wheeling below it? Nope, sorry. Unless the brakes are applied, in which case you aren't taking off anyway.
Either way, the wheels are (nearly) frictionless and unaffected by the motion of the belt.
The. Plane. Takes. Off.
I'm with Putney on the practical issues. The landing gear are in no way "frictionless", and there will be sufficient friction between the landing gear and the belt in pretty much any system for the belt's drag to keep the plane from moving forward under thrust, especially with a mere ultralight engine providing that thrust. There are two ways to get the plane to take off:
1) Reduce the friction between belt and plane. Magical frictionless bearings in the wheels would work, but the only real world option would probably be some sort of air-cushion/"hover" landing gear.
2) Overcome the friction between belt and plane, with sufficiently powerful engines or rockets.
"What force would be counteracting the forward thrust from the engine?"
Friction in the wheels.
The whole idea here is that the plane is moving very fast with respect to the treadmill, but at zero velocity with respect to the ground and the surrounding air (except for whatever boundary layer is whipped up by the treadmill).
The funny thing is, people seem to think that the plane can have an arbitrarily high propeller velocity, and that at some point the thrust from the propeller will be enough to overcome the friction in the wheels. Friction, though, is a harsh mistress, and increases as lubricants break down and internal temperatures increase.
The wheels could be frictionless, and that ideal but not realistic expectation would make it perfectly evident that the forward thrust is being translated by those wheels, right into the tread which moves at equal but opposit speed realitive to the plane. The energy required to lift the plane in now bulding up as heat in the tread and will need to be disapated with air movement and some good liquid coolers as well.
People need to be reminded here that there are TWo -- count 'em, TWO -- questions floating around the Internets.
Treadmill matches WHEEL speed.
Treadmill matches PLANE speed.
Even the original BoingBoing article quoting David Pogue glosses over that the question had changed. Pogue asks about the treadmill matching WHEEL speed. Yet the BoingBoing article was updated with a Straight Dope article with the treadmill matching PLANE speed.
The outcome of the Mythbusters test is going to rely on which question they actually tested, since there are two questions with very important differences.
If the treadmill matches PLANE speed, then the treadmill can go 100 mph backward while the plane goes 100 mph (relative air speed) forward and the wheels spin at 200 mph. The airplane is therefore moving, can get lift, and can take off.
If the treadmill matches WHEEL speed, then the treadmill can go 100 mph backward while the wheels go 100 mph forward and the plane goes 0. The treadmill can go 200 mph backward while the wheels go 200 mph forward and the plane still goes 0. At any point, if the plane starts moving forward and the wheels are still in contact with the treadmill, then the wheels will have to be moving faster than the treadmill. At that point, it's not about physics anymore, it's about semantics. You immediately violate the rules of the question and the experiment if, at any time, the wheels move at a different speed than the treadmill, which is the only way the airplane could gain relative air speed and lift.
#48, 49: Good lord.
You're saying that if the treadmill either
a. Rotates opposite the motion of the aircraft at equal speed rotating the wheels at twice the speed of the aircraft
or
b. Rotates at the same speed as the aircraft, keeping the wheels from rotating
that will somehow generate as much force as the propulsion from the aircraft engine?
The thought experiment says NOTHING about the motion of the aircraft with respect to the ground. It says the treadmill moves with respect to the WHEELS OF THE PLANE. It also handily can be read two different ways.
Again to be clear: THE TREADMILL SPEED IS NOT RELATED TO THE GROUND SPEED OF THE AIRCRAFT. "speed of the wheels" is NOT "speed of the aircraft".
Either way you read it, the plane takes off. Either with the wheels spinning at 2x the ground speed or spinning at 0 RPM.
High school physics 101.
Here is my 0,02€:
"The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction."
Assuming we can disregard the friction of the wheels, here is what will happen:
Lets say the thrust of the engine gives the airplane an airspeed of 30 mph (not enough to take off), and holds that speed.
That means the wheels are moving at 30 mph. So the conveyor has to move in the opposite direction, and starts and speeds up to 30 mph.
But since the airspeed still is 30 mph (remember we are holding that speed), what happens is this:
While the conveyor belt accelerates from 0 to 30 mph, the wheels accelerate from 30 to 60 mph (the speed of the conveyor belt + the air speed).
So the conveyor belt accelerates from 30 to 60 mph, while the wheels accelerate from 60 to 90 mph.
So the conveyor belt accelerates from 60 to 90 mph, while the wheels accelerate from 90 to 120 mph.
And so on and so on...
And thats at a fixed speed!
If the airplane accelerates, the numbers 'accelerate' as well.
In an ideal world, this is what would happen:
The conveyor belt is able to react to any changes in speed, instantly, and the microsecond the thrust of the engine tries to push the airplane forward with any speed (0,0001 mph), the conveyor belt will accelerate to that speed, the wheels will accelerate (0,0001 + 0,0001 mph), the conveyor belt will accelerate further and so on, towards the speed of light.
In the real world, this will happen:
The conveyor belt is a bit slower to react (i.e. slower than 'instantly'), and will be a little behind in accelerating to "exactly match the speed of the wheels, moving in the opposite direction."
Thus the plane will move forward and take off.
But the rule:
"The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction."
is not met, because it is practically impossible for the conveyor belt instantly and all ways exactly match the speed of the wheels.
Wheel bearings are made specifically to reduce friction between the vehicle and it's wheels. They are very good, and very low in friction, especially on something like an aircraft. If there was a lot of friction in the bearing, the aircraft would have trouble overcoming it without the presence of a conveyor belt. They do not have that trouble, and freely take off all the time. Even doubling the friction, which is roughly what the case would be with our hypothetical situation, would not significantly slow the aircraft.
The conveyor belt will "exactly match the speed of the wheels," which would merely double the speed of the wheels. That would not hold the aircraft still.
@40
"The term working mass is used primarily in the aerospace field. In more "down to earth" examples the working mass is typically provided by the Earth, which contains so much momentum in comparison to most vehicles that the amount it gains or loses can be ignored. However in the case of an aircraft the working mass is the air, and in the case of a rocket, it is the rocket fuel itself."
reaction mass = working mass, rockets and planes both
The plane's forward motion is provided by something that doesn't touch the ground, so being on a treadmill doesn't make any difference.
Imagine if you were sitting on a skateboard on a sheet of ice, or a treadmill, or whatever. Imagine you had a rope tied to a tree and in your hands. If you pull on the rope you will move forward, no matter what the wheels are doing, no matter what the ground is doing, no matter anything.
Kevtastic said, "You immediately violate the rules of the question and the experiment if, at any time, the wheels move at a different speed than the treadmill, which is the only way the airplane could gain relative air speed and lift."
It should be obvious that if the plane moves faster than the tread mill, it will run off the mill and roll onto solid tarmac, probably catastrophically.
KEVTASTIC:
That "wheel speed" interpretation is meaningless and collapses to infinity very quickly. If that's what the statement is intended to be interpreted as you mine as well argue about "what happens when you accelerate when you are traveling at the speed of light?"
if I shoot an arrow at a tortoise, will it ever hit it?
@rv9factory: so that's where the confusion lies. I always understood the problem to mean: the conveyor moves back at a speed that is high enough to keep the plane stationary. No moving of wings through air -> no lift -> no flight.
Are there any pilots out there? Air speed over wing surface. You know: http://en.wikipedia.org/wiki/Bernoulli's_principle
You can stand right next to a planes that's thrusting it's ass off while on a treadmill. You hold out your hand and feel the air doing what over the wing's surface? The air isn't moving, and the plane isn't flying.
Arguing the physics of wheel bearings and friction and the like obfuscates all points.
Grab a Hot Wheels car, set it on a treadmill, and prove both questions instantly.
(a) While your finger's resting atop the Hot Wheels car, the treadmill speed is matching the WHEEL speed and there's no forward motion of the car. The car is going ZERO. Turn the treadmill on a higher speed. The treadmill and the wheels will both spin faster, but there will still be zero movement by the car, and thus no lift and no flight.
(b) Push the Hot Wheels car forward, the equivalent of thrust. The wheels spin freely, and so long as they're in contact with the treadmill, they will travel exactly the combined speed of (1) the forward motion of the car, plus (2) the reverse motion of the treadmill. When the treadmill matches/is identical to the speed of the PLANE, the wheel speed is essentially double the individual speed of either the plane or the treadmill. Notably, the car moves forward quite easily, and therefore can generate lift, proving it can take off. Also worth noting is that when the car starting moving forward from "thrust," the wheel speed no longer matched the treadmill speed (it instead was the treadmill speed plus the plane speed), violating the rules of the version of the question that requires the treadmill matches the WHEEL speed.
GUYSMILEY:
That, of course, is the problem with the whole scenario.
The original question floating around for years and years and years has always been about whether the treadmill matches the PLANE speed. Aviation magazines I've read answered this question and ignored wheel speed questions. Straight Dope did likewise.
It seems to be a relatively recent phenomenon that this turned into a wheel speed question, which IMO is a semantics question, not a physics question.
"Last night I spoke to Adam Savage..."
That's got to be a great sentence to type.
how do float planes take off?
how do helicopters and VTOLs like the Osprey convert to forward flight from a hover?
Tanner, BitTorrent and/or NZB+USENET could be your very good friends, when it comes to watching that particular episode of MythBusters, as well as MythBusters in the future.
Indeed, using this link:
http://www.cnet.com/8301-13739_1-9775271-46.html
you can automate the whole downloading procedure and have every new episode of MythBusters delivered to your computer every week.
DO read the comments, the author of the article left out an important step in the process, the needed step is mentioned in the comments.
Thrust from the plane is acting against the air, not the pavement. The propeller/jet will move the plane forward whether or not there is a treadmill underneath it.
The speed it achieves will be unrelated to the treadmill, because the wheels are only there to keep the plane from damaging itself while on the ground. I hope Mythbusters have a really really long treadmill, or else the plane will simply pull itself off the end and continue along on solid ground.
The way I envision it is swimming in a fast moving current. You may not be able to swim fast enough to move against the current, but if you had an anchored rope you could pull yourself along. The rope is the air, the water is the treadmill.
On a tangential note.
Here is what will happen. The plane will lift off of the treadmill a few inches and then fall back down. Because it is an ultralight it may even lift off several feet, stall out and fall back to earth because it will have zero air speed.
Imagine a plane resting on a treadmill, both are at rest. The plane does not take off because there is no such thing as magic. You start the engine and the plane begins to accumulate forward momentum but it doesn't move with respect to the ground because the treadmill applies an equal but opposite force to the plane's forward momentum. If the equipment is well designed the plane will remain stationary with respect to the ground. As the propeller gathers speed there may by enough air volume flowing over the wings to create some lift but because it has no air speed it can only lift straight up, stall out and then fall back down.
If this was possible we would already be doing it since it would be a great way to launch an airplane or jet from a ship. The fact that we are not doing this is strong circumstantial evidence that it doesn't work. You'll notice that we use a catapult, the exact opposite of a treadmill, on aircraft carriers.
Case closed.
Noen, your "case closed" doesn't counteract your misunderstanding of the physics involved.
The treadmill has no means with which to "[apply] and equal but opposite force." It is only contacting the wheels, which are more or less free to spin however fast they wish.
A treadmill does not move the air around it except for a minuscule amount against its surface. Why do you think it would?
As Mr. Miyagi might say:
"Ice... not fight back!"
"the pilot guessed wrong" - the plane must fly then since pilots are not very smart.
The genius addition in the question is the airplane. If it was a jet engine strapped to a soap-box car and people were asked if it would remain stationary there would be far less debate.
A sad comment on the state of the teaching of science given some of the answers above.
Just two days until the mystery will be revealed once and for all, and still another discussion erupts ... I'm surprised, but I very much suspect that I shouldn't have been.
Once again, quite a few people here are confused. Maybe that's because they've never seen the actual airplane-on-a-treadmill brain-teaser, but instead the bastardised nonsense version that was previously featured on bb.
Back then, I said:
If you're interested and don't mind a spoiler, go to the original bb story and read the full version of my explanation, quoted under the heading "Nex says". If you want no spoiler at all, you should have stopped reading about 10 lines above :-) If you want just a little hint: I'm quite sure my explanation is correct, because I've posted it to several lenghty forum threads chock full with people who enjoy logic puzzles, and no one ever refuted it, though some actual pilots (or so they said) did try. And lo and behold, Mr. Savage says "the pilot guessed wrong." I think that's a bit scary, actually.
@72 -Noen:
"Case closed."? Perhaps you'll learn why you're wrong once you realize that you're not necessarily right...
@72-Noen:
>>>Case closed
Well geez, that settles it then. Someone with no grasp of the mechanics and physics of what's going on has declared the debate over. LOL.
Once again, what does the wheels' interaction with the treadmill have to do with how fast it is moving in relation to the ground and/or the air? The answer is nothing. The wheels spin freely and do not impede the aircraft's forward motion through the air.
#73 dculberson
"The treadmill has no means with which to "[apply] and equal but opposite force." It is only contacting the wheels, which are more or less free to spin however fast they wish."
The wheels are connected to the plane are they not? They are therefore a part of the equation. If the forward force is 8ft per second and the reverse force is -8ft per second the combined forward force is zero and the plane stays where it is. It is assumed that the treadmill is designed so that it exactly counters any forward momentum. Good wheel bearings should be able to handle anything a prop can throw at them.
"A treadmill does not move the air around it except for a minuscule amount against its surface. Why do you think it would?"
I never said it would. You are misreading me.
#74 trieste
"If it was a jet engine strapped to a soap-box car and people were asked if it would remain stationary there would be far less debate."
Not the same at all. Some of the thrust from a jet exhaust comes from the force of the exhaust leaving the body of the jet and some comes from the exhaust pushing against the mass of the surrounding air. I think that a jet on this treadmill would take off because it is pushing against everything, the air, the ground and the treadmill. A comparable analogue would be to place a large vacuum directly behind the jet exhaust and designed so that it sucks in the exhaust gases as fast as they are expelled. In that hypothetical, the jet would not move or only move very little.
I just have to get that off my chest...I live in Alaska, and I hate when people refer to the weather in Alaska as if the whole place is exactly the same. It's twice the size of Texas, and for the record it was a balmy 32 degrees in my hometown last week when I left to go back to school.
http://dilldoe.org/blog/?p=119
Comic strip which shows the outcome of a Plane on a Treadmill!
I'm quite convinced that this experiment will cause the universe to implode. Hurry Up!
Noen, the problem does not specify that the airplane remains stationary. Where do you see that specified?
Also, where do you see it specified that it is a propeller airplane?
#28: Phlogiston.
#78 Noen: I await in eager anticipation your explanation of how rockets work in space.
I was going to write some uniformed opinion, but I see . . . oh, nevermind . . . I'll wait until Wednesday evening to find out what Jamie and Adam come up with.
#76= EdT.
""Case closed."? Perhaps you'll learn why you're wrong once you realize that you're not necessarily right..."
Oh come on, I said that mainly to get people's goat. This is not that big of a deal. Chill.
For this MythBusters example the plane is a prop plane and it will not move forward because a prop generates forward momentum by the screw action of the propeller. Imagine that you are on a treadmill and you try to move forward by pulling on an infinite rope. You will not move forward because for every step and pull on the rope the treadmill moves back. It would be a different matter if the rope were attached to the treadmill. In that case you could pull yourself forward. In this example it isn't connected to the treadmill and you stay put.
This reminds me of a critical thinking word problem like we had in grade school. The purpose of those problems is to let people condense out the important information and discard the rest.
Since a plane must move forward through the air to generate lift in order to take off, The question can be reduced as follows:
"Can a plane that is on a treadmill that keeps it from moving forward move forward?"
The answer becomes obvious: "No".
The problem states that the conveyor belt matches the speed of the wheels, only in reverse. It doesn't say whether this is practical, it just states it. Since the wheels can't move forward, and the wheels are attached to the plane, then the plane can't move forward. If the plane can't move forward, then it can't take off. I don't know of any plane that can take off with zero forward movement. If there is one, then for that specific plane the answer is the "The plane takes off". For all other planes, which I believe is all of them, the answer is "The plane does not take off".
Done.
plane takes off
#63 -
FWIW, Jeff, Bernoulli's Principle does not explain airfoil lift. The Wikipedia article you cited actually explains this rather well:
A common misconception about wings
I never heard this before, but right off I don't understand why there is even any discussion. I picture the runway as a maglev track (like the trains) and the plane has superconductor magnets instead of wheels. It will float above the runway and take off, when thrust is applied. It doesn't matter what the ground is doing. Why would the wheels behave any differently?
An important part of 'critical thinking word problems' is comprehending the meaning of the words. This is tricky when there is no sensible meaning and you're trying hard to interpret the question in a meaningful way. proper critical thinking gets you there, though.
This argument seems to make people go crazy. Half the people here are right and half are hopelessly tied up in knots. As soon as the plane starts to move forward at all (which it must do, with nothing actually holding it back), the wheels and the treadmill will both immediately accelerate to infinity. The premise is flawed.
Consider the inverse: if the plane isn't moving forward, then it follows that the wheels aren't moving and the treadmill isn't moving either. Does it make sense to visualise a plane sitting there, blasting its engines on full thrust with nothing happening?
I'm glad we sorted that then.
I can't help but recall my high-school physics here. Primarily because my physics teacher was a fan of the SIN contest (http://sin.uwaterloo.ca/index.php). Anyhow, he always preached "draw the FBD!" Which is a free-body diagram. Ba