I did. You are just not paying attention.
Like I said, if Cecil cannot explain it, what good can I do?
You said a bunch of stuff about the belt being impossible to build and aircraft carriers. If you wrote that answer on a test you would get a big fat honking F. Just, cmplete the sentence:
The airplane accelerates with a magnitude of _____.
I don’t care about whether its possibile to build the belt or orders of magnitude or any of that stuff. Just finish the sentence.
Well if you don’t care what is possible then sure. And pigs can fly too.
:dubious:
Its almost as though you are avoiding the question becuase you can’t answer it.
**“A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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). Can the plane take off?” ** *the original question by Berj A. Doudian
*
This problem gives us a “unique” conveyor belt, not a unique plane. If we use a “real world” airplane, the plane will not take flight. Berj Doudian stated that the wording of the question is difficult, “but the spirit of the situation is clear.” Doudian at the end of his question to Cecil added frictionless wheels, which knocked out the whole spirit issue. And since then, other posters have added monstrous engines, new laws of motions, and even Albert Einstein. If the question includes frictionless wheels, I’ll have to agree with Berj and Cecil…but if it doesn’t, then the conveyor belt and the plane are not independent.
Wheels on planes, as Cecil stated, are to reduce friction…but that doesn’t equal frictionless. In order to be frictionless, the plane itself would need to be weightless. If the plane is weightless, why does it need wheels, it’s flying already. This appears to be the way Frankenberrycat (post #19) sees it with the post “if the engines weren’t on and the wheels were free-rolling, the plane could sit in one spot without moving.” It also appears that the wheels are flying too because (under the current laws of physics) if the wheels, the plane, or any other object is sitting on the belt it would move with it, not spin unless held aloft.
Cecil’s example about the health club treadmill, rollerblades, and rope is an interesting example, but didn’t give me any guidance on how fast I need to climb off the treadmill for flight. But yet I have no stall speed, planes do. A plane would have to exit the belt with enough speed for takeoff. I would guess that I, on a treadmill, could pull not only myself but brother (who doubles my weight) at the same time. Does most planes have that much excess power? This draws a question…As takeoffs are when planes need the most power, do we have a big market for aircraft that produce way more power than they will need? It’s not like every airport has treadmill runways. Putting a big powerful engine in a small plane might sound fun but between the weight and the fuel cost to run it, it wouldn’t be very efficient. On the other hand, a 747 only needs all its engines on takeoff, it could cruise on half, but I don’t think it has very much excess takeoff power seeing it weighs 200 tons empty. When NASA piggybacks the Shuttle (which combined is still well below the 747s maximum takeoff weight) they need an extra 1000ft to become airborne. Once airborne, it can only fly(with all four engines) at 15,000 not the normal 30,000. It doesn’t appear that even a 747 has grossly more power than it needs, does a Cessna or Piper?
As the scenario, as ask, described the conveyor belt but not the plane, I think it is running against the “spirit” of the question to make this some futuristic plane. To keep to the spirit of the question we should use a “stock” plane that you would find at any real airport. Real planes don’t have frictionless wheels and overly powerful engines. Planes are design to have enough power to takeoff, climb, and stay aloft. A plane normally is at full throttle at takeoff, so the idea of some “magical” overdrive that kicks in on a conveyor belt is adding something to the scenario. As for the wheels, they are not only reducing friction, but also help maintain the planes momentum. If we replace the conveyor belt with an inclined runway you will see that not only are the wheels transferring more friction onto the plane, but the wheels are not helping to build momentum. While at first glance this might appear like a ramp, it strains the engines to get the plane aloft. This was experimented in World War II and since, the FAA has refused to certify any new inclined runways.
Anyway, it is the adding facts not given in the question I see as the problem. That is like answering the question “Could you shot someone on the moon if you had a long enough barrel?” with YES, because you can use a laser guided missile bullet. The question didn’t ask you to use a special bullet just a special barrel. Same on the original question, special conveyor belt not special plane.
I answered your first question, “Where would the plane go?”
Why do you think the force of the belt even matters? Force equals mass times acceleration. It is the velocity or speed of the belt that is supposed to be able to hold the airplane back by applying enough force through the wheels according to you. But that is not even what the puzzle states as written in Cecil’s column.
Any force it can apply to the airplane can only be applied through the near frictionless wheel bearings. Any force transference would be miniscule. In fact it would be many orders of magnitude lower than the force being used to move the belt. I’m sorry if you don’t like that, but it is true. Like I said, it is not possible for a belt to be built that can move at a speed fast enough to transfer enough force through the wheels to the airplane to hold it still.
But like I said. You apparently don’t care what is possible.
It is like asking, what happens if you accelerate yourself to the speed of light? Well, that is not possible either.
I’m sorry I must have missed it. Can you just give me a number, it seems I am having difficulties wading through your post to find the aanswer. Just a number would be nice.
I will answer your question if you tell me how much force you believe will be transferred through the wheels from the belt to the body of the airplane including the wheels. And then you must tell me if you believe that it is possible to build a belt that can do that.
If you believe that it is possible to build a belt can transfer through its free spinning wheels enough force to hold back the airplane and its wheels, then you are wrong.
But like I said, you don’t seem to care what is possible.
You want a magic belt that can do the impossible.
Fe, for a jet? No, but then again I don’t think we could build a mile long treadmill that could support a jet period.
Ok, so once again, the magnitude of the acceleration is ____
Why do you need a mile long treadmill. If it is supposed to hold the airplane still, it only needs to be long enough to fit the wheels of the airplane on it.
Your question is unanswerable until you tell me exactly how much force you believe can be transfered through the wheels to the entire airplane? And remember it has to be a real world possible number. In order to hold the airplane still, it must equal the force of the engines. That is not possible in the real world. Maybe in your make believe world. I live in the real world and deal with what is possible.
In the real world, the airplane will move forward at almost the same velocity as it would on a stationary runway.
Don’t ask me what would happen in an impossible situation. The answer is not a number. The answer is, it cannot happen in this world.
The point is that its a thought exercise. Depending on how you read the question the airplane either takes off or it doesn’t. The fact that we can’t build a proper treadmill in no way diminishes in the case of the airplane taking off from the fact that the plane does take off.
This is part of the problem you have to solve.
Dude quit bullshitting. I’ve given you every piece of informaiton that you need to answer the question. You keep talking about the real world in a discussion about launching airplanes off of treadmills for god’s sake. This is simply a thought experiment in an ideal world. I have a magic treadmill, indestructable wheels and the whole shebang.
I am going to say it flat out, I think you are wrong and I simply don’t think you can answer the question correctly. Stop with the real world concerns and just do a calculation for an ideal system. I accelerate my treadmill at a rate of 2*(Force of the engines)/(Mass of tires) in the opposite direction of the Force of the Engines. What is my acceleration?
I meant to say it would move forward with almost the same acceleration as a stationary runway.
It will reach takeoff speed in the real world.
I’m sorry, but I will not entertain magical conveyer belts.
Magic does not exist in the real world.
Cecil almost had it when he cited the lack of lift for the wings, but it got away from him. The correct answer is that the plane could take flight, but it would take one heckuva big engine to do it. You don’t need an imaginary treadmill or tether or any other device to solve this one. Consider this - imagine the plane has no wings. After all according to Cecil’s analysis it is the engines/jets that are providing the power of flight independent of the wings. If a 747 simply had its standard jet engines strapped to its side with no wings (and we assume for the sake of argument that balance is not an issue) could the plane take flight. Of course not, contrary to Cecil’s conclusion the lift of the wings is needed, the engines alone would have to be rocket boosters to get the plane of the ground without any assistance.
Christi7df, you and treis are talking at cross purposes. The cause of this is that there are various versions of the question and there are various assumptions one can make about the conditions pertaining to the scenario set up by the question. treis has stated this several times and I have no difficulty at all understanding the version and conditions he is talking about. But you are not likewise clearly stating your position.
1/ treis is talking about the version of the question where the conveyor accelerates to match the tangential speed of the outer edge of the wheels of the plane, whatever that speed may be.
2/ treis is talking about a thought experiment in which one assumes that it is possible to build a conveyor that will be able to carry out 1/. Practical difficulties notwithstanding.
If you are talking about some other version of the question, or if you are wanting to bring the real world difficulties with building such a conveyor into it, then you are simply not on the same page as treis.
treis has stated certain conclusions about a certain question based on certain assumed conditions. There is little point in you denying his conclusion, while talking about a different question based on different assumed conditions.
Are you on the same page as treis as regards 1/ and 2/ or not? Yes or no?
And I see that you have now clearly stated you are not on the same page as treis.
Why you would spend post after post arguing at cross purposes with someone who has clearly stated they are acting on certain assumptions while you act on certain other assumptions, I have no idea.
I know exactly what treis is talking about. Treis is saying if you have a magic conveyer belt that can move fast enough to completely cancel out the force of the airplane engines through only the free spinning wheels of the airplane, then the belt would hold the airplane still. I say that is not possible in the real world.
I take the spirit of the puzzle as stated in Cecil’s column. I hold to the matching forward motion in reverse as being the reality part of the puzzle, but some people interpret this as meaning that the airplane is held still on the belt by the reverse motion of the belt.
If you want to twist it into the ridiculous, then you can torture the puzzle to say that the belt can move at ridiculous speeds as Cecil stated. But if you can give the belt ridiculous magic capability to hold the plane back, then why can’t I give the airplane ridiculous magic capability to overcome the force of the magic conveyer?
If he can do that with the belt, then why can’t I do the same with the airplane? Then you have a paradox as Cecil stated.
I prefer to stick to real world possibilities.
You have misunderstood Cecil’s answer. The aircraft moves forwards using the engines, it then attains airspeed and therefore airflow over the wings. It then flies.
Some Guy, an airplane’s engines do indeed power it, just as Cecil said. Cecil never said that an airplane’s engines provide lift, because in fact, they don’t. And I don’t think that anyone in this thread will dispute that an ordinary airplane with no wings won’t take off (though I think that’s about the only thing that nobody in this thread will dispute).
Meanwhile, it is true that due to the rotational inertia of the wheels, a treadmill constantly accelerating at a ludicrous rate could stop the airplane. However, the acceleration needed for such a feat is far beyond the conception of the original question, and the treadmill would quickly reach such a speed that air entrained to the treadmill would be sufficient to lift the plane. And, of course, once the plane is off the treadmill, it’s going to stay off, ludicrous acceleration or no.
Thats pretty weak. It also rings pretty hollow that you say you don’t deal with magic conveyer belts in a thread about airplanes taking off of treadmills. You say that a plane can take off of a treadmill like this but thats not in the real world either. There is no treadmill in the world that is 11,000 feet long, can move 500 mph and support a 400 ton plane. I don’t know why you can deal with this magical treadmill but not another.
Its unfortunate though, you have done a lot in this thread to further the cause of ignorance and its unfortunate you won’t even fight it in yourself.
I have been very patient and answered every question you have asked of me. The minimum polite thing to do is answer the one question I have posed to you.