Plane on a coneyor belt
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beautyful sketch, David!
I like the political criticism
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David_H : exceptionally funny!
Modelhead : exceptionally concise!
pav
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To all the non believers, i will attempt to demonstrate why you are wrong.
Imagine a rocket flying parallel to the ground. Now imagine it has some wheels attached, everything's good so far. Now imagine the wheels happened to be touching a conveyor belt moving in the opposite direction to the rocket, the rocket will still be moving forwards and the wheels spinning but at a speed greater than that of the rocket.
Make any sense?
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Jakob,
Yes it would move backwards if it's engines weren't running, but the point is that as soon as they are...and it overcomes a slight amount of inertia in the wheel bearings...then whatever the conveyor is doing has no relevance at all to the plane. It will move forwards through the air at pretty much normal speed...however fast the conveyor is moving....and eventually take off.The problem with the question is that it presupposes that the plane would be able to be kept at a single point in space with its throttle wide open, simply by the conveyor moving backwards matching its speed. This simply isn't true; the wheels effectively freewheel, so the motion of the runway is irrelevant. The thrust of the engines would not just compensate for the backward motion of the ground; the plane would HAVE to move forward....and it doesn't matter that the conveyor increases its speed accordingly
You have to kind of imagine the plane being suspended by a wire. The only real mechanical interaction it has with the conveyor is the relatively small amount of drag in the wheels. Once the forward thrust is sufficient to make that drag relatively insignificant, it will simply take off as normal with its wheels spinning madly. The plane is not clawing itself forward aginst the conveyor; it's clawing its way forwards through the air, which is not affected by the conveyor. In other words it's motive power is not connected to the conveyor at all...hence the analogy of the winch.
Once it is accelerating it only has to overcome the relatively tiny amount of friction in the wheels. This friction is largely unaffected by how fast the conveyor is travelling, which is why I said in my original post that the conveyor could be travelling at light speed...it makes no difference.
Modelhead...absolutely right...in reverse.
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Alan, I see it better now after that last explanation of yours. Its not about the lift. Planes don't need lift to start moving forward. And once they are moving forward, their ability to gain more speed has nothing to do with the speed at which their wheels are turning, since the wheels just spin freely. The belt can keep speeding up, but the wheels just spin faster, but they don't hinder the forward movement of the plane.
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ok. we agree with the fact that the plane, if it's engines are turned off and the conveyor is moving, will be transported backwards, right?
so if the plane accelerates and tries to move forward, the conveyor accelerates likewise. wo we have a forward-motion of the plane and a backward-motion of the conveyor, which results in the plane being stationary compared to the people standing arround the whole machinery.
this is exactly like pav running on his treadmill. no matter how fast he runs, the treadmill is always keeping him in the same place.we know the plane can only be lifted up by air pressing aginst it's wing (because that is how planes fly).
we also know, that the plane stays stationary in relation to the spectators around (otherwise the conveyor would have failed it's purpose and the whole experiment would have gone wrong).
and because the air arround the spectators is stationary (unless there is a fierce wind due to weather), it does not move in respect to the plane (except the air, that is sucked in and blown out by the jet enginge of course. but that does not pass by the wings of the plane).regarding these facts we have prove that there is no upwards thrust of the plane's wings by air.
and that means, that the plane is not able to fly
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The conveyor does not push backwards against the plane, only the wheels which can spin freely. So the conveyor does not inhibit the planes forward progress as it would a car.
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so if we apply this to cars. . .what does that do for the price at the pump?
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It gets expensive insert a plane maetaphor/similie/pun
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Ive got a variation on this conundrum, if anyones interested...
Its a bit trickier than this one
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oh, ok another thought. Its not at all like a treadmill in that sense. Think of it as being on treadmill on a skateboard. If you use your legs to push against the treadmill to move at the same speed as the treadmill, then you stills stay stationary. A plane however is not using the wheels to accelerate. It is using something not connected to the ground - the air. So its more like standing on the sakteboard and pulling forward with your arms. No matter how fast the conveyor moves below your board, and how fast your wheels turn, it is the speed at which your arms can pull that move you forward. One arm pull distance is the same whether the conveyor is running or not. The same as the plane pulling itself forward through the air.
Maybe thats a better analogy?
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@plot-paris said:
ok. we agree with the fact that the plane, if it's engines are turned off and the conveyor is moving, will be transported backwards, right?
so if the plane accelerates and tries to move forward, the conveyor accelerates likewise. wo we have a forward-motion of the plane and a backward-motion of the conveyor, which results in the plane being stationary compared to the people standing arround the whole machinery.
this is exactly like pav running on his treadmill. no matter how fast he runs, the treadmill is always keeping him in the same place.we know the plane can only be lifted up by air pressing aginst it's wing (because that is how planes fly).
we also know, that the plane stays stationary in relation to the spectators around (otherwise the conveyor would have failed it's purpose and the whole experiment would have gone wrong).
and because the air arround the spectators is stationary (unless there is a fierce wind due to weather), it does not move in respect to the plane (except the air, that is sucked in and blown out by the jet enginge of course. but that does not pass by the wings of the plane).regarding these facts we have prove that there is no upwards thrust of the plane's wings by air.
and that means, that the plane is not able to fly
you're wrong
pav
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I think Plot-Paris is right. The plane does not fly . ..
I mean --not if it's a Delta Flight . ..and you don't get peanuts either.
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You can't keep a powered plane in one place by pulling the ground from under it....even at a matched speed.
Even the pilot himself thinks he's going to sit there like a brick.
http://www.youtube.com/watch?v=YORCk1BN7QY&feature=related -
Well that answers that.
Next!
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you know. .. Dave Barry had the theory that flight as in airplane flight was all a myth. .. and was really jsut an example of mass-hallucination. . .his thinking was that an airplane is much heavier than say . .. a piano . .. and yet when he and a bunch of his friends pushed a piano down the runway they couldnt get it to take off, so logic being what it is, a plane can't fly either.
Hmmm.
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ok. so one half of us approached the whole experiment theoretically (with a high speed conveyor that uses the minimal frictional resistance of the tyres to keep the plane in place)
and the other half expected the plane to actually move forward (because an ordinary conveyor can't put up enough speed)...next challenge!
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You asked for it: imagine the same situation again (plane on a conveyor belt) except this time the conveyor belt matches the speed of the planes wheel rather than the speed of the plane itself.
Can it take off?
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yes!
As it uses its jets to propel forward, even if it had it's brakes on the jet would take off.
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well, because the wheels have no motor, the speed of the wheels is defined by the speed the conveyor has to put up to keep the plane (that is trying to move forward) in place...
this time the plane does not take off
and now it is official, I completely misunderstood the first question
[Edit] understood the first question like the second one is meant...[]
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