As I said from the beginning, the original problem description presents an impossible scenario if the jet engines are on. Therefore, the jet engines are off and the jet does not take off. And please don't ask me to explain my reasoning again. Just re-read my comments in this thread.
Actually, it does. Information cannot travel faster than the speed of light.
-Phil
Say you have 10 LEDs spaced 100000 miles apart in outerspace. They are set off uS after each other that is actually faster than they could communicate with each other at speed of light .To the viewer on earth this "information" is faster than speed of light . Same as you painting the sky with a laser pointer, no photons ever traveled from each endpoint of the line you draw
Good for your physics teacher! These are the kinds of chestnuts that sharpen one's intuition about how things work. No formulas needed!
I had a physics prof that enjoyed torturing us with questions like that. Here's one of my favorites:
You're standing on earth shining a flashlight into space, sweeping the beam back and forth. Beyond some very distant point, the beam will be sweeping through space faster than the speed of light. How is this possible, given the limits of relativity?
-Phil
A sweeping beam is an illusion. If you wave a light back and forth, each photon of light will begin its trip in one direction at the speed of light and will continue doing just that.
It's the human mind that creates the "beam" from seeing the small fraction of photons that reflect back at any instant, and mentally extends that beam to infinity.
I have a different take on this. We are told " Ignoring any friction in the wheel bearings...".
That implies the wheels can have no effect on the speed of the plane.
Therefore if the motors are up to power the plane will move forwards. Newton says so ( F = ma )
After enough acceleration we will have reach take off speed.
The conveyor and its control system can do whatever they like, forwards or backwards and any speed, spinning the wheels cannot slow or speed the plane.
Admittedly the conveyor, it's control/drive system may explode and destroy the plane before it flies as they try to reach infinite speed in one scenario or another. Or perhaps the spinning wheels disintegrate.
Say you have 10 LEDs spaced 100000 miles apart in outerspace. They are set off uS after each other that is actually faster than they could communicate with each other at speed of light .To the viewer on earth this "information" is faster than speed of light . Same as you painting the sky with a laser pointer, no photons ever traveled from each endpoint of the line you draw
But if the LED's are turned on, by a timer, independent of one another, what information is conveyed? Well, perhaps "Hey Look! An LED turned on."
If there is any force at all the jet will move.
If the jet moves the wheels will turn.
If the wheels turn the conveyor will move in the oposite direction at the same speed that the circumference of the wheel is moving.
If the conveyor moves at the same speed as the circumverence of the wheels the wheel axis will remain stationary.
If the wheel axis remains stationary the jet will not move.
If the jet does not move there can't be any force on it.
If there is no force and the jet is not moving the jet will not take off.
So the only result that satifies the problem description and the laws of Physics is that the engines exert no force on the jet, and the jet does not move.
Any other result either violates the problem description or the laws of Physics.
>what information is conveyed
The left to right "movement" (knight rider style)
The same as the information of a beam movement from the laser pointer being pointed and moved between points in the sky that are farther apart than the speed of light.
Side note, a shadow is not faster than speed of light. After you block the light the photons will still travel and the shadow will not appear until the last photon have reached the destination.
Ok as there are some not satisfied about the control system for the runway that I described perhaps if I say that some form of control makes sure that the dot on the wheel and the dots on the runway stay in sync so as the plane starts its engines and tries to move forward this control mechanism starts sensing the dots moving apart and immediately compensates. the runway accelerates or decellerates to maintain the synchronisation.
some form of control makes sure that the dot on the wheel and the dots on the runway stay in sync ...
Well, as long as the wheel and runway are in contact, that would be a pretty simple control. It wouldn't have to do anything -- or it could go forward or backward -- no difference.
But will that change others view now? Some others were saying my original post was flawed in it's description, i'm hoping that this amendment will generate a different approach to the problem from some here.
But as the imagery wheels don't have any friction a weak sawing thread could hold the plane in place even if the conveyer belt spinned backwards at full speed.
And if someone pulled on this thread the plane would move forward, and plane engines have more power than a thin tread.
The original problem statement is so full of holes there probably is no correct answere.
Now we have these dots which might confuse me even more.
What if I have a single mark on the wheel at the contact point of of wheel and conveyor and a single mark on the conveyor at the same contact point. Same for the other wheels.
Is the conveyor supposed to keep those marks in contact with each other? In which case the conveyors top surface will move forward with the plane to prevent the marks from separating. The wheels will never rotate. The plane will accelerate and when it takes off both plane and conveyor are moving forward at the same speed and the wheels never turned.
skylight, the dots just mean that there is no slippage between the tires and the runway. With this change in the description the jet will take off. However, it is unknown how fast the runway moves, or whether it moves at all.
I think someone need to contact mythbusters and have them redo the experiment!!! Only with a jet, yeah that would be cooler! lol Dont the wheels on a plane move at the same speed as the ground only in reverse on a normal take off anyways?
You have a jet plane sitting on a runway which happens to be a conveyor belt, the wheels of the jet plane has encoders that relay wheel speed to the conveyor runway which instantaneously matches the speed but in the reverse direction to the way the jet plane is facing, Ignoring any friction in the wheel bearings would the jet be able to take off?
and Dave Hein said something where each logical step seems correct and then heater articulated it more precisely
What if I have a single mark on the wheel at the contact point of of wheel and conveyor and a single mark on the conveyor at the same contact point. Same for the other wheels.
Is the conveyor supposed to keep those marks in contact with each other? In which case the conveyors top surface will move forward with the plane to prevent the marks from separating. The wheels will never rotate.
Ok, let's assume the encoder keeps the marks in contact. Assume that the plane has taxiied to the runway, parked on the treadmill, the engines have been turned off and the brakes are off and that there is no wind and the runway is perfectly flat. The plane does not move. The pilot now turns on the engines which produce a backward force. The plane begins to roll forward, say one degree as measured on the wheel. The plane has moved forward 1cm relative to the ground. The plane has moved forward 1cm relative to the treadmill track. The treadmill system measures this and moves the track forwards 1cm to compensate. The wheel and the treadmill are now back in alignment. However, the plane has still moved forward 1cm relative to the ground.
Now repeat until the plane takes off. At the moment of takeoff, the wheel is not moving relative to the track, and (for a 747) the track is moving forwards at 280kilometres per hour.
But if there are marks on the wheel and track then this part doesn't happen
which instantaneously matches the speed but in the reverse direction to the way the jet plane is facing
.
So - let's think it through with the conveyor moving as described.
So - the plane rolls forward and the wheel moves 1cm. The plane moves forward 1cm relative to the ground. The conveyor must work in the reverse direction because that is what the question says. So it moves back 1cm relative to the ground. At this point the plane has moved forward 1cm relative to the ground, and rotated 2cm relative to the conveyor and the conveyor has moved backwards 1cm relative to the ground. The encoder system interprets that as more wheel movement and speeds up the conveyor to match. That makes the wheel move faster in a positive feedback loop. So the plane ends up taking off normally, and the conveyor destroys itself within the first few seconds as the positive feedback speeds it up backwards as fast as it can go?
A jet is accellerated by the thrust of the engine. If the wheels are moving or not makes no difference there.
A jet does not move because the wheels are turning, it is just the other way around. So if the runway moves the wheels are just turning faster. But the plane will still behave as usual.
Even without wheels the engine will provide the thrust to move the jet.
Using a car or a motorbike is another question. But airplanes do not need wheels at all to provide momentum.
Usually they are used to brake while landing. But for takeoff?
I have heard different arguments to this over the years and one suggestion is as the wheels have no bearing(pun intended) to whether the plane takes off or not to imagine the wheels are a ski with a surface that is frictionless in relation to the surface of the runway.
I'm not sure if adding those marks to the wheels has confused the issue as they were merely intended as an indication that the runway was trying to keep up with wheel rotation.
To try to stop this positive feedback loop that "destroys" the runway or wheels, may I suggest that the runway almost instantaneously compensates?
Although it may sound like i'm moving the goalposts some what i'm trying to iron out any flaws that have been picked up from the original post,
Anyhow some interesting replies, please keep them coming.
Although interesting the videos homebrew experimentation cannot compete with the theoretical or hypothetical situation posed as there are limitations to those experiments such as speed control, you saw the model aircraft moving nackwards suggesting the feedback loop is not efficient.
As for the mythbusters experiments they have little if any feedback as to whether the runway is matching wheel speed.
The whole point whether misleading or not is that the runway is supposed to attempt to stop the plane moving, Does it achieve that is the original argument?
It would just do that if the pilot hits the brakes. Then the wheels would stop the plane from moving foreward like it has to beeing pushed by the thrust of the jet-engine.
Comments
....unless the flaps aren't down.....
....unless, unless, unless....
This can go on forever. The original problem states the plane is SITTING.
A sweeping beam is an illusion. If you wave a light back and forth, each photon of light will begin its trip in one direction at the speed of light and will continue doing just that.
It's the human mind that creates the "beam" from seeing the small fraction of photons that reflect back at any instant, and mentally extends that beam to infinity.
I have a different take on this. We are told " Ignoring any friction in the wheel bearings...".
That implies the wheels can have no effect on the speed of the plane.
Therefore if the motors are up to power the plane will move forwards. Newton says so ( F = ma )
After enough acceleration we will have reach take off speed.
The conveyor and its control system can do whatever they like, forwards or backwards and any speed, spinning the wheels cannot slow or speed the plane.
Admittedly the conveyor, it's control/drive system may explode and destroy the plane before it flies as they try to reach infinite speed in one scenario or another. Or perhaps the spinning wheels disintegrate.
Yup, you're both right: it's just an illusion.
-Phil
But if the LED's are turned on, by a timer, independent of one another, what information is conveyed? Well, perhaps "Hey Look! An LED turned on."
If the jet moves the wheels will turn.
If the wheels turn the conveyor will move in the oposite direction at the same speed that the circumference of the wheel is moving.
If the conveyor moves at the same speed as the circumverence of the wheels the wheel axis will remain stationary.
If the wheel axis remains stationary the jet will not move.
If the jet does not move there can't be any force on it.
If there is no force and the jet is not moving the jet will not take off.
So the only result that satifies the problem description and the laws of Physics is that the engines exert no force on the jet, and the jet does not move.
Any other result either violates the problem description or the laws of Physics.
The left to right "movement" (knight rider style)
The same as the information of a beam movement from the laser pointer being pointed and moved between points in the sky that are farther apart than the speed of light.
Side note, a shadow is not faster than speed of light. After you block the light the photons will still travel and the shadow will not appear until the last photon have reached the destination.
-Phil
And if someone pulled on this thread the plane would move forward, and plane engines have more power than a thin tread.
Now we have these dots which might confuse me even more.
What if I have a single mark on the wheel at the contact point of of wheel and conveyor and a single mark on the conveyor at the same contact point. Same for the other wheels.
Is the conveyor supposed to keep those marks in contact with each other? In which case the conveyors top surface will move forward with the plane to prevent the marks from separating. The wheels will never rotate. The plane will accelerate and when it takes off both plane and conveyor are moving forward at the same speed and the wheels never turned.
and Dave Hein said something where each logical step seems correct and then heater articulated it more precisely
Ok, let's assume the encoder keeps the marks in contact. Assume that the plane has taxiied to the runway, parked on the treadmill, the engines have been turned off and the brakes are off and that there is no wind and the runway is perfectly flat. The plane does not move. The pilot now turns on the engines which produce a backward force. The plane begins to roll forward, say one degree as measured on the wheel. The plane has moved forward 1cm relative to the ground. The plane has moved forward 1cm relative to the treadmill track. The treadmill system measures this and moves the track forwards 1cm to compensate. The wheel and the treadmill are now back in alignment. However, the plane has still moved forward 1cm relative to the ground.
Now repeat until the plane takes off. At the moment of takeoff, the wheel is not moving relative to the track, and (for a 747) the track is moving forwards at 280kilometres per hour.
But if there are marks on the wheel and track then this part doesn't happen .
So - let's think it through with the conveyor moving as described.
So - the plane rolls forward and the wheel moves 1cm. The plane moves forward 1cm relative to the ground. The conveyor must work in the reverse direction because that is what the question says. So it moves back 1cm relative to the ground. At this point the plane has moved forward 1cm relative to the ground, and rotated 2cm relative to the conveyor and the conveyor has moved backwards 1cm relative to the ground. The encoder system interprets that as more wheel movement and speeds up the conveyor to match. That makes the wheel move faster in a positive feedback loop. So the plane ends up taking off normally, and the conveyor destroys itself within the first few seconds as the positive feedback speeds it up backwards as fast as it can go?
A jet is accellerated by the thrust of the engine. If the wheels are moving or not makes no difference there.
A jet does not move because the wheels are turning, it is just the other way around. So if the runway moves the wheels are just turning faster. But the plane will still behave as usual.
Even without wheels the engine will provide the thrust to move the jet.
Using a car or a motorbike is another question. But airplanes do not need wheels at all to provide momentum.
Usually they are used to brake while landing. But for takeoff?
Back to physics 101 guys...
Enjoy!
Mike
I'm not sure if adding those marks to the wheels has confused the issue as they were merely intended as an indication that the runway was trying to keep up with wheel rotation.
To try to stop this positive feedback loop that "destroys" the runway or wheels, may I suggest that the runway almost instantaneously compensates?
Although it may sound like i'm moving the goalposts some what i'm trying to iron out any flaws that have been picked up from the original post,
Anyhow some interesting replies, please keep them coming.
As for the mythbusters experiments they have little if any feedback as to whether the runway is matching wheel speed.
The whole point whether misleading or not is that the runway is supposed to attempt to stop the plane moving, Does it achieve that is the original argument?
It would just do that if the pilot hits the brakes. Then the wheels would stop the plane from moving foreward like it has to beeing pushed by the thrust of the jet-engine.
clueless why anyone even thinks the other way?
Mike