You have a plane and a conveyor belt.

[Tom]

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: PurdueRy

Originally posted by: Tom

Originally posted by: jagec

Originally posted by: Tom
As I stated before, the plane engines are not an external force. They are part of the plane .

If this was a car and not a plane, I bet you would readily concede that it wouldn't matter how powerful the car was, that as long as the conveyor matched the speed of the wheels in the opposite direction, that the car would not move relative to anything but the belt.

The airplane behaves exactly the same way. It doesn't matter that the wheels are not "driven" by the powerplant, the fact is the wheels have to move for the plane to move, and they can't move relative to anything but the conveyor belt, just like in the car scenario.

Part of the plane or not, the engines don't affect the wheels of a plane directly. Instead, they make the plane move forward, and friction with the ground makes the wheels turn. The gravity example is valid, since gravity makes his example car's wheels turn in exactly the same fashion.

The wheels DON'T have to move for the plane to move. And the wheels can move without the plane moving. My example of the plane landing on the conveyor should show that...if you match the speed of the conveyor to the plane, the plane can land and then take off, without the wheels moving one bit. Yet the plane's travelling at 300MPH relative to a stationary object. If you put the plane on the conveyor, and hold it steady, you can get the wheels to move without the plane moving.

Wheel motion isn't directly tied to plane motion! In real life situations, we often find the two linked, but that doesn't make it a scientific law!

Are you talking about wheel movement, or rotation ? I am talking about movement, the plane cannot move forward unless the wheels go with it.

And in this case the wheels can only move forward relative to the conveyor, movement relative to everythnig else does not exist because the conveyor moves relative to everything else, in the opposite direction the plane moves relative to the conveyor.

So the movement, not rotation, of the wheels, relative to everything but the conveyor, is zero. And since the wheels are attached to the plane the movement of the plane, relative to everythnig but the conveyor, is also zero.

This is where my toy car example helps. Those wheels are rotating, yet no matter how fast the conveyor spins, it cannot stop the forward movement of the toy car....why? Because wheel RPM and place speed are not directly related in a problem like this.

Your toy car example does not illustrate anything related to the original question, it's a completely different situation. All you toy car example says is gravity makes things fall down, nobody disputes that.

However, if you stick to the original question's parameters, where the conveyor's speed will match the speed of the rotating wheels, no matter what, then even in your toy car example the car would not move forward relative to anythnig but the belt. This would require an even more hypothetical conveyor belt though, because the wheels and belt would accelerate forever.

You shoot down the example, yet you offer no proof of how it does not fit this scenario. You also don't answer how the conveyor causes a force to oppose the thrust of the engine.

We will be going in circles until you can come to the conclusion on these.

Gravity causes things to fall down, engines cause things to go forward, the result is the same in this case. Both cause an unopposed force forward resulting in the acceleration of the plane. You say it is completely unrelated, but if you would think about the FBD of this situation you would realize both illustrate the same situation.

As I said before, if the original question's premise is maintained, about the conveyor movement always matching the wheel movement, in the opposite direction, then in either the original case, or your gravity case, the car will only move relative to the conveyor.

So I'm only shooting down your example because it doesn't change anything, not because it refutes my point.

So the car won't move relative to ground in the toy car case!?!

Not as long as the theoretical conveyor belt and the wheel can continously accelerate and maintain exactly opposite motion.

 

[KK]

Originally posted by: JujuFish

Originally posted by: NanoStuff
The plane increases it's thrust and the wheels begin to rotate. The belt compensates for the rotation of the wheels in reverse, as in the belt moves in reverse exactly as fast as the wheels move forward.

The conveyor belt does not prevent the wheels from moving forward. They just move in the opposite direction with the same speed. This is where I believe you've misunderstood the problem. In your interpretation, the wheels remain fixed while the conveyor belt moves, all relative to the ground. In actuality, the conveyor belt moves backward as much as the wheels move forward, thereby matching its speed. Thus, the wheels ARE moving forward relative to the ground, and consequently the plane takes off.

If you're still not convinced, look at the converse of the situation you've created in your mind, and you might see the flaw: The conveyor belt is maintained in a stationary position-relative to the ground-by the forward movement of the wheels. Obviously, this is not the case, but this is the same way you're interpretting the original scenario.

Edit: Editted to kill the nested quotes.

If the conveyor belt was actually able to match the forward velocity then the wheel would never move physically foward. But it's impossible for the conveyor to keep up since the engine acting indepedently of the wheels would force the plane forward.

 

[jagec]

Originally posted by: Tom
It doesn't matter how fast the treadmill is going, what matters is it will ALWAYS go exactly the right speed to keep the wheels from moving relative to anything but the conveyor, that is the FACT we are given.


"The engines cause an unopposed force relative to ground and the plane will begin to creep forward. "

EXACTLY WHAT I HAVE BENN SAYING ALL ALONG. THE PROBLEM IS THE CONVEYOR BELT IS THE GROUND.

The plane doesn't have to creep forward, it can accelerate like a scalded ape. But all of the motion is irrelevant to it's movement through the air, because in this case the ground is not tied to the air, it is in fact tied to the movement of the plane instead.

I don't dispute at all that the "unopposed force" makes the plane move, I've said that at least a dozen times. The point that I haven't heard anyone refute is, the movement is all relative to the conveyor belt, which is itself moving relative to the rest of the world.

Reference frames 101: All motion is relative to EVERY OTHER POINT IN THE UNIVERSE. We're setting our reference point as the rest of the world (ie not the conveyor belt, not the airplane). You're setting your reference point as the conveyor...however, realize that if the conveyor is your reference point, it has a velocity of ZERO. Frankly, I can see how you're confused...with that reference frame, the ground (when I say "ground", I mean the rest of the world, NOT the conveyor) will be moving forward, and thanks to the stipulations of the OP, the plane will be moving forwards at twice that speed.

Try setting your reference frame to something besides the conveyor, say, Mars, and reconsider the problem.

 

[PurdueRy]

Originally posted by: Tom

Originally posted by: PurdueRy

Originally posted by: Tom

Several of you don't seem to grasp that in this problem, the "ground" has been removed from the frame of reference of the air. All of the forces and motion occur relative to this special reference point of the conveyor belt, because a special condition has been placed on the relative motion of the wheel, that it can only move relative to the belt. Since the wheel can only move relative to the belt, necessarily the same thing applies to the entire entity the wheel is a part of, namely the plane.


Water does not behave anything like a conveyor belt like this, so that comment has no bearing.

I fully grasp what you are struggling with and that's why we are trying to help you understand.

Explain to me this one situation:

The place in on the treadmill. The treadmill speeds up to 1 billion RPM(just for kicks). Neglecting friction, what happens? Nothing, the plane stays still.

Now with the treadmill rotating like crazy, turn on the engines. What happens? Well the treadmill is already going much faster than the plane, it has no affect on the planes speed relative to ground. The engines cause an unopposed force relative to ground and the plane will begin to creep forward.

The treadmill can spin as fast as it wants, just like in the toy car example, it cannot change the planes speed relative to ground...which is what matters.

It doesn't matter how fast the treadmill is going, what matters is it will ALWAYS go exactly the right speed to keep the wheels from moving relative to anything but the conveyor, that is the FACT we are given.


"The engines cause an unopposed force relative to ground and the plane will begin to creep forward. "

EXACTLY WHAT I HAVE BENN SAYING ALL ALONG. THE PROBLEM IS THE CONVEYOR BELT IS THE GROUND.

The plane doesn't have to creep forward, it can accelerate like a scalded ape. But all of the motion is irrelevant to it's movement through the air, because in this case the ground is not tied to the air, it is in fact tied to the movement of the plane instead.

I don't dispute at all that the "unopposed force" makes the plane move, I've said that at least a dozen times. The point that I haven't heard anyone refute is, the movement is all relative to the conveyor belt, which is itself moving relative to the rest of the world.

Whenever I said "Ground" that means...the actual ground...not a moving treadmill.

The plane is stationary no matter how fast the treadmill goes with the engines off. This is stationary relative to the stationary air...not the treadmill. Now create a unopposed force...your telling me nothing happens? The treadmill is already going much faster than the planes wheel speed...so its already matched that. Where does this new force factor in then? Acceleration...and this is acceleration relative to the AIR, not the treadmill.

 

[PurdueRy]

Originally posted by: Tom

Not as long as the theoretical conveyor belt and the wheel can continously accelerate and maintain exactly opposite motion.

You need to think about this longer then, that situation should be pretty easy to see what would result. Bed time for me. Good luck man!

 

[Tom]

"The conveyor belt is maintained in a stationary position-relative to the ground-by the forward movement of the wheels."

I don't have the slightest idea what you are talking about with that statement.


I didn't quote the rest of the message, but based on what you say about what I've said, I guess you don't understand anything I've said so far..

In the original question, the conveyor belt IS the ground. The only other relevant part of the enviroment is the air, because we are talking about an airplane.


The airplane does move, relative to the ground, but the ground IS a conveyor belt; that WILL move relative to the air, in exactly the opposite direction and speed that the plane moves relative to the ground(the conveyor belt is the ground, remember ?)

The net effect of

A. the plane moving relative to the ground(conveyor)

and

B.the ground(conveyor) moving in precisely the opposite way, relative to the AIR


Is that the plane does not move relative to the air.

Hence it cannot take off.

 

[Tom]

Originally posted by: PurdueRy

Originally posted by: Tom

Not as long as the theoretical conveyor belt and the wheel can continously accelerate and maintain exactly opposite motion.

You need to think about this longer then, that situation should be pretty easy to see what would result. Bed time for me. Good luck man!

Um, we are talking about theory here, not the real world. In theory nothing would happen except the wheels would spin faster and the conveyor would go faster to match.

 

[hehatedme]

Here's a simple way to look at it. Take the wheels off the plane, and have it lie on the ground. Turn on the engines and the plane will move forward (even though it will be scraping the ground, it will take off). Adding wheels to the plane won't change this phenomenom.

 

[PurdueRy]

Originally posted by: hehatedme
Here's a simple way to look at it. Take the wheels off the plane, and have it lie on the ground. Turn on the engines and the plane will move forward (even though it will be scraping the ground, it will take off). Adding wheels to the plane won't change this phenomenom.

actually it does friction becomes huge in that case and can't be neglected

 

[PurdueRy]

Originally posted by: PurdueRy

The plane is stationary no matter how fast the treadmill goes with the engines off. This is stationary relative to the stationary air...not the treadmill. Now create a unopposed force...your telling me nothing happens? The treadmill is already going much faster than the planes wheel speed...so its already matched that. Where does this new force factor in then? Acceleration...and this is acceleration relative to the AIR, not the treadmill.

Think about this Tom and respond to it directly.

G'night

 

[JujuFish]

Originally posted by: Tom

"The conveyor belt is maintained in a stationary position-relative to the ground-by the forward movement of the wheels."

I don't have the slightest idea what you are talking about with that statement.


I didn't quote the rest of the message, but based on what you say about what I've said, I guess you don't understand anything I've said so far..

In the original question, the conveyor belt IS the ground. The only other relevant part of the enviroment is the air, because we are talking about an airplane.


The airplane does move, relative to the ground, but the ground IS a conveyor belt; that WILL move relative to the air, in exactly the opposite direction and speed that the plane moves relative to the ground(the conveyor belt is the ground, remember ?)

The net effect of

A. the plane moving relative to the ground(conveyor)

and

B.the ground(conveyor) moving in precisely the opposite way, relative to the AIR


Is that the plane does not move relative to the air.

Hence it cannot take off.

When I say ground, I mean the earth. If you want to remove earth from the equation, then ground means air, as they are stationary (for the purposes of this scenario) to each other. So, look at my statement as:
"The conveyor belt is maintained in a stationary position-relative to the air-by the forward movement of the wheels."

Your arguments about the plane moving relative to the ground, and the ground moving relative to the air is equal to saying that the ground moves relative to the wheels, and the wheels are moving relative to the air. The net effect of these two, using your same logic, produces flight. Now do you see what I am getting at?

 

[JujuFish]

Originally posted by: PurdueRy

Originally posted by: hehatedme
Here's a simple way to look at it. Take the wheels off the plane, and have it lie on the ground. Turn on the engines and the plane will move forward (even though it will be scraping the ground, it will take off). Adding wheels to the plane won't change this phenomenom.

actually it does friction becomes huge in that case and can't be neglected

In the hypothetical situation given by the OP, the plane has infinite thrust. The conveyor is limited by the motion of the plane, and is therefore finite before the plane takes off. So, hehatedme's statement still stands.

 

[dug777]

I don't get why you are arguing?

No air movement=no lift=no flight.

 

[Tom]

Originally posted by: hehatedme
Here's a simple way to look at it. Take the wheels off the plane, and have it lie on the ground. Turn on the engines and the plane will move forward (even though it will be scraping the ground, it will take off). Adding wheels to the plane won't change this phenomenom.

As I just said, in this scenario the real issue is the "ground" and the air are not relative to the plane in an expected way.


Wheels, water, gravity, none of this has much of anything to do with it.

 

[Tom]

Originally posted by: JujuFish

Originally posted by: PurdueRy

Originally posted by: hehatedme
Here's a simple way to look at it. Take the wheels off the plane, and have it lie on the ground. Turn on the engines and the plane will move forward (even though it will be scraping the ground, it will take off). Adding wheels to the plane won't change this phenomenom.

actually it does friction becomes huge in that case and can't be neglected

In the hypothetical situation given by the OP, the plane has infinite thrust. The conveyor is limited by the motion of the plane, and is therefore finite before the plane takes off. So, hehatedme's statement still stands.

That is just baloney. There is nothing "finite" about the speed of the conveyor.

Hehateme's statement has absolutley nothing to do with the conveyor belt scenario, he thinks the wheels are the relevant point, when what makes the question unique is the ability of the ground to move, relative to the air.

 

[jagec]

Originally posted by: dug777
I don't get why you are arguing?

No air movement=no lift=no flight.

The plane will take off. There is no question about it. In a real world situation, in an idealized situation with frictionless bearings, with a jet, on a bet, with a prop, with a mop, on a boat, with a goat, the plane WILL take off.

Originally posted by: Tom
As I just said, in this scenario the real issue is the "ground" and the air are not relative to the plane in an expected way.

True! The conveyor (your ground) is moving past the air, and the plane is moving past the conveyor twice that fast, which means it is moving through the air at the speed of the conveyor. Now we just feed some more power to the engines to speed up the plane, the conveyor matches that speed...once we get to the right speed, the plane takes off.

In your reference frame, that would mean that the "ground" (once again, your conveyor) is at V=0 (reference), the air is moving at Vto (takeoff speed), and the plane is moving at 2*Vto, which means the motion of the plane through the air is 2*Vto - Vto, or the TAKEOFF SPEED!!! Yay!

Alright, I'm out. I've tried to make my point, but it's like hitting my head against a brick wall. It doesn't matter if I'm in my reference frame, or that of the brick wall, it's gonna hurt

 

[mchammer]

Originally posted by: Zenmervolt
The conveyor belt cannot ever match the speed at which the wheels rotate. It's simply not physically possible given that an airplane produces thrust by virtue of Newton's Third Law relative to the air, not to the ground. To borrow the explanation from The Straight Dope:

"...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."

ZV

The wording of the question doesn't matter, the plane will take off. Mr. Straight Dope makes as subtle error above where I have bolded. Turning would be measured by RPM and is irrelevant anyway. To find the RPM the tire was rotating would require knowing the diameter of the wheel.

(edited to correct an error)

 

[JujuFish]

Originally posted by: Tom

Originally posted by: JujuFish

Originally posted by: PurdueRy

Originally posted by: hehatedme
Here's a simple way to look at it. Take the wheels off the plane, and have it lie on the ground. Turn on the engines and the plane will move forward (even though it will be scraping the ground, it will take off). Adding wheels to the plane won't change this phenomenom.

actually it does friction becomes huge in that case and can't be neglected

In the hypothetical situation given by the OP, the plane has infinite thrust. The conveyor is limited by the motion of the plane, and is therefore finite before the plane takes off. So, hehatedme's statement still stands.

That is just baloney. There is nothing "finite" about the speed of the conveyor.

Hehateme's statement has absolutley nothing to do with the conveyor belt scenario, he thinks the wheels are the relevant point, when what makes the question unique is the ability of the ground to move, relative to the air.

You just aren't getting the point. Either that, or you are one of those people who can't stand being wrong and will argue til the end of time no matter how foolish your statements may be.

Yes, the conveyor belt moves relative to the air. But so do the wheels and the plane. What hehatedme is saying is that the plane will move relative to the air regardless of whether there are wheels or not.

 

[Tom]

Originally posted by: JujuFish

Originally posted by: Tom

"The conveyor belt is maintained in a stationary position-relative to the ground-by the forward movement of the wheels."

I don't have the slightest idea what you are talking about with that statement.


I didn't quote the rest of the message, but based on what you say about what I've said, I guess you don't understand anything I've said so far..

In the original question, the conveyor belt IS the ground. The only other relevant part of the enviroment is the air, because we are talking about an airplane.


The airplane does move, relative to the ground, but the ground IS a conveyor belt; that WILL move relative to the air, in exactly the opposite direction and speed that the plane moves relative to the ground(the conveyor belt is the ground, remember ?)

The net effect of

A. the plane moving relative to the ground(conveyor)

and

B.the ground(conveyor) moving in precisely the opposite way, relative to the AIR


Is that the plane does not move relative to the air.

Hence it cannot take off.

When I say ground, I mean the earth. If you want to remove earth from the equation, then ground means air, as they are stationary (for the purposes of this scenario) to each other. So, look at my statement as:
"The conveyor belt is maintained in a stationary position-relative to the air-by the forward movement of the wheels."

Your arguments about the plane moving relative to the ground, and the ground moving relative to the air is equal to saying that the ground moves relative to the wheels, and the wheels are moving relative to the air. The net effect of these two, using your same logic, produces flight. Now do you see what I am getting at?

I am not removing earth from the equation, the original question does that. In every respect what would usually be the "earth" has been replaced by a conveyor belt.

You basically throw the whole question out the window if you don't acknowledge the particulars of the problem presented, it is clear that as far as the plane is concerned the "ground" is the conveyor belt. And this belt very definately is NOT stationary with the air, in fact it is the whole crux of the matter that in this particular scenario it is not in the same frame of reference as the air.

When you redescribe what I said, you omit the fact that the movement of the ground relative to the air, is in the opposite direction from the movement of the plane relative to the ground, so they do not add together to produce flight, but do exactly the opposite and cancel each other out.

 

[Tom]

Originally posted by: jagec

Originally posted by: dug777
I don't get why you are arguing?

No air movement=no lift=no flight.

The plane will take off. There is no question about it. In a real world situation, in an idealized situation with frictionless bearings, with a jet, on a bet, with a prop, with a mop, on a boat, with a goat, the plane WILL take off.

Originally posted by: Tom
As I just said, in this scenario the real issue is the "ground" and the air are not relative to the plane in an expected way.

True! The conveyor (your ground) is moving past the air, and the plane is moving past the conveyor twice that fast, which means it is moving through the air at the speed of the conveyor. Now we just feed some more power to the engines to speed up the plane, the conveyor matches that speed...once we get to the right speed, the plane takes off.

In your reference frame, that would mean that the "ground" (once again, your conveyor) is at V=0 (reference), the air is moving at Vto (takeoff speed), and the plane is moving at 2*Vto, which means the motion of the plane through the air is 2*Vto - Vto, or the TAKEOFF SPEED!!! Yay!

Alright, I'm out. I've tried to make my point, but it's like hitting my head against a brick wall. It doesn't matter if I'm in my reference frame, or that of the brick wall, it's gonna hurt

The plane is not moving "twice that fast" releative to the conveyor, the plane moves in the opposite direction relative to the ground. Yuo are doubling the velocity when the correct answer is the veolcities cancel each other out, relative to the air.

The plane goes 200mph north, relative to the ground(conveyor) at exactly the same time the conveyor(ground) goes 200mph south relative to the air.

This gives a velocity of the plane relative to the air of 0 mph, not 400 mph.

 

[JujuFish]

Originally posted by: Tom

Originally posted by: JujuFish

Originally posted by: Tom

"The conveyor belt is maintained in a stationary position-relative to the ground-by the forward movement of the wheels."

I don't have the slightest idea what you are talking about with that statement.


I didn't quote the rest of the message, but based on what you say about what I've said, I guess you don't understand anything I've said so far..

In the original question, the conveyor belt IS the ground. The only other relevant part of the enviroment is the air, because we are talking about an airplane.


The airplane does move, relative to the ground, but the ground IS a conveyor belt; that WILL move relative to the air, in exactly the opposite direction and speed that the plane moves relative to the ground(the conveyor belt is the ground, remember ?)

The net effect of

A. the plane moving relative to the ground(conveyor)

and

B.the ground(conveyor) moving in precisely the opposite way, relative to the AIR


Is that the plane does not move relative to the air.

Hence it cannot take off.

When I say ground, I mean the earth. If you want to remove earth from the equation, then ground means air, as they are stationary (for the purposes of this scenario) to each other. So, look at my statement as:
"The conveyor belt is maintained in a stationary position-relative to the air-by the forward movement of the wheels."

Your arguments about the plane moving relative to the ground, and the ground moving relative to the air is equal to saying that the ground moves relative to the wheels, and the wheels are moving relative to the air. The net effect of these two, using your same logic, produces flight. Now do you see what I am getting at?

I am not removing earth from the equation, the original question does that. In every respect what would usually be the "earth" has been replaced by a conveyor belt.

You basically throw the whole question out the window if you don't acknowledge the particulars of the problem presented, it is clear that as far as the plane is concerned the "ground" is the conveyor belt. And this belt very definately is NOT stationary with the air, in fact it is the whole crux of the matter that in this particular scenario it is not in the same frame of reference as the air.

When you redescribe what I said, you omit the fact that the movement of the ground relative to the air, is in the opposite direction from the movement of the plane relative to the ground, so they do not add together to produce flight, but do exactly the opposite and cancel each other out.

I am not omitting particulars, you are inventing ones that are not there.
The statement says that the conveyor belt matches the speed of the wheels. Your interpretation of this means that the conveyor belt moves relative to air. However, conversely, the wheels are simultaneously matching the speed of the conveyor belt. By your own logic, the wheels must then also be moving relative to the air. Understand?

 

[mchammer]

Originally posted by: Tom

Originally posted by: JujuFish

Originally posted by: Tom

"The conveyor belt is maintained in a stationary position-relative to the ground-by the forward movement of the wheels."

I don't have the slightest idea what you are talking about with that statement.


I didn't quote the rest of the message, but based on what you say about what I've said, I guess you don't understand anything I've said so far..

In the original question, the conveyor belt IS the ground. The only other relevant part of the enviroment is the air, because we are talking about an airplane.


The airplane does move, relative to the ground, but the ground IS a conveyor belt; that WILL move relative to the air, in exactly the opposite direction and speed that the plane moves relative to the ground(the conveyor belt is the ground, remember ?)

The net effect of

A. the plane moving relative to the ground(conveyor)

and

B.the ground(conveyor) moving in precisely the opposite way, relative to the AIR


Is that the plane does not move relative to the air.

Hence it cannot take off.

When I say ground, I mean the earth. If you want to remove earth from the equation, then ground means air, as they are stationary (for the purposes of this scenario) to each other. So, look at my statement as:
"The conveyor belt is maintained in a stationary position-relative to the air-by the forward movement of the wheels."

Your arguments about the plane moving relative to the ground, and the ground moving relative to the air is equal to saying that the ground moves relative to the wheels, and the wheels are moving relative to the air. The net effect of these two, using your same logic, produces flight. Now do you see what I am getting at?

I am not removing earth from the equation, the original question does that. In every respect what would usually be the "earth" has been replaced by a conveyor belt.

You basically throw the whole question out the window if you don't acknowledge the particulars of the problem presented, it is clear that as far as the plane is concerned the "ground" is the conveyor belt. And this belt very definately is NOT stationary with the air, in fact it is the whole crux of the matter that in this particular scenario it is not in the same frame of reference as the air.

When you redescribe what I said, you omit the fact that the movement of the ground relative to the air, is in the opposite direction from the movement of the plane relative to the ground, so they do not add together to produce flight, but do exactly the opposite and cancel each other out.

I think you are getting closer. Think of it this way, is it possible to affect the speed of a wheeled object on a treadmill by manipulating the speed of the treadmill? Answer should be no. (we will leave out any possible wheel bearing friction)

 

[Tom]

Originally posted by: mchammer

Originally posted by: Zenmervolt
The conveyor belt cannot ever match the speed at which the wheels rotate. It's simply not physically possible given that an airplane produces thrust by virtue of Newton's Third Law relative to the air, not to the ground. To borrow the explanation from The Straight Dope:

"...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."

ZV

The wording of the question doesn't matter, the plane will take off. Mr. Straight Dope makes as subtle error above where I have bolded. Turning would be measured by RPM and is irrelevant anyway. To find the MPH the edge of the tire was moving would require knowing the diameter of the wheel and again is irrelevant.

You can eliminate concerns about rpms and diamters, by just refering to the movement of the wheel and the conveyor belt in velocities.


But the thing you quoted is interesting, because it is completely preposterous. The wheel's movement is relative to the conveyor, the conveyor's movement is relative to the air there is no issue of the wheel having to go perpetually faster to match the conveyor, because the poster of that misunderstands what the speed of the conveyor is relative to.

 

[KK]

if the plane on this conveyor belt was hooked to a truck that was not on the conveyor belt, what would happen if the truck pulls the plane? Is the conveyor going to continue to match the wheel speed of the plane? Is this a magical conveyor belt, did the OP get this conveyor belt from the same place jack got his beans?

The truck pulling is going to exert(sp?) the same forces on the plane as the air is forcing on the plane going thru the engine.

 

[Tom]

Originally posted by: JujuFish

Originally posted by: Tom

Originally posted by: JujuFish

Originally posted by: Tom

"The conveyor belt is maintained in a stationary position-relative to the ground-by the forward movement of the wheels."

I don't have the slightest idea what you are talking about with that statement.


I didn't quote the rest of the message, but based on what you say about what I've said, I guess you don't understand anything I've said so far..

In the original question, the conveyor belt IS the ground. The only other relevant part of the enviroment is the air, because we are talking about an airplane.


The airplane does move, relative to the ground, but the ground IS a conveyor belt; that WILL move relative to the air, in exactly the opposite direction and speed that the plane moves relative to the ground(the conveyor belt is the ground, remember ?)

The net effect of

A. the plane moving relative to the ground(conveyor)

and

B.the ground(conveyor) moving in precisely the opposite way, relative to the AIR


Is that the plane does not move relative to the air.

Hence it cannot take off.

When I say ground, I mean the earth. If you want to remove earth from the equation, then ground means air, as they are stationary (for the purposes of this scenario) to each other. So, look at my statement as:
"The conveyor belt is maintained in a stationary position-relative to the air-by the forward movement of the wheels."

Your arguments about the plane moving relative to the ground, and the ground moving relative to the air is equal to saying that the ground moves relative to the wheels, and the wheels are moving relative to the air. The net effect of these two, using your same logic, produces flight. Now do you see what I am getting at?

I am not removing earth from the equation, the original question does that. In every respect what would usually be the "earth" has been replaced by a conveyor belt.

You basically throw the whole question out the window if you don't acknowledge the particulars of the problem presented, it is clear that as far as the plane is concerned the "ground" is the conveyor belt. And this belt very definately is NOT stationary with the air, in fact it is the whole crux of the matter that in this particular scenario it is not in the same frame of reference as the air.

When you redescribe what I said, you omit the fact that the movement of the ground relative to the air, is in the opposite direction from the movement of the plane relative to the ground, so they do not add together to produce flight, but do exactly the opposite and cancel each other out.

I am not omitting particulars, you are inventing ones that are not there.
The statement says that the conveyor belt matches the speed of the wheels. Your interpretation of this means that the conveyor belt moves relative to air. However, conversely, the wheels are simultaneously matching the speed of the conveyor belt. By your own logic, the wheels must then also be moving relative to the air. Understand?

No, it's exactly the opposite because the directions of the two motions are in opposite directions.

If Vw relative to Vc is 200mph north vector

and Vc relative to Va is 200mph south vector

then Vw to Va is 0. not 400mph.