Welcome to the MacNN Forums.

Teronzhul · 05:00 AM

Except that link primarily answers a differently worded question, one that says the belt movement is inverse to that of the plane speed, and not that of its wheels. It does address this as well though.

However, 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.

If the speed of the belt were constantly matching the rotational (which is how I initially read the question) speed of the wheels of the plane, the plane would never move.

I think most of us are still arguing two different questions.

Physicists here be damned

d4nth3m4n · Jan 31, 2007, 05:22 AM

Originally Posted by Teronzhul

If the speed of the belt were constantly matching the rotational (which is how I initially read the question) speed of the wheels of the plane, the plane would never move.

nope, re-read the thread and then get back to me.

Teronzhul · Jan 31, 2007, 06:11 AM

I quite understand what has transpired in the thread, but as I said I misread the original question as both the speed of the wheels and the conveyor to be rotational.

Going back to the skate on a string example. Of course pulling the skate forward would cause it to move and at the same time increase the rotational velocity of its wheels in comparison to that of the treadmill. Its wheels would have to move faster than the conveyor for it to make any forward progress.

If the treadmill were to react to counter its forward motion though, it would of course have to increase its speed to match that of the skate's wheels, and through its contact with the skate would cause an additional increase in speed to the wheels of the skate. Thus the paradox described above.

It calls me a fool for thinking of it this way, but I think the wording of the original question lead more than me to be confused. Were it to function this way, any movement of the plane would cause the conveyor and then the wheels to increase their speed exponentially as soon as the plane overcame its momentum and started to move. Within a few seconds the speed would exceed the operational capacity of the wheels causing them to melt, the plane to fall to the conveyor, and then for it to be launched backwards at 20000 miles per hour. Knowing all of this, the pilot certainly would never start the engines as he wouldn't want to die, and the plane would never move.

Assuming absolutely frictionless wheels, then it could, although I would of course have to play the fool as it would indeed require a conveyor that could operate at infinite speed.

phantomdragonz · Jan 31, 2007, 06:15 AM

hahaha!!! read this on another board a year ago... although they are all not as smart as the people here...

here is the secret...

OPEN LOOP VS. CLOSED LOOP...

(I think thats what it's called anyways)

Oh, and it will only fly if it knows the unladen velocity of a swallow!

HAHAHAHA!!!

Zach

JohnM15141 · Jan 31, 2007, 10:38 AM

yes, if you choose to interpret the question that way, but then that would make the question pointless, the intention of the questioner is to present an airplane that is stationary relative to an object next to the conveyor belt, splitting hairs over the poor choice of words diverts the point of the problem.

Another given in the problem is as you state it is that on the moving conveyor belt runway, the tangential speed of the plane's wheels can greatly exceed their linear velocity. This is true for all airplanes rolling down any runway: The rotational velocity of the wheels greatly exceed the speed of the airplane rolling over the runway.

equate this to your typical 1st year physics problem where there are massless objects, light speed of 100 mph, etc. etc., in order to frame the question to illuminate a particular result regardless that it is "illogical" to have a massless object.

Originally Posted by f1000

John, when the ground is stationary, the tangential speed of the plane's wheels will equal their linear velocity. In the case of our moving conveyor belt runway, however, the tangential speed of the plane's wheels can greatly exceed their linear velocity.

It's somewhat absurd to interpet "wheel speed" as meaning tangential speed. If we did so, then we'd find that our conveyer belt runway could never match the tangential speed of an already moving wheel. As the conveyor belt began to move, the wheel would spin faster, thereby always remaining beyond the belt's reach. Our conveyer belt would only be able to maintain reverse tangential speed if the wheel's axle were stationary in the first place; thus, it's only logical that we interpret wheel speed to mean linear velocity (relative to our stationary observer).

f1000 · Jan 31, 2007, 11:19 AM

Originally Posted by JohnM15141

yes, if you choose to interpret the question that way, but then that would make the question pointless, the intention of the questioner is to present an airplane that is stationary relative to an object next to the conveyor belt, splitting hairs over the poor choice of words diverts the point of the problem.

No, I answered the problem as it could only properly be interpreted. You're trying to answer an absurd problem. You might as well be trying to "explain" Xeno's Paradox.

Originally Posted by JohnM15141

This is true for all airplanes rolling down any runway: The rotational velocity of the wheels greatly exceed the speed of the airplane rolling over the runway.

Now you're confusing tangential with rotational speeds: CHAPTER 6.

This is the important take away point-> "Furthermore, it should be obvious that the tangential speed is equal to the linear velocity v of the wheel."

Dakar² · Jan 31, 2007, 11:20 AM

Originally Posted by phantomdragonz

hahaha!!! read this on another board a year ago... although they are all not as smart as the people here...

here is the secret...

OPEN LOOP VS. CLOSED LOOP...

(I think thats what it's called anyways)

Oh, and it will only fly if it knows the unladen velocity of a swallow!

HAHAHAHA!!!

Zach

European or African?

f1000 · Jan 31, 2007, 11:33 AM

Originally Posted by Teronzhul

If the speed of the belt were constantly matching the rotational (which is how I initially read the question) speed of the wheels of the plane, the plane would never move.

I think you mean the tangential velocity: rotational velocity is in units like radians/s and is not directly comparable with linear velocity.

In any case, nowhere does the problem state that the treadmill-runway must "counter" the plane's forward motion. Countering the forward motion is different from simply matching the tangential (linear) velocity in reverse.

Sealobo · Jan 31, 2007, 12:36 PM

i can't believe that this thread can get to page 3....

JohnM15141 · Jan 31, 2007, 12:55 PM

No, you are missing the "spirit" of the question! Just because the author used the word "wheels" instead of "airplane" in the question:

Imagine an airplane 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 airplane take off?

Perhaps you need an additional paramater such as the radius of the wheels equal zero?

The intent was to present an airplane not moving, but at takeoff power, on a treadmill, stationary, relative to an object standing next the runway.

You are losing sight of the purpose of the question due to the vagueness of the question being in laymans terms and not "Physics"-speak as it where

You are not "properly" answering the question. You are simply choosing to answer it that way, regardless of the questioners intent. Declaring that there is only one proper way to answer the question does not make it so...

If you stick to your choice of interpreting the terms so rigidly and ignore the intent of the questioner then we have no question at all! In your "interpretation" there will always be forward movement of the airplane and at some point there will be enough airflow to create lift and the airplane will fly...but the intent was to present an airplane with no forward movement.

Originally Posted by f1000

No, I answered the problem as it could only properly be interpreted. You're trying to answer an absurd problem. You might as well be trying to "explain" Xeno's Paradox.

This isn't important because its not fundamentally part of the question...

Originally Posted by f1000

Now you're confusing tangential with rotational speeds: CHAPTER 6.

This is the important take away point-> "Furthermore, it should be obvious that the tangential speed is equal to the linear velocity v of the wheel."

d4nth3m4n · Jan 31, 2007, 01:08 PM

Originally Posted by JohnM15141

No, you are missing the "spirit" of the question! Just because the author used the word "wheels" instead of "airplane" in the question:

Perhaps you need an additional paramater such as the radius of the wheels equal zero?

The intent was to present an airplane not moving, but at takeoff power, on a treadmill, stationary, relative to an object standing next the runway.

You are losing sight of the purpose of the question due to the vagueness of the question being in laymans terms and not "Physics"-speak as it where

You are not "properly" answering the question. You are simply choosing to answer it that way, regardless of the questioners intent. Declaring that there is only one proper way to answer the question does not make it so...

If you stick to your choice of interpreting the terms so rigidly and ignore the intent of the questioner then we have no question at all! In your "interpretation" there will always be forward movement of the airplane and at some point there will be enough airflow to create lift and the airplane will fly...but the intent was to present an airplane with no forward movement.

why would the questioner make up such a no-brainer of a question if the "spirit" of the question was nothing more than asking "if a plane is tied to an anchor that absolutely holds it in place, can it take off?" seems a little foolish to me. agreed?

or are you just trolling because you're bored and need to get your postcount up?

Mrjinglesusa · Jan 31, 2007, 01:19 PM

The answer is very simple. Are you ready?

The plane WILL take off if it can generate air flow over the wings.

If there is no air flow over the wings, it will NOT take off.

Period.

Doesn't matter how fast the treadmill goes. Doesn't matter how much thrust the plane has. If the combination of all factors does not allow air flow over the wings, the plane cannot take off.

Dakar² · Jan 31, 2007, 01:22 PM

Groundbreaking. Everyone is arguing whether the plane is achieving that airflow or not.

JohnM15141 · Jan 31, 2007, 01:26 PM

I disagree, the question is intended to be thought provoking however "foolish" it is phrased.

If you percieve the question as the aircraft is tied to an anchor, then there is no need for a treadmill. Since the airplane is going nowhere regardless if the engines are on or off the, and the conveyor belt doesnt have to move at all.

The thought provoking part is the that the airplane is doing everything it would normally do to fly, with the exception that it is not moving forward...

I'd say your post fits the defination of "trolling" as it appears to provoke rather then illuminate on the topic...agreed?(Rhetorical question)

Originally Posted by d4nth3m4n

why would the questioner make up such a no-brainer of a question if the "spirit" of the question was nothing more than asking "if a plane is tied to an anchor that absolutely holds it in place, can it take off?" seems a little foolish to me. agreed?

or are you just trolling because you're bored and need to get your postcount up?

d4nth3m4n · Jan 31, 2007, 01:34 PM

hey man, i'm just trying to get to page 10.

Gossamer · Jan 31, 2007, 01:41 PM

Originally Posted by JohnM15141

I disagree, the question is intended to be thought provoking however "foolish" it is phrased.

If you percieve the question as the aircraft is tied to an anchor, then there is no need for a treadmill. Since the airplane is going nowhere regardless if the engines are on or off the, and the conveyor belt doesnt have to move at all.

The thought provoking part is the that the airplane is doing everything it would normally do to fly, with the exception that it is not moving forward...

I'd say your post fits the defination of "trolling" as it appears to provoke rather then illuminate on the topic...agreed?(Rhetorical question)

But it is moving forward. Imagine a plane on a giant treadmill at rest. Suddenly the treadmill is instantly at max speed. What does the plane do? Move very little, only because of friction in the wheel bearings, but for the most part it's moving very slowly with respect to outside objects, even though the wheels are spinning furiously. The treadmill can do VERY VERY VERY little to affect the overall speed of the plane. Thrust IS COMPLETELY INDEPENDENT OF THE WHEELS. It doesn't matter what the treadmill is doing, the jet engine acts like someone pushing on a skate on a tread mill. The wheels will spin even more furiously, but it will still move forward.

JohnM15141 · Jan 31, 2007, 01:53 PM

Yes, you are correct, it will move forward, relative to the treadmill.

And, yes thrust is completely independant of the wheels.

But, whether you are being propelled by thrust, a stick, muscle power etc. the effect is to move across the surface of the treadmill and in this case you appear stationary to a viewpoint not on a treadmill.

Its the same as a man running on a treadmill, or a car driving on it, or even a rocket powered car. Work is being done to maintain position over the treadmill which causes them to stay in place relative to us the observer...

If the airplane where to do nothing on the treadmill then, as the treadmill moved, it would move with it. If the airplane started its engine and produced thrust to compensate, it would move forward over the treadmill and its wheels would turn. In this case the thrust produced is equal to move the plane forward to compensate for the amount the treadmill is trying to move it backwards. Just like a man standing on a treadmill, if he did not start walking he would fall off the back of the treadmill, if he walked to fast he would fall of the front, but if he walked the same speed as the treadmill, he would stay in one place over the treadmill...

Originally Posted by Gossamer

But it is moving forward. Imagine a plane on a giant treadmill at rest. Suddenly the treadmill is instantly at max speed. What does the plane do? Move very little, only because of friction in the wheel bearings, but for the most part it's moving very slowly with respect to outside objects, even though the wheels are spinning furiously. The treadmill can do VERY VERY VERY little to affect the overall speed of the plane. Thrust IS COMPLETELY INDEPENDENT OF THE WHEELS. It doesn't matter what the treadmill is doing, the jet engine acts like someone pushing on a skate on a tread mill. The wheels will spin even more furiously, but it will still move forward.

Gossamer · Jan 31, 2007, 01:58 PM

Originally Posted by JohnM15141

Its the same as a man running on a treadmill, or a car driving on it, or even a rocket powered car. Work is being done to maintain position over the treadmill which causes them to stay in place relative to us the observer...

No no no no no. Both the man and the car rely on friction between themselves and the treadmill for propulsion.

If the airplane where to do nothing on the treadmill then, as the treadmill moved, it would move with it.

Only if the treadmill accelerated slowly enough that the friction in the wheel bearings kept the wheels from spinning. If the acceleration was instant, the airplane would not move relative to outside objects, due to its inclination to remain at rest.

JohnM15141 · Jan 31, 2007, 02:14 PM

This is true, but the result is the same whether you are pushing against the friction of the treadmill with shoes or wearing a pair of roller skates and hurling heavy objects to move forward. The "equal and opposite reaction" will move you over the treadmill.

Originally Posted by Gossamer

No no no no no. Both the man and the car rely on friction between themselves and the treadmill for propulsion.

Think of a car sitting on that same treadmill, it will move backwards too, even if the treadmill starts quickly and causes the wheels to turn, eventually the car will move backwards with the treadmill unless something happens to propel the car such as its motor turning the wheels, or a rocket pushing the car. If the motor turns the wheels too fast, it will drive off the front of the treadmill, or if the same rocket attached to the car provides too much thrust it will go off the front of the treadmill. So to keep it on the treadmill, the driver should press the accelerator pedal enough to keep the wheels turning as fast as neccessary to be stationary, the rocket should be throttled to provide enough thrust to stay stationary...

Originally Posted by Gossamer

Only if the treadmill accelerated slowly enough that the friction in the wheel bearings kept the wheels from spinning. If the acceleration was instant, the airplane would not move relative to outside objects, due to its inclination to remain at rest.

f1000 · Jan 31, 2007, 03:22 PM

John, why don't you try setting the system up for us using Lagrangian mechanics? Then we'll be able to see mathematically what's going on. I'm on a train right now and I probably won't be able to get back to this until next week.

JohnM15141 · Jan 31, 2007, 03:42 PM

That is unneccessary since the system is restricted to the reaction is equal to and opposite of the action. i.e. the action of the airplane is equal to and opposite of the conveyor belt so that, as stated, in the question, there is no motion relative to a stationary object. Whether you go through the trouble to describe it in terms of Lagrangian mechanics or newtonian physics, the input parameters needs to be such that the action equals the reaction to fit the model described in the question. A lot of unneccesary work to come to the same conclusion: If I put a sopwith Camel on a treadmill and started it up and moved the treadmill backwards the same amount the propellor is trying to move the plane forward until the sopwith camel engine is turning as fast as it would to fly, it won't fly. Granted that treadmill is going have to be working outrageously hard, but that is how the problem is presented...

Originally Posted by f1000

John, why don't you try setting the system up for us using Lagrangian mechanics? Then we'll be able to see mathematically what's going on. I'm on a train right now and I probably won't be able to get back to this until next week.

TETENAL · Jan 31, 2007, 04:10 PM

Originally Posted by JohnM15141

Granted that treadmill is going have to be working outrageously hard, but that is how the problem is presented...

A treadmill working outrageously hard would spin the airplane's wheels outrageously fast, but outrageously fast spinning wheels will not move the plane. The propeller or jet engines of the plane pushing against the still standing surrounding air will move the plane forwards. So it will take off.

JohnM15141 · Jan 31, 2007, 04:30 PM

No, the airplane thrust is pushing against the surrounding airmass to over come the friction between the treadmill and the wheels that is keeping the plane stationary. Thus there is no relative wind over the wings to create lift. Airplane engines be it jets or propellors cannot move the surrounding airmass enough to create a wind. And remember the surrounding airmass is not a single rigid mass, so when the thrust begins pushing against the airmass, the entire airmass does not push back, only a fraction of the airmass is affected. The thurst is enough to overcome the friction of the treadmill. If the airplane where not on a treadmill, then the thrust would overcome the friction, the airplane would move forward, lift would be generated, and the airplane would fly. But, in our problem the treadmill is moving such that it prevents the airplane from moving forward due to the friction of the wheels. The force that is preventing the airplane from moving backwards on the treadmill is the thrust generated by the propellor/jet moving a portion of the airmass.

Originally Posted by TETENAL

A treadmill working outrageously hard would spin the airplane's wheels outrageously fast, but outrageously fast spinning wheels will not move the plane. The propeller or jet engines of the plane pushing against the still standing surrounding air will move the plane forwards. So it will take off.

slpdLoad · Jan 31, 2007, 04:40 PM

Airplane engines be it jets or propellors cannot move the surrounding airmass enough to create a wind.

Wait...what? That's absurd. How do you think a plane generates any forward movement at all then?
The jets/propellers move air.

THE WHEELS ARE FREE SPINNING AND DO NOT EFFECT THE PLANE AT ALL.

jokell82 · Jan 31, 2007, 04:43 PM

Originally Posted by JohnM15141

Its the same as a man running on a treadmill, or a car driving on it, or even a rocket powered car. Work is being done to maintain position over the treadmill which causes them to stay in place relative to us the observer...

But take the skate on the treadmill as an example. If you push it forward, are its wheels all of a sudden moving faster than the treadmill? Or is it impossible to push it forward since the treadmill is acting against the wheels?

Even though the wheels spin at the same rate as the treadmill, the skate can still move forward. The same goes with a plane. The wheels DO NOT MOVE THE PLANE FORWARD.

CollinG3G4 · 04:59 PM

Originally Posted by kc311v2

I read this off another forum and I can't find the link, but the question went like this:

Imagine an airplane 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 airplane take off?

<didn't read the entire topic>

Sure, it can "take off", but it won't fly since the engines are not running! And to that, to climb, a plane would need more energy/thrust/whatever than the initial take off provided.

JohnM15141 · Jan 31, 2007, 04:56 PM

When I said "wind" I was referring to the movement of an entire airmass.

When an airplane sitting on a runway generates thrust, the thrust overcomes the resistance of the wheels on the runway. When the thrust is greater than that resistance the airplane begins to move forward over the runway.

The wheels definately affect the airplane due the friction of the wheels against the runway. The weight of the airplane increases that friction. That friction, and rotational friction, and the inertia of the airplane all come into play when the airplane tries to move.

Originally Posted by slpdLoad

Wait...what? That's absurd. How do you think a plane generates any forward movement at all then?
The jets/propellers move air.

THE WHEELS ARE FREE SPINNING AND DO NOT EFFECT THE PLANE AT ALL.

Gossamer · Jan 31, 2007, 05:00 PM

Originally Posted by JohnM15141

This is true, but the result is the same whether you are pushing against the friction of the treadmill with shoes or wearing a pair of roller skates and hurling heavy objects to move forward. The "equal and opposite reaction" will move you over the treadmill.

Think of a car sitting on that same treadmill, it will move backwards too, even if the treadmill starts quickly and causes the wheels to turn, eventually the car will move backwards with the treadmill unless something happens to propel the car such as its motor turning the wheels, or a rocket pushing the car. If the motor turns the wheels too fast, it will drive off the front of the treadmill, or if the same rocket attached to the car provides too much thrust it will go off the front of the treadmill. So to keep it on the treadmill, the driver should press the accelerator pedal enough to keep the wheels turning as fast as neccessary to be stationary, the rocket should be throttled to provide enough thrust to stay stationary...

You're joking right?

Stoooooooooooooooooooooooooooooooooooooooooooooooo p comparing friction-driven cars and people to thrust-driven airplanes. Noooooooooooooooooooooooot the same thing at alllllllllllllllllllllllllllllllll.

JohnM15141 · Jan 31, 2007, 05:03 PM

Yes and no,

if you push the skate it will move forward as long as the force pushing it is greater than the friction caused by the treadmill moving the opposite direction. For the airplane, the "thrust" is the pushing force. If it is greater than the force of friction on the treadmill moving the opposite direction then it will move forward too. However, in our question the treadmill works harder when the thrust increases to maintain the airplane in position, relative to a stationary object next to the treadmill.

And yes, the wheels do not move the plane forward, the are spinning freely under the plane. The wheels are spinning due to the friction of the treadmill. The airplane is using thrust to overcome this friction, if it didnt then it would move backwards with the treadmill.

Originally Posted by jokell82

But take the skate on the treadmill as an example. If you push it forward, are its wheels all of a sudden moving faster than the treadmill? Or is it impossible to push it forward since the treadmill is acting against the wheels?

Even though the wheels spin at the same rate as the treadmill, the skate can still move forward. The same goes with a plane. The wheels DO NOT MOVE THE PLANE FORWARD.

JohnM15141 · Jan 31, 2007, 05:10 PM

You are right they are not the same thing but the result is the same. Work is being done, whether it s a spinning tire using friction or a rocket blowing air, it is a force moving in an opposite direction, in those terms they are exactly the same!

How the force is created is different.

But force is force...apply that force to move an object over distance and you have work being done.

So, no I'm not joking. Just like you can move a car with a rocket, a car with wings will fly once the wings begin to generate lift. However once the tires leave the ground the force to move the "car with wings" will be removed and the car will slow down and when lift decreases it will come back down to the gound where it can apply its force again.

Originally Posted by Gossamer

You're joking right?

Stoooooooooooooooooooooooooooooooooooooooooooooooo p comparing friction-driven cars and people to thrust-driven airplanes. Noooooooooooooooooooooooot the same thing at alllllllllllllllllllllllllllllllll.

jokell82 · Jan 31, 2007, 05:13 PM

Originally Posted by JohnM15141

Yes and no,

if you push the skate it will move forward as long as the force pushing it is greater than the friction caused by the treadmill moving the opposite direction. For the airplane, the "thrust" is the pushing force. If it is greater than the force of friction on the treadmill moving the opposite direction then it will move forward too. However, in our question the treadmill works harder when the thrust increases to maintain the airplane in position, relative to a stationary object next to the treadmill.

And yes, the wheels do not move the plane forward, the are spinning freely under the plane. The wheels are spinning due to the friction of the treadmill. The airplane is using thrust to overcome this friction, if it didnt then it would move backwards with the treadmill.

Back to the skate on the treadmill. What you're saying is that if the treadmill kept going faster and faster as you try to push it forward you'd never be able to move the skate forward???

Dakar² · Jan 31, 2007, 05:14 PM

I just need to see this happen on MythBusters and I'll be happy.

JohnM15141 · Jan 31, 2007, 05:16 PM

No, I'm not saying that...

Originally Posted by jokell82

Back to the skate on the treadmill. What you're saying is that if the treadmill kept going faster and faster as you try to push it forward you'd never be able to move the skate forward???

jokell82 · Jan 31, 2007, 05:19 PM

Originally Posted by JohnM15141

No, I'm not saying that...

Then if the skate can move forward, so can the plane.

JohnM15141 · Jan 31, 2007, 05:40 PM

Not as described in the question, the question says the conveyor moves to oppose the airplanes movement.

Originally Posted by jokell82

Then if the skate can move forward, so can the plane.

jokell82 · Jan 31, 2007, 05:51 PM

Originally Posted by JohnM15141

Not as described in the question, the question says the conveyor moves to oppose the airplanes movement.

No, the conveyor moves to oppose the spin of the wheels, which in no way opposes the airplanes movement.

JohnM15141 · Jan 31, 2007, 05:53 PM

Now we are back to a Glass is half empy, Glass is half full argument

If the question is viewed in reference to the wheels only then we have no question at all...

Originally Posted by jokell82

No, the conveyor moves to oppose the spin of the wheels, which in no way opposes the airplanes movement.

analogika · Jan 31, 2007, 07:34 PM

Originally Posted by JohnM15141

Yes, you are correct, it will move forward, relative to the treadmill.

And, yes thrust is completely independant of the wheels.

But, whether you are being propelled by thrust, a stick, muscle power etc. the effect is to move across the surface of the treadmill and in this case you appear stationary to a viewpoint not on a treadmill.

No.

Because since, as you yourself have realized, the thrust is completely independent of the wheels, thrust is also COMPLETELY independent of the treadmill. The treadmill is completely irrelevant. Its sole purpose in this "problem" is to confuse people like you, who are too easily distracted from simple reasoning.

Thrust is relative only to the air that surrounds the aircraft.

Going back to the roller-skate example:

It's been established that if you're pulling a roller skate on a conveyor belt by a little string, it will move forward, completely regardless of how fast or slow the conveyor belt might be moving.

Now imagine strapping a rocket to the roller skate. (rocket = jet = propeller in terms of how thrust works.)

analogika · Jan 31, 2007, 07:39 PM

Originally Posted by JohnM15141

You are right they are not the same thing but the result is the same. Work is being done, whether it s a spinning tire using friction or a rocket blowing air, it is a force moving in an opposite direction, in those terms they are exactly the same!

How the force is created is different.

Not necessarily. That's a completely irrelevant point, and it's wrong, anyway:

A combustion engine in a propeller plane is pretty much exactly the same thing as a combustion engine in a car.

The point you're not getting is that how the force is APPLIED is different.

Jets, propellers, and rockets apply thrust AGAINST THE AIR BEHIND THEM.

Wheels turned by engines apply force AGAINST THE GROUND, relying upon friction. Thus, wheel-driven vehicles are directly affected by whether the ground is moving.

Thrust-driven vehicles are not. They are affected by whether the AIR is moving.

JohnM15141 · Jan 31, 2007, 08:11 PM

That is just not true!

You strap a rocket on that roller skate, the friction of the wheels on the treadmill are still a factor and the rocket has to over come that friction to get the skate moving. In our example if the rocket does not over come that friction when the tread mill starts moving, the skate will move backwards with the treadmill.

A Jet engine is not influenced by the air around it at all. Its the thrust from propelling air behind it that moves the Jet forward. Regardless of the airmass behind it, beside it all around it. That same jet engine would be propelled in a vacuum if it had a supply of air to compress and accelerate out the back end of it.

The thrust is not completely independent of the wheels. When an airplane rolls down the runway, one of the forces it must overcome is the friction of the wheels against the runway.

On a treadmill, regardless of how you create the pushing force, if it is equal to the force of the treadmill pushing the airplane backwards, then rhe airplane will remain stationary to an observer even though the wheels are spinning due to friction of the treadmill.

Originally Posted by analogika

No.

Because since, as you yourself have realized, the thrust is completely independent of the wheels, thrust is also COMPLETELY independent of the treadmill. The treadmill is completely irrelevant. Its sole purpose in this "problem" is to confuse people like you, who are too easily distracted from simple reasoning.

Thrust is relative only to the air that surrounds the aircraft.

Going back to the roller-skate example:

It's been established that if you're pulling a roller skate on a conveyor belt by a little string, it will move forward, completely regardless of how fast or slow the conveyor belt might be moving.

Now imagine strapping a rocket to the roller skate. (rocket = jet = propeller in terms of how thrust works.)

slpdLoad · Jan 31, 2007, 08:15 PM

Originally Posted by JohnM15141

You strap a rocket on that roller skate, the friction of the wheels on the treadmill are still a factor and the rocket has to over come that friction to get the skate moving. In our example if the rocket does not over come that friction when the tread mill starts moving, the skate will move backwards with the treadmill.

I think you are grossly underestimating the power of a jet engine, and even some type of rocket for that matter. Do you really think the friction of some ball-bearings in a wheel are enough to stop a plane or skateboard if a jet or rocket (respectively) is strapped to it?

JohnM15141 · Jan 31, 2007, 08:17 PM

No, you have it totally wrong, rockets gain thrust from the momentum of the mass they are throwing behind them, thats why they work in a vacuum. Jets do the same, they compress air and release it at a high velocity behind them. The mass leaving the jet engine is countered with a force in the opposite direction, that is thrust. The air behind a jet engine has nothing to do with that thrust.

How the force is applied is irrelevant, as long as a force is applied to counter the force from the treadmill moving backwards.

Originally Posted by analogika

Not necessarily. That's a completely irrelevant point, and it's wrong, anyway:

A combustion engine in a propeller plane is pretty much exactly the same thing as a combustion engine in a car.

The point you're not getting is that how the force is APPLIED is different.

Jets, propellers, and rockets apply thrust AGAINST THE AIR BEHIND THEM.

Wheels turned by engines apply force AGAINST THE GROUND, relying upon friction. Thus, wheel-driven vehicles are directly affected by whether the ground is moving.

Thrust-driven vehicles are not. They are affected by whether the AIR is moving.

JohnM15141 · Jan 31, 2007, 08:20 PM

Of course not, but in the case of this problem it is stated that the airplanes thrust is being matched by that of the conveyor belt. That is all. The imaginary part in this model, is this extremely powerful conveyor belt that is moving beneath the airplane countering the thrust propelling the airplane forward such that the airplane does not move relative to a stationary observer.

Originally Posted by slpdLoad

I think you are grossly underestimating the power of a jet engine, and even some type of rocket for that matter. Do you really think the friction of some ball-bearings in a wheel are enough to stop a plane or skateboard if a jet or rocket (respectively) is strapped to it?

slpdLoad · Jan 31, 2007, 08:25 PM

But no matter how fast or strong the conveyor is, it can only affect the wheels of the plane (which are just free spinning on the axles). It cannot effect the fact that the plane will still be propelled forward by the jet engines, which will create lift and the plane will take off.

(Note: I'm not trying to make fun of you or anyone else or troll here. It's just not that often that you get into an argument where the answer seems to obvious to you and yet somebody else does not see it at all) Anywho...

JohnM15141 · Jan 31, 2007, 08:35 PM

Yes the treadmill can only affect the wheels of the airplane, but those wheels are attached to that airplane thus they affect the airplane as well. It is the thrust of the airplanes engine that must overcome the friction on the wheels.

If you place an airplane on a treadmill with its engines off and the treadmill begins to move, the airplane will move with the treadmill. There is always friction.

It takes a force to overcome that friction. The force will come from the propellor or jet on the airplane. On our treadmill, if the engines begin to apply force, the airplane will stop moving with the treadmill the wheels will begin to spin and the airplane begin to move over the conveyor belt. Of course if the thrust is increased the airplane will begin to move forward, but in our model the treadmill increase its speed to counter that thrust(the friction becomes greater and more thrust from the airplane is required to overcome that force.)

By your reasoning here, an airplane will never move on a treadmill no matter how fast the conveyor belt is moving because the friction of the wheels is irelevant? Thats just not so...

Originally Posted by slpdLoad

But no matter how fast or strong the conveyor is, it can only affect the wheels of the plane (which are just free spinning on the axles). It cannot effect the fact that the plane will still be propelled forward by the jet engines, which will create lift and the plane will take off.

(Note: I'm not trying to make fun of you or anyone else or troll here. It's just not that often that you get into an argument where the answer seems to obvious to you and yet somebody else does not see it at all) Anywho...

Doofy · Jan 31, 2007, 08:38 PM

It'll fly. Unless the cabin crew are on strike.

smacintush · Jan 31, 2007, 08:38 PM

Originally Posted by JohnM15141

Of course not, but in the case of this problem it is stated that the airplanes thrust is being matched by that of the conveyor belt.

I t doesn't say that. How can you argue the answer when you can't even understand the question?

Calimus · Jan 31, 2007, 08:38 PM

Originally Posted by slpdLoad

But no matter how fast or strong the conveyor is, it can only affect the wheels of the plane (which are just free spinning on the axles). It cannot effect the fact that the plane will still be propelled forward by the jet engines, which will create lift and the plane will take off.

So you are saying that if you park an airplane on a stopped treadmill, then start the treadmill up, the plane will not begin moving?

Because that's essentially what you're describing. While the force due to friction in reality will be dwarfed by the force of the engines, enough friction on the wheels COULD balance the treadmill and the engines.

JohnM15141 · 08:46 PM

No, it does say that. There are vageries in the wording of the question but for all intensive purposes the questioner is trying describe a model where an airplane is on a rolling treadmill and is not moving, when it increases its thrust to move forward, the treadmill counters it. That is the crux of the problem an airplane at full take off power but not moving because its on a giant conveyor belt.

Originally Posted by smacintush

I t doesn't say that. How can you argue the answer when you can't even understand the question?

analogika · Jan 31, 2007, 08:50 PM

Originally Posted by JohnM15141

No, you have it totally wrong, rockets gain thrust from the momentum of the mass they are throwing behind them, thats why they work in a vacuum. Jets do the same, they compress air and release it at a high velocity behind them. The mass leaving the jet engine is countered with a force in the opposite direction, that is thrust. The air behind a jet engine has nothing to do with that thrust.

I'm sorry - you are indeed correct with regard to thrust in rockets. Jets and props do, however, depend upon air.

The rest of my point stands completely untouched.