# MythBusters tackles "plane on a conveyor belt problem"

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davemania

Answer is there. ^^ (Y)

Speed ing wheels dont + jet engines dont make the plane fly.

Here is an example.

If you run on a tread mill hold a bad beside you open side facing in the direction you run...you will notice there is no wingd filling the bag.

Do the same not on the treadmill..wou will see the bag fill up.

You need this factor to lift the plane.

Thats a wrong anaolgy, your legs are propelling your body forward. That is not the case for the plane. If the wheel if free from the plane, than the thrust of the engine will move the plane forward. If it moves, there is lift.

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jackwanders

the question isn't particularly clear or well-defined, at least in its original form. for instance, it does not say that wheels do not provide propulsion or that the plane has an engine (it could very well be a glider) that provides all the propulsion. not to mention the fact that it argues over the question of wheels moving as fast as the treadmill, etc.

from what i understand, whoever thought up this question was simply wondering what would happen if an airplane on a treadmill were moving such that the treadmill would cancel out any forward movement.

let's say, in one case, that the wheels do provide propulsion. suppose a plane at rest. this situation is exactly the same as though the plane were moving at some speed matched by the treadmill. if the engine can pull the plane forward when the plane is at rest, it will surely do it when the plane's speed matches that of the treadmill, and there will be air moving over the wing.

but if the treadmill can match the speed of the plane, engines and all, such that it does not move, there will be no air flow over the wing.

Damn, dreamz, you just convoluted this issue much more than necessary. First, the original post does state that the plane is either propeller or jet-engine based. No gliders.

Second, no plane in the world (that I know of) has powered wheels that provide the main source of propulsion.

Third, we already concluded that saying "the treadmill moves as fast as the wheels" is a paradox, so therefore the question should be "the treadmill matches the plane's speedometer".

Fourth, it is impossible for the treadmill to prevent the plane's forward motion, as the plane does not push against the ground to propel itself forward. The treadmill's motion is inconsequential.

Therefore, there is only one solution. The plane does move forward and take off.

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snyper
Thats a wrong anaolgy, your legs are propelling your body forward. That is not the case for the plane. If the wheel if free from the plane, than the thrust of the engine will move the plane forward. If it moves, there is lift.

no, thats not the comparsion im making..its showing how on a threadmill leg (wheel) movement does not create lift..ie fill the bag.

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jackwanders

But that's irrelevant, as planes don't use their wheels to produce lift. The better analogy is the skater.

put on a pair of rollerblades and get on a treadmill. hold a rope that's attached to the wall in front of you. when you turn the treadmill on, you are stationary as the treadmill moves under you. However, if you pull on the rope, you will move forward, as the treadmill cannot counter this new source of propulsion.

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Menge

no, thats not the comparsion im making..its showing how on a threadmill leg (wheel) movement does not create lift..ie fill the bag.

yes. but dreamz post assumes that the plane is propelled by the wheels - which is NOT how planes work.

planes are propelled by pushing air - not the ground below them.

so making an analogy with a car or a person doesn't work for planes.

the plane flies, assuming the treadmill is as big as a runway (which is how far it will need to go before acquiring proper speed to take off, like it were on asphalt).

to a plane, it doesn't really make a difference if the ground is pushing it forwards or backwards... only difference the ground makes to a plane is that it's fine as long as the plane doesn't "sink" into it - which is the purpose of the wheels!

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dreamz

Damn, dreamz, you just convoluted this issue much more than necessary.

i don't think so, but i'll get into it in this post.

First, the original post does state that the plane is either propeller or jet-engine based. No gliders.

i was going on what i've seen on the internet (e.g., other forums, other supposedly definitive answers, etc.). i meant the original creator of the question.

the glider case is significant in that it illustrates the principles of 1) the source of lift, and 2) net movement.

Second, no plane in the world (that I know of) has powered wheels that provide the main source of propulsion.

it's a thought experiment. it helps to point out the underlying structure or flaw in certain arguments.

Third, we already concluded that saying "the treadmill moves as fast as the wheels" is a paradox, so therefore the question should be "the treadmill matches the plane's speedometer".

i was merely pointing out the problem with the original question (cf. point 1).

Fourth, it is impossible for the treadmill to prevent the plane's forward motion, as the plane does not push against the ground to propel itself forward. The treadmill's motion is inconsequential.

it does push on the ground, given the mass and inertia of the plane, simply because the plane isn't suspended in the air. clearly, a propellor spinning at 1hz is not going to move the plane forward. any treadmill that pulls the plane back at a greater rate will overcome any possible forward movement.

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jackwanders

We're not dealing with the "original question" though, we're dealing with this question. Going on about other variations of the question isn't helping anything. Using a glider in the question defeats the purpose of the question altogether.

Also, in order for a treadmill to cancel out ALL forward motion of the plane, the friction generated in the wheel bearings would have to be so great that it overcomes the thrust produced by the engines.

Let's take a smaller example. Put a matchbox car on a treadmill and start it up. The car will move backwards, due to the inertia of the car and the friction of the wheel axles. However, as soon as you place a finger behind the car, it will stop moving, and its wheels will begins spinning at the speed of the treadmill. Lightly placing your finger has overcome all the force driving the car back. In fact, if you ever so slightly push your finger forward, the car will move forward on the treadmill. That's all the force it took to overcome the backward force of the treadmill. Why? Because the wheels are free-spinning, there is no engine driving the wheels.

Now go back to the plane. The plane is being pushed by a huge finger, one generating ridiculous amounts of thrust. Its wheels are also free-spinning. The treadmill simply cannot go fast enough to overcome that. The wheels would explode before that. So your observation, while valid, is irrelevant to the question. This is taking the question to a place it was never meant to go.

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snyper

everyones missing the point.

Aeroplane wont take off on a treadmill because ther is NO LIFT UNDER THE WINGS.

This can only be created by MOVING FORWARD.

No matter how fast you gon on a treadmill there is no moving forward.

FORGET WHAT PROPELS THE PLANE!

Its the theory of taking off from a treadmill

Remember MOVING FORWARD. . .

ok im going to stop yelling now :x

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Fred Derf

everyones missing the point.

Aeroplane wont take off on a treadmill because ther is NO LIFT UNDER THE WINGS.

This can only be created by MOVING FORWARD.

No matter how fast you gon on a treadmill there is no moving forward.

FORGET WHAT PROPELS THE PLANE!

Its the theory of taking off from a treadmill

Remember MOVING FORWARD. . .

ok im going to stop yelling now :x

The point was made that the plane will easily move forward regardless of the speed of the treadmill.

The treadmill could crank itself up to twice the velocity of the aircraft and it wouldn't make a difference (assuming the wheels can handle the friction and they don't overheat).

The plane will move forward.

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dreamz

The treadmill simply cannot go fast enough to overcome that. The wheels would explode before that. So your observation, while valid, is irrelevant to the question. This is taking the question to a place it was never meant to go.

it's completely relevant. no one ever said the treadmill couldn't go that fast. it's supposed to be a hypothetical situation, and no one has specified the limits of the experiment.

let's stick with the plane example, but cut out all variables.

put a plane on a treadmill and leave the engine off. start the treadmill. the plane will move backward because it's resting on a treadmill. so far, no problems. in fact, to begin with, turn it up so high that the plane will retreat at an astounding speed.

now, turn on the engine and assume that the propellor speed, design, etc. allows for a continuous increase (if an increase is desired) in forward movement. think of this as a continuous function over the closed interval [0, a], where 'a' is max(engine design). this is uncontroversial. there is no instantaneous (i.e., discontiuous) path. then for all points less than a given point, the plane will continue to retreat, and there will be another where it will stand still. this is like balancing a molecule between gravity and em.

there are two things to consider, the propulsion and the treadmill. we can consider all combinations of propulsion (high and low) and treadmill speeds (high and low). just as we can match a low propulsion with a low treadmill speed, we can match a high propulsion with a high treadmill speed.

now, to say that the plane will always pull forward is to say that the propellor is strong enough to overcome all of the factors considered.

getting back to the plane at rest, since a plane can propel itself from rest (with the use of its engines or whatever) and since rest is just like a plane being pushed along at some speed that is matched by the treadmill, and if we assume the engine is capable of additional thrust (so that it can move to the right of this point), the plane will move forward.

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jackwanders

everyones missing the point.

Aeroplane wont take off on a treadmill because ther is NO LIFT UNDER THE WINGS.

This can only be created by MOVING FORWARD.

No matter how fast you gon on a treadmill there is no moving forward.

FORGET WHAT PROPELS THE PLANE!

Its the theory of taking off from a treadmill

Remember MOVING FORWARD. . .

ok im going to stop yelling now :x

I hate to do this, but you're missing the point.

When a person runs, his forward motion is caused by pushing against the ground. Relatively speaking, he is pushing the ground backwards (equal and opposite reaction).

When a car drives, its forward motion is caused by the same force. The engine drives the axle, which spins the wheels. The wheels push against the ground, which in turn pushes the car forward.

Why does this work? Because the ground is stationary. You can't actually push the earth backwards, so instead you push yourself forward. The car's wheels cannot make the earth spin, so the car is pushed forward.

However, on a treadmill, we change that. The 'ground' is constantly moving backwards. So when a person runs on a treadmill, the ground is already being pushed backwards, so there is no forward movement. When a car is on a treadmill (or a dyno), there is already backwards motion so the car goes nowhere.

Now let's look at a plane. A plane is completely different from a person or a car. A plane does not propel itself by pushing the ground backwards. It propels itself by pushing the air backwards, either through a propeller or through a jet engine.

Let's look at another example. Ever see "Back to the Future"? In the scene where Marty is running away from Biff, he hops on a skateboard and grabs onto a jeep. Whereas normally his propulsion would be created by pushing off the ground, now he is being propelled by the force of the car pulling him (disregard the source of the car's propulsion, it's irrelevant). If the ground underneath Marty's skateboard were to start moving backwards, would he slow down? No, because the force that's pulling him along wouldn't change. The only thing that would happen would be that the wheels on the skateboard would spin faster.

Similarly, a plane "grabs" onto the air around it and pulls itself forward along the air, even when it's on the ground. Therefore, if the ground underneath the plane begins moving backwards (as with a treadmill), the plane would not slow down or stop. The wheels would spin faster, but the plane would continue moving forward. This forward motion would create lift, and the plane will take off.

I'm running out of ways to explain this...

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snyper

erm..your efforts are not in vain, i totally agree with you... you are focussing on the enegry, im just focusing on the "lift aspect"

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jackwanders

it's completely relevant. no one ever said the treadmill couldn't go that fast. it's supposed to be a hypothetical situation, and no one has specified the limits of the experiment.

let's stick with the plane example, but cut out all variables.

put a plane on a treadmill and leave the engine off. start the treadmill. the plane will move backward because it's resting on a treadmill. so far, no problems. in fact, to begin with, turn it up so high that the plane will retreat at an astounding speed.

now, turn on the engine and assume that the propellor speed, design, etc. allows for a continuous increase (if an increase is desired) in forward movement. think of this as a continuous function over the closed interval [0, a], where 'a' is max(engine design). this is uncontroversial. there is no instantaneous (i.e., discontiuous) path. then for all points less than a given point, the plane will continue to retreat, and there will be another where it will stand still. this is like balancing a molecule between gravity and em.

there are two things to consider, the propulsion and the treadmill. we can consider all combinations of propulsion (high and low) and treadmill speeds (high and low). just as we can match a low propulsion with a low treadmill speed, we can match a high propulsion with a high treadmill speed.

now, to say that the plane will always pull forward is to say that the propellor is strong enough to overcome all of the factors considered.

getting back to the plane at rest, since a plane can propel itself from rest (with the use of its engines or whatever) and since rest is just like a plane being pushed along at some speed that is matched by the treadmill, and if we assume the engine is capable of additional thrust (so that it can move to the right of this point), the plane will move forward.

Yes, I understand all that, which should be obvious after my posts here. But this is like taking the "rooster lays eggs on the roof" question and debating whether or not we have the ability to genetically modify a rooster so it can lay eggs.

The entire point of this question is to see if you can stop thinking of a plane like you would a car. That's really about it.

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Fred Derf

Why does this work? Because the ground is stationary. You can't actually push the earth backwards, so instead you push yourself forward. The car's wheels cannot make the earth spin, so the car is pushed forward.

Technically, at least to a physicist, you do push the earth backwards ever so slightly (to the point where it could not be measured). The same thing happens with gravity. If you jump up you will soon realize that the earth will pull you back down but technically there is an insignificant amount of gravity pulling the earth back up to you as well.

For all practical purposes however... no.

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vincent
everyones missing the point.

Aeroplane wont take off on a treadmill because ther is NO LIFT UNDER THE WINGS.

This can only be created by MOVING FORWARD.

No matter how fast you gon on a treadmill there is no moving forward.

FORGET WHAT PROPELS THE PLANE!

Its the theory of taking off from a treadmill

Remember MOVING FORWARD. . .

ok im going to stop yelling now :x

The thrust will help the plane overcome the restriction of the treadmill, thus the plane will move forward. I think a jet will have a better chance than a propellar based plane.

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snyper
The thrust will help the plane overcome the restriction of the treadmill

Perhaps im missing the point, my interpretation of the question was it the treadmill was to match the speed createed by the thrust., therefore cancel out the speed create by the trust of the engine, the question was to see if the wheels were moving why could the plane not take off, hence my answer ...no lift.

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jackwanders

Technically, at least to a physicist, you do push the earth backwards ever so slightly (to the point where it could not be measured). The same thing happens with gravity. If you jump up you will soon realize that the earth will pull you back down but technically there is an insignificant amount of gravity pulling the earth back up to you as well.

For all practical purposes however... no.

well, yes, but i'm trying to keep things as simple as possible.

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dreamz

erm..your efforts are not in vain, i totally agree with you... you are focussing on the enegry, im just focusing on the "lift aspect"

snyper, think about it this way.

hang a plane on a string from the ceiling (this is hypothetical). turn on the engines. does the plane move forward? of course.

Yes, I understand all that, which should be obvious after my posts here. But this is like taking the "rooster lays eggs on the roof" question and debating whether or not we have the ability to genetically modify a rooster so it can lay eggs.

The entire point of this question is to see if you can stop thinking of a plane like you would a car. That's really about it.

the whole point of my post was that it requires the confluence of two factors, the engine and the inertia (mass, friction, and all related factors), and that there are certain points (to the left of a certain critical point or below a certain design level) such that the plane will not take off. the original problem does NOT specify the engine, which is why i had to point it out, especially since this is what people consider.

but for a sufficiently strong engine (beyond the critical point), forward movement is not a problem. that a plane can accelerate from rest (on the ground) or move in the air without its wheels is evidence of this.

it is always important to consider 1) what is possible in a current state, and 2) what is required for a certain state to be possible. explaining both leads to a better understanding.

Perhaps im missing the point, my interpretation of the question was it the treadmill was to match the speed createed by the thrust., therefore cancel out the speed create by the trust of the engine, the question was to see if the wheels were moving why could the plane not take off, hence my answer ...no lift.

thrust is a function of air, not wheels (that's why a plane can fly through the air without using its wheels). you can't match the thrust of the engines by changing its wheels, but by countering the air (e.g., use a tornado, vacuum, etc.).

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Laurë

everyones missing the point.

Aeroplane wont take off on a treadmill because ther is NO LIFT UNDER THE WINGS.

This can only be created by MOVING FORWARD.

No matter how fast you gon on a treadmill there is no moving forward.

FORGET WHAT PROPELS THE PLANE!

Its the theory of taking off from a treadmill

Remember MOVING FORWARD. . .

ok im going to stop yelling now :x

The way the plane propels itself forward is important. If it used it's wheels like a car it could not move forward on the treadmill.

It uses air instead - which means it can move forward relative to the air and treadmill.

erm..your efforts are not in vain, i totally agree with you... you are focussing on the enegry, im just focusing on the "lift aspect"

How can you agree with something you do not understand?

Lift is created because the plane can move forward. It moves forward because of the method it uses to propel itself forward. It uses air, not the ground. Therefore, it doesn't matter what the ground is doing.

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snyper
thrust is a function of air, not wheels (that's why a plane can fly through the air without using its wheels). you can't match the thrust of the engines by changing its wheels, but by countering the air (e.g., use a tornado, vacuum, etc.).

oh..ok..the penny has just dropped..and now i kno what ripgut meant by sayind the propellor engine would have less effect.....

Regardless of the wheel speed on the threadmill it would be the thrust of the engine that would power the engine.. i think the plane on the piec of string did it ;)

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Laurë

Yeah I didn't get it straight away.

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jackwanders

Perhaps im missing the point, my interpretation of the question was it the tread mill was to match, therefore cancel out the speed create by the trust of the engine, the question was to see if the wheels were moving why could the plane not take off, hence my answer ...no lift.

This goes to the fact that this 'puzzle' is diffcult to word correctly, so as to avoid paradox and confusion.

There's a different between having the treadmill "match" and "cancel out" the forward speed of the plane.

For the treadmill to "match" the speed of the plane, the treadmill would simply go however fast the plane's speedometer read. However, this would have little overall effect on the forward speed of the plane, for reasons we've already discussed at length. The main effect would be that the wheels would be spinning twice as fast as they usually would.

For the treadmill to "cancel out" the speed of the plane, we'd have to, as dreamz said, have a treadmill move so fast that the frictional forces cancel the thrust of the plane's propeller or jet engine. In this case, the treadmill would be going many, many times faster than what the speedometer on the plane reads.

The only correct way to state the problem is as follows (imo):

A plane (either propeller or jet-engine based) is sitting on a treadmill. As the plane begins to move and speed up, the treadmill's speed will match that of the plane's speedometer.

Now, will the plane be able to take off?

Speaking of matching the wheel's speed, or cancelling out the speed of the plane are only going to confuse people and incite argument.

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snyper
Lift is created because the plane can move forward.

I know how lift is created.. ever stick ur hand out the window of a movin car at an angle?

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.fury

How are people still convinced it would fly, I don't know...

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vincent
How are people still convinced it would fly, I don't know...

Red Herring :sleep:

Please demonstrate why it will not fly...

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