[Guest]

This is a very highly debated topic, even though scaled tests have been conducted. The popular Mythbusters show on discovery did it full scale, but some still dispute the way they did it. You can probably find thousands of forums that host this same question, and I wouldn't be suprised if it has even been posted here before. So here goes!

A large passenger jet sits on a tredmill the size of a normal runway that they would normally take off on. When the jet starts up, it climbs in speed slowly. The tredmill matches the jet's speed EXACTLY. For every mile an hour the jet climbs, the tredmill goes in REVERSE that exact amount as-well.

The grand question: Will the plane take-off?

Assume that:

The wind is at 0 MPH

The Jet is powered by turbines

Edited September 6, 2008 by RedRum

[Guest]

just a question prime - have you wondered what the minimum take off speed is - btw, the experiment has been done and light was not involved - well, it was a 'light-aircraft' using a propeller

I think I'm done with this topic! Interesting this one was on here a long tome ago, just they get lost in amongest so many others.

[Prime]

just a question prime - have you wondered what the minimum take off speed is - btw, the experiment has been done and light was not involved - well, it was a 'light-aircraft' using a propeller

I think I'm done with this topic! Interesting this one was on here a long tome ago, just they get lost in amongest so many others.

I was talking about the speed of the rotation of the wheels, not airplane ground speed. Sorry, if that was unclear.

[Guest]

I think it has been done by the great BN

There is something missing to the explanation - when will it take off?

It will take off at its normal takeoff speed...

[Guest]

How can that be possible? - If the treadmill matches the speed exactly, the plane will remain motionless -

If the plane is motionless, what is its speed?

[Guest]

I think everyone who disagreed with the answer stopped posting about 4 pages ago. It's amazing how long people can argue with each other about something they agree on.

[Guest]

I think the plane would take off, regardless of what the "ground" (in this case, the treadmill) is doing.

Like others have surely stated, the wheels on an airplane have no effect on what the plane's thrust. I think posters before me have said that the turbines will "push off air," but in a physics sense, it doesn't push off the air (what air is there for a spacecraft "push off of" in space?). It's pushing itself (citing Newton's third law)! There may have been a better way to say that.

The treadmill can be moving thousands of times faster than the airplane would move (in order to take off), but all that would happen is that the wheels would turn really fast, and the turbines of the plane would make available a force that would propel the plane forward through space. If it wasn't for gravity, planes wouldn't NEED wheels. Hmm. Would it even be called a plane?

[Guest]

In the model, where the conveyor matches the wheels rotation, and the plane does not take off -- it is not just friction that counteracts the force of the jets. As the plane edges forward, the belt has to move faster to match the wheels' rotation. The wheels rotate faster. There is an acceleration in RPMs. Acceleration requires force. As speed becomes comparable to the speed of light the acceleration becomes more and more costly in terms of energy required.

None of this makes a lick of sense. You agree that a plane will take off if the conveyor belt is calibrated to move at the same speed but opposite direction of the plane. Why do you think it will be any different if the treadmill moves any faster? The wheels will just spin faster.

Why does the treadmill need to go the speed of light if it matches the wheels' speed? I already mentioned it's not possible for the treadmill to match the speed of the wheels. As the plane moves forward at 20MPH, the wheels are also moving 20 MPH. How fast is the treadmill going in the opposite direction? If it's matching the speed of the wheels, it must be going at 20MPH. But if the plane is moving forward at 20MPH, and the treadmill is moving backwards at 20MPH, the wheels are really spinning at 40MPH. See the problem?

You say,

"If the belt matches the rotation of the wheels -- the plane does not take off."

The plane takes off if the treadmill is going the same speed as the plane or even five times faster. While the conveyor does exert some modest backward force on the plane, that force is easily overcome by the thrust of the engines pulling the plane ahead. The plane moves forward at roughly its usual speed relative to the ground and air, generates lift, and takes off.

I think posters before me have said that the turbines will "push off air," but in a physics sense, it doesn't push off the air (what air is there for a spacecraft "push off of" in space?). It's pushing itself (citing Newton's third law)!

There is no air for the props or turbine engines to shove backwards, that's why these planes won't work in outer space. With no atmosphere, you'd need liquid or solid rockets to push stored materials in one direction to experience a thrust force in the opposite direction.

[Guest]

In the model, where the conveyor matches the wheels rotation, and the plane does not take off -- it is not just friction that counteracts the force of the jets. As the plane edges forward, the belt has to move faster to match the wheels' rotation. The wheels rotate faster. There is an acceleration in RPMs. Acceleration requires force.

After some thought I think Prime is right. If energy being put into accelerating the wheels is the same as the energy output of the engines then the plane would stay stationary, and the rotation speed of the wheels would match the speed of the conveyor belt. The belt would have to be constantly accelerating. Of course, the mass of the wheels is so insignificant compared to the force produced by the engine that the conveyor belt would have to be moving ridiculously fast (and then continue accelerating indefinitely). So if the conveyor speed matches the wheel speed the plane won't take off. Of course the much more practical question is if the conveyor speed matches the plane speed, in which case the plane will take of easily.

[Guest]

After some thought I think Prime is right. If energy being put into accelerating the wheels is the same as the energy output of the engines then the plane would stay stationary, and the rotation speed of the wheels would match the speed of the conveyor belt. The belt would have to be constantly accelerating. Of course, the mass of the wheels is so insignificant compared to the force produced by the engine that the conveyor belt would have to be moving ridiculously fast (and then continue accelerating indefinitely). So if the conveyor speed matches the wheel speed the plane won't take off. Of course the much more practical question is if the conveyor speed matches the plane speed, in which case the plane will take of easily.

Accelerating the wheels - where is the drive shaft? the wheels frees spin on the ground, ice, water treadmill or anything, if in the air, they do not spin -so what is propelling the plane - the engines force is not applied to the wheels at all they are simply an undercarriage to allow landing and for rolling while gathering speed from engine propulsion.

put the plane on ice with skies - how is it gonna take off without wheels?

[Guest]

After some thought I think Prime is right. If energy being put into accelerating the wheels is the same as the energy output of the engines

What does that even mean? A plane moves forward because its engines and the wheels spin as a result. A plane on a treadmill moving backwards will have its wheels spinning faster, but the energy to do so comes from the treadmill. The effect it has on the plane being propelled forward is so minimal it's barely measurable.

the rotation speed of the wheels would match the speed of the conveyor belt.

It would? How? The planes turbines are turned on full force. It shoves air backwards with enough force to propel the plane forward. The plane and its wheels in the first split second are clocked at 2 MPH. How fast is the conveyor belt moving in your scenario?

[Guest]

Accelerating the wheels - where is the drive shaft? the wheels frees spin on the ground, ice, water treadmill or anything, if in the air, they do not spin -so what is propelling the plane - the engines force is not applied to the wheels at all they are simply an undercarriage to allow landing and for rolling while gathering speed from engine propulsion.

put the plane on ice with skies - how is it gonna take off without wheels?

Energy is consumed in order to accelerate the wheels because of Newton's first law. If the conveyor belt were moving at a constant speed then no energy would be consumed and the plane would move forward without hindrance from the conveyor. The specific math is (moment of inertia) x (rotational acceleration) = (force from the jets). If this happens then the energy being consumed forward and the energy backward will be the same, and the plane will be stationary. As I said before though, this type of acceleration is way beyond any practical limits, so this is a purely theoretical situation. Also, if the wheels were massless then you would be correct, the plane would take off fine.

[Guest]

Energy is consumed in order to accelerate the wheels because of Newton's first law. If the conveyor belt were moving at a constant speed then no energy would be consumed and the plane would move forward without hindrance from the conveyor.

The treadmill accelerates as the plane accelerates. This causes the wheels to spin faster, but the plane still takes off. The reasons why have been explained numerous times.

The specific math is (moment of inertia) x (rotational acceleration) = (force from the jets). If this happens then the energy being consumed forward and the energy backward will be the same, and the plane will be stationary.

What are you talking about? Are you saying if it takes as much energy to move a treadmill as it does to move a plane, the plane can't take off? All of the energy used to propel the plane is in one direction, so I can't see what else you may be talking about. It doesn't matter if this treadmill needs a serious grease job and consumes twice as much energy as the plane. The plane knows nothing about what energy the treadmill consumes to move and spin its wheels faster.

Also, if the wheels were massless then you would be correct, the plane would take off fine.

I know I'm gonna regret asking this but... why?

[Guest]

I'm not sure what your physics knowledge is but heres a quick overview of how this works. It takes energy in order to make anything move. In order to change the velocity of a plane a certain amount it takes a set amount of energy. The equation for this is .5*(mass of plane)*(final velocity-initial velocity). It also takes energy in order to make anything with mass rotate. Just like with the plane, in order to change the rotational velocity of the wheel it takes a set amount of energy. The equation for this is .5*(moment of inertia)*(final rotational velocity-initial rotational velocity). The energy actually detracts from the work that the engine does. Energy is consumed by the wheels whenever a plane takes off, it just makes such a small difference that it doesn't impact anything. But if the change in the rotational velocity is extremely great then this energy is no longer negligible. The faster the conveyor accelerates the larger the change in rotational velocity of the wheels will be, and the more energy is consumed. Also, if the mass in 0 then the moment of inertia will be 0, meaning that it won't take any energy to accelerate the wheels.

[Guest]

hmmmm..., would you mind answering my questions starting in post #111 so we can actually get someplace? Otherwise we're just spinning our wheels.

Energy is consumed by the wheels whenever a plane takes off, it just makes such a small difference that it doesn't impact anything.

Right, such a small amount, as you say "it doesn't impact anything". Since it's such a small amount, would it impact anything if the treadmill was moving twice as fast? No.

You said, "So if the conveyor speed matches the wheel speed the plane won't take off."

Why? Answer the question I asked you earlier:

"The planes turbines are turned on full force. It shoves air backwards with enough force to propel the plane forward. The plane and its wheels in the first split second are clocked at 2 MPH. How fast is the conveyor belt moving in your scenario?"

Edited September 9, 2008 by Scraff

[Guest]

hmmmm..., would you mind answering my questions starting in post #111 so we can actually get someplace? Otherwise we're just spinning our wheels.

Right, such a small amount, as you say "it doesn't impact anything". Since it's such a small amount, would it impact anything if the treadmill was moving twice as fast? No.

You said, "So if the conveyor speed matches the wheel speed the plane won't take off."

Why? Answer the question I asked you earlier:

"The planes turbines are turned on full force. It shoves air backwards with enough force to propel the plane forward. The plane and its wheels in the first split second are clocked at 2 MPH. How fast is the conveyor belt moving in your scenario?"

The energy consumed by the wheels accelerating in a real life scenario is minimal. The scenario I am talking about probably has the conveyor belt accelerating at over 10,000 m/s/s, (this is just an estimate, I don't know without real numbers). Which means that after even just after a few seconds it is moving several tens of thousands m/s. Like I said, purely theoretical. The much more realistic question is if the conveyor matches the plane's speed. Which, as has been said, the plane will take off at. I hope this answers your questions.

Edited September 9, 2008 by hmmmm...

[Guest]

hmmmm..., the way discussions work on message boards when two disagree, is when one asks questions, the other answers. I asked you to directly respond to my questions. If you're not going to do that, you're wasting my time.

The scenario I am talking about probably has the conveyor belt accelerating at over 10,000 m/s/s.

Why? You said, "So if the conveyor speed matches the wheel speed the plane won't take off".

I'll ask a third time:

"The planes turbines are turned on full force. It shoves air backwards with enough force to propel the plane forward. The plane and its wheels in the first split second are clocked at 2 MPH. How fast is the conveyor belt moving in your scenario (the conveyor speed matches the wheel speed)?"

If the planes just started moving (2 MPH), and "the conveyor speed matches the wheel speed", are wheels spinning so fast the conveyor is going "over 10,000 m/s/s"?

In the first split second when the plane is clocked at 2 MPH, how fast are the wheels going and how fast is the treadmill going?

[Guest]

hmmmm..., the way discussions work on message boards when two disagree, is when one asks questions, the other answers. I asked you to directly respond to my questions. If you're not going to do that, you're wasting my time.

Why? You said, "So if the conveyor speed matches the wheel speed the plane won't take off".

I'll ask a third time:

"The planes turbines are turned on full force. It shoves air backwards with enough force to propel the plane forward. The plane and its wheels in the first split second are clocked at 2 MPH. How fast is the conveyor belt moving in your scenario (the conveyor speed matches the wheel speed)?"

If the planes just started moving (2 MPH), and "the conveyor speed matches the wheel speed", are wheels spinning so fast the conveyor is going "over 10,000 m/s/s"?

In the first split second when the plane is clocked at 2 MPH, how fast are the wheels going and how fast is the treadmill going?

I apologize, I have misunderstood your question and your confusion. You're making an incorrect assumption with the question though. The plane will never be clocked at 2 mph. The plane never moves an inch. The whole point of my scenario is that it doesn't shove air backwards with enough force to propel the plain forward. After a second the treadmill will be moving at 10,000 m/s and the wheel will be rotating at 10,000 m/s. Sorry, hope this answers the question. If not maybe try asking in a different way?

[Guest]

I apologize, I have misunderstood your question and your confusion.

Some apology. My confusion? I have been asking you the same direct questions multiple times!

You're making an incorrect assumption with the question though. The plane will never be clocked at 2 mph. The plane never moves an inch. The whole point of my scenario is that it doesn't shove air backwards with enough force to propel the plain forward. After a second the treadmill will be moving at 10,000 m/s and the wheel will be rotating at 10,000 m/s.

I have a feeling you're being difficult on purpose. If you weren't, you would just explain "why" and not make me ask.

If the plane is shoving air backwards with enough force to propel the plane forward if it were on a normal runway, why would it not do the same on your treadmill?

Why are the wheels rotating at 10,000 m/s if the plane is still? Please answer this question. If the plane is still, what force caused the wheels to spin? Remember, it can't be the treadmill, because it won't move until the wheels do because, "the conveyor speed matches the wheel speed".

Please answer these questions also, they're important for us to move forward:

What about a split second after the plane's thrusters are on? If the problem is that the treadmill and/or wheels cant go 10,000 m/s, what about after that split second later when the treadmill is going 100 mph? Is the plane still? What's causing the wheels to spin so fast that the treadmill is going so fast to match its speed? Again, remember, the wheels of an aircraft only spin if it's moving forward and your treadmill only moves if there are wheels spinning.

Sorry, hope this answers the question. If not maybe try asking in a different way?

If you actually answered my questions or asked me to re-phrase the several times I asked the same ones earlier and there was a communication problem, that would be acceptable. We could then maybe ascertain that I need to ask them in another way. But what you did was ignore them. Me not asking in a different way was not the issue.

[Guest]

Man, if that last post didn't just put the nail in the coffin, I'm not sure what will. Nice job, Scraff!

[Guest]

Some apology. My confusion? I have been asking you the same direct questions multiple times!

I have a feeling you're being difficult on purpose. If you weren't, you would just explain "why" and not make me ask.

If the plane is shoving air backwards with enough force to propel the plane forward if it were on a normal runway, why would it not do the same on your treadmill?

Why are the wheels rotating at 10,000 m/s if the plane is still? Please answer this question. If the plane is still, what force caused the wheels to spin? Remember, it can't be the treadmill, because it won't move until the wheels do because, "the conveyor speed matches the wheel speed".

Please answer these questions also, they're important for us to move forward:

What about a split second after the plane's thrusters are on? If the problem is that the treadmill and/or wheels cant go 10,000 m/s, what about after that split second later when the treadmill is going 100 mph? Is the plane still? What's causing the wheels to spin so fast that the treadmill is going so fast to match its speed? Again, remember, the wheels of an aircraft only spin if it's moving forward and your treadmill only moves if there are wheels spinning.

If you actually answered my questions or asked me to re-phrase the several times I asked the same ones earlier and there was a communication problem, that would be acceptable. We could then maybe ascertain that I need to ask them in another way. But what you did was ignore them. Me not asking in a different way was not the issue.

We have a difference in assumptions about the question. I was thinking that the treadmill could accelerate simultaneously with the wheels. I feel like as long as the treadmill's speed matches the wheel's speed that satisfies the OP. In fact that is the only way that the treadmill's speed can match the wheel's speed, because the wheels speed are based on the treadmill's speed (assuming they don't slip). So based on these assumptions, it is the treadmill that is causing the wheels to rotate so fast. I'm imagining a scenario where the force that is produced on the plane from the rotation of the wheels is identical to the force applied to the plane by the engine, with the treadmill moving simultaneously with the wheels. And once again I am sorry that I didn't answer your question. I misunderstood it and thought I was answering when in fact it wasn't.

[Guest]

We have a difference in assumptions about the question. I was thinking that the treadmill could accelerate simultaneously with the wheels.

No different assumption at all. I'm assuming the same.

I feel like as long as the treadmill's speed matches the wheel's speed that satisfies the OP.

No, the OP is about the treadmill matching the forward speed of the plane. We both agreed the plane would take off in that scenario. I'm talking about your scenario:

"So if the conveyor speed matches the wheel speed the plane won't take off".

Those are your words! I have copy and pasted them repeatedly and questioned you on those very words repeatedly - you knew darn well what you said and it was those words I was debating you on!

What's your explanation for this (your words):

"The whole point of my scenario is that it doesn't shove air backwards with enough force to propel the plain forward. After a second the treadmill will be moving at 10,000 m/s and the wheel will be rotating at 10,000 m/s."

Why will the plane be still in that scenario? What's causing the wheels to spin if the plane is still? What will be causing the wheels to spin a split second later when the wheels are going 100 MPH and the plane is miraculously still?

It's obvious you're trying to save face at this point, hmmmm..., and try to pin this on a misunderstanding between us and not you just being plain incorrect. If that's the case, re-think just admitting your wrong, save us both a lot of aggravation, and you'll actually look better for it.

If I'm wrong about that last assumption, then please answer for me why a plane that has not moved an inch has spinning wheels.

[Guest]

It's obvious you're trying to save face at this point, hmmmm..., and try to pin this on a misunderstanding between us and not you just being plain incorrect. If that's the case, re-think just admitting your wrong, save us both a lot of aggravation, and you'll actually look better for it.

This is good advice. Those who admit they're wrong look a lot better than those that try to wiggle out of not being perfect by continually saying "I thought you meant this...it's not that my logic has failed in any way". Otherwise, I too would like to know why that still airplane's wheels are spinning.

[Guest]

In fact that is the only way that the treadmill's speed can match the wheel's speed, because the wheels speed are based on the treadmill's speed (assuming they don't slip).

What are you talking about? Then what is it that's determining the treadmill's speed?

Once again, you said,

"So if the conveyor speed matches the wheel speed the plane won't take off".

The conveyor (in your own scenario) matches the wheel speed. The wheel speed of course is determined by the plane's forward movement. Where are you getting the super-fast treadmill speed from? What determines its speed if its not the wheel speed?

[Guest]

No different assumption at all. I'm assuming the same.

No, the OP is about the treadmill matching the forward speed of the plane. We both agreed the plane would take off in that scenario. I'm talking about your scenario:

"So if the conveyor speed matches the wheel speed the plane won't take off".

Those are your words! I have copy and pasted them repeatedly and questioned you on those very words repeatedly - you knew darn well what you said and it was those words I was debating you on!

What's your explanation for this (your words):

"The whole point of my scenario is that it doesn't shove air backwards with enough force to propel the plain forward. After a second the treadmill will be moving at 10,000 m/s and the wheel will be rotating at 10,000 m/s."

Why will the plane be still in that scenario? What's causing the wheels to spin if the plane is still? What will be causing the wheels to spin a split second later when the wheels are going 100 MPH and the plane is miraculously still?

It's obvious you're trying to save face at this point, hmmmm..., and try to pin this on a misunderstanding between us and not you just being plain incorrect. If that's the case, re-think just admitting your wrong, save us both a lot of aggravation, and you'll actually look better for it.

If I'm wrong about that last assumption, then please answer for me why a plane that has not moved an inch has spinning wheels.

As I said before, the wheels are spinning because the treadmill is moving underneath them. As long as there is equal force pushing the plane forward and backward the plane will not move. The engines start up and at the exact same moment that the treadmill starts moving. The treadmill will create a force on the plane no matter how fast it accelerates. Its simply a matter of accelerating fast enough so that the forces are equal. So the engines will run and the treadmill will move, keeping the plane stationary. But since the treadmill is accelerating so will the wheels. The wheels rotating speed is not impacted by the plane's speed. I don't see what you aren't understanding about this scenario. You haven't actually attacked any of the points that I've made either. You've just asked me questions, and then gotten mad at me. If you see something that I have said that is wrong just point that out to me.

Edited September 9, 2008 by hmmmm...

[Guest]

Where are you getting the super-fast treadmill speed from? What determines its speed if its not the wheel speed?

I'm sure there's one of those evil personal trainers behind the curtain. They love doing that.