A jet plane on a large treadmill

What this thread needs is a big snapper.

http://adv.alsscan.com/alsscan/tya5/trish179.jpg

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Originally Posted by Big Blue

I feel like I'm taking crazy pills. This is so mind-bogglingly retarded. I've completely lost respect for some individuals here.

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So have we

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Originally Posted by Big Blue

The plane moves forward with respect to a fixed point (say a tree on the edge of the runway). The runway moves backward at an equal speed. So, as the plane is doing 50 mph with respect to the tree, the runway is doing negative 50 mph with respect to the tree. So the wheels are spinning at 100mph.

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There are too many unstated vaiables, ie: weight of the plane, power generated by the engines, frictional resistance between the wheels of the plane and the conveyor belt, whether or not there is an instantaneous reaction by the belt, etc.

Beaver's got it all correct, absolutely. Everyone who says, oh try jumping on a treadmill and see what happens when speeds change is missing the fact it is the friction force of your feet to the treadmill that will send you flying backwards.

The only contact the jet has to the treadmill is wheels that spins essentially freely and without friction (relative to the massive thrust force the jet can generate). Apply X million pounds of force to something that can roll freely and it will move. The only change that occurs by being on a treadmill that rolls backwards at same speed the jet moves forwards is that it's wheels will spin at double speed.

Oh my god, some of you are really fucking stupid. :nonono2:

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Originally Posted by DJSapp

Ummm, Beaver is right. The only thing slowing the 747's velocity RELATIVE TO AIR is the rolling friction from the treadmill.

Simple example: A person is on a treadmill wearing roller skates and a rocket pack. The tredmill starts moving, and the person moves backward slowly due to rolling friction. The person flips on the rocket pack and blasts forward off the front. Why? The propulsion does not come from the wheels, it comes from the rocket pack.

Complex example: If the minimum takeoff speed for the 747 required 200 mph airspeed (speed of air moving over wing), the treadmill will be moving at 200 mph, the wheels will be spinning at 400 mph. The plane will be slowed by friction so the engines will need to overcome this. In theory the plane will take off.

Now to actually answer this in a more real situation I need to know what the strength and rolling resistance of the tires are, and the maximum thrust of the 747's engines. There are two ways the plane would not take off: 1. The tires blow out because they are rotating too fast to achieve the required airspeed, or 2. The rolling resistance is too great for the engines to overcome.

Got to go, Ogre is knocking on my door.

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I've only read up to DJ's post and I don't have any time to read any further right now. But...

My physics prof would have ripped everyone, who posted up to this point, a new one for even attempting to answer a question without having enough information to answer it. Too many assumptions, too many unknowns. DJ has provided examples that do have enough information to come up with an answer.

That is all.

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Originally Posted by Cornholio

But, as I read it, the treadmill functions to make the plane's velocity, relative to the control tower, zero.

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This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

It says nothing about making the speed zero. the plane has to have some velocity for the treadmill to have a velocity. If the plane stays still, the treadmill doesn't move.

C'mon, Beav - this thread beat any of the Ogre threads to hell and now I can't participate...

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Originally Posted by Big Blue

I feel like I'm taking crazy pills. This is so mind-bogglingly retarded. I've completely lost respect for some individuals here.

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Seconded. I've only made one post and I give up already.

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Originally Posted by MeatPuppet

My physics prof would have ripped everyone, who posted up to this point, a new one for even attempting to answer a question without having enough information to answer it. Too many assumptions, too many unknown

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I dunno, it took the Science Teachers forum 133 pages :nonono2:

Forward Force doesn't produce lift; airflow over the wing does. You need to be moving relative to the air to acheive lift. As long as the plane is never moving relative to the air (and if it's on a treadmill, it's position is stationary) it has no lift - it stays on the ground. It's speed relative to the treadmill will approach the speed of light/failure speed of the treadmill because there is no friction, BUT IT WON'T FUCKING TAKE OFF!

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Originally Posted by MeatPuppet

My physics prof would have ripped everyone, who posted up to this point, a new one for even attempting to answer a question without having enough information to answer it.

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Heh.

I think you'll find that you never have all the information you need to solve a problem. Assumptions are the only way we find the mathematical approximations that we do.

Beaver is right. I had to think about it for a bit. Imagine a 3 wheeler with the front (middle, non-drive) wheel on a treadmill and two rear wheels (drive wheels) on the regular ground.

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Originally Posted by mcsquared

Beaver is right. I had to think about it for a bit. Imagine a 3 wheeler with the front (middle, non-drive) wheel on a treadmill and two rear wheels (drive wheels) on the regular ground.

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You need to be moving relative to the air to acheive lift. As long as the plane is never moving relative to the air (and if it's on a treadmill, it's position is stationary) it has no movement relative to air. Therefore no lift - it stays on the ground. It's speed relative to the treadmill will approach the speed of light/failure speed of the treadmill because there is no friction, BUT IT WON'T FUCKING TAKE OFF!

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Originally Posted by cj001f

You need to be moving relative to the air to acheive lift. As long as the plane is never moving relative to the air (and if it's on a treadmill, it's position is stationary) it has no movement relative to air. Therefore no lift - it stays on the ground. It's speed relative to the treadmill will approach the speed of light/failure speed of the treadmill because there is no friction, BUT IT WON'T FUCKING TAKE OFF!

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There's no need to yell.

It's just a different reading of the problem.

I think it's all relative to the stationary object, like the control tower.

They think differently. (although I still don't see what they're measuring speed against.)

The more I think about it, the more feasible Beaver's assessment of this problem seems to be. Imagine if the treadmill were rotating in the opposite direction, it wouldn't make it any easier to take off (assuming 0 friction in the wheel bearings). The plane would just sit motionless on the treadmill until it throttled up at which time the jets would push against the air causing it to move forward until eventually reaching takeoff speed.

:rolleyes:

Lets think about this for a second... If you have EVER studied the premise on which modern airplanes manage to stay in the air you know that it is due to the shape of the wing. The straight bottom of the wing creates high air pressure whilst the rounded top of the wing creates low pressure because the air takes longer to get over it. Without airflow, that bitch is going no where. Unless of course you strap an ariane 5 to it :eek:

Since there is no air flow over the wing (which, btw is what an airplane requires to takeoff, not just thrust. That's also why gliders fly!!!) You are technically doing the same thing as parking a 747 at the start of a run way and flicking the thrust onto max, is the plane gonna take off right then and there? nope. It needs to get up to a FORWARD MOVING speed so that there is enough airflow over the wing to create lift. unless we are talking rocket or space-shuttle type thrust on the 747 (which it simply does not have, sorry) it still needs to roll. And if you COULD theoretically get the conveyor belt to work, don't you think that your local international airport would have implemented that already (or at least Japan)?


Here's your sign.

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Originally Posted by belgian

:rolleyes:

Lets think about this for a second... If you have EVER studied the premise on which modern airplanes manage to stay in the air you know that it is due to the shape of the wing. The straight bottom of the wing creates high air pressure whilst the rounded top of the wing creates low pressure because the air takes longer to get over it. Without airflow, that bitch is going no where. Unless of course you strap an ariane 5 to it :eek:

Since there is no air flow over the wing (which, btw is what an airplane requires to takeoff, not just thrust. That's also why gliders fly!!!) You are technically doing the same thing as parking a 747 at the start of a run way and flicking the thrust onto max, is the plane gonna take off right then and there? nope. It needs to get up to a FORWARD MOVING speed so that there is enough airflow over the wing to create lift. unless we are talking rocket or space-shuttle type thrust on the 747 (which it simply does not have, sorry) it still needs to roll. And if you COULD theoretically get the conveyor belt to work, don't you think that your local international airport would have implemented that already (or at least Japan).

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OK, this doesn't make much sense.

We all agree that if there's no airflow (forward velocity) then there's no takeoff.

The question is if there's forward velocity.

I say no, because that's how I read the problem.

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Originally Posted by SnowRider4Life

The more I think about it, the more feasible Beaver's assessment of this problem seems to be. Imagine if the treadmill were rotating in the opposite direction, it wouldn't make it any easier to take off (assuming 0 friction in the wheel bearings). The plane would just sit motionless on the treadmill until it throttled up at which time the jets would push against the air causing it to move forward until eventually reaching takeoff speed.

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clap clap clap clap clap clap,

and for the people that think that the plane will not take off, what are the JET engines pushing against?

that is the treadmill pushing against?, no its not the plane.

The correct answer.

this question has caused so much chaos all over the internet, im glad it is doing the same here.

I think a lot of you have been tricked by the wording of the question. It says nothing about the conveyor keeping the plane stationary, just moving at the same speed as the plane in the opposite direction. As long as the plane's wheels can spin at twice the normal take-off speed without frying the bearings it will take off.

edit: also, fastest thread to 5 pages ever.

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Originally Posted by Dantheman

I think a lot of you have tricked by the wording of the question. It says nothing about the conveyor keeping the plane stationary, just moving at the same speed as the plane in the opposite direction. As long as the plane's wheels can spin at twice the normal take-off speed without frying the bearings it will take off.

edit: also, fastest thread to 5 pages ever.

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See, that's not how I read it. If you read it like you just said, then the plane will take off.

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Originally Posted by Cornholio

See, that's not how I read it. If you read it like you just said, then the plane will take off.

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Of course, since the jet engines don't push against the treadmill, but the stationary air. Thank you for simplifying the answer, DTM.

Beaver, why not get rid of the big dripping pussy now, hmmmmm?

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Originally Posted by Tippster

Of course, since the jet engines don't push against the treadmill, but the stationary air. Thank you for simplifying the answer, DTM.

Beaver, why not get rid of the big dripping pussy now, hmmmmm?

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No, it has nothing to do with what's pushing the plane forward. Well, not much.

It's more to do with how the plane's speed is measured.

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Originally Posted by Cornholio

It's more to do with how the plane's speed is measured.

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Yup.

If the planes speed is measured against the treadmill belt, and the treadmill belt matchs the speed in the opposite direction it will never, ever takeoff unless the treadmill as a whole is moving.

If the planes speed is measured against the air, it will take off when it reachs takeoff speed, no matter what speed the treadmill is moving at. This is why airplanes takeoff facing into the wind - lower groundspeed required, shorter takeoff distance.

To answer this question once-and-for-all I think we're gonna have to call in some pros.

http://imageserver4.textamerica.com/...1700130517.jpg

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Originally Posted by Cornholio

No, it has nothing to do with what's pushing the plane forward. Well, not much.

It's more to do with how the plane's speed is measured.

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Originally Posted by cj100f

Yup.

If the planes speed is measured against the treadmill belt, and the treadmill belt matchs the speed in the opposite direction it will never, ever takeoff unless the treadmill as a whole is moving.

If the planes speed is measured against the air, it will take off when it reachs takeoff speed, no matter what speed the treadmill is moving at. This is why airplanes takeoff facing into the wind - lower groundspeed required, shorter takeoff distance.

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Didn't I say this somewhere around page 2?

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Originally Posted by DJSapp

Didn't I say this somewhere around page 2?

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yeah, but that didn't leave enough room for insults, fueds, pontification, half baked theories, porn, jokes, random pictures... you know, the meat of TGR

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Originally Posted by The Big One

A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

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"tracks the planes speed" is where people are hung up. How do you measure the speed if it's not moving? I think the logical assumption he means, is the conveyer belt will replicate (in the opposite direction) the ground speed that a 747 normally accelerates to in relation to the amount of thrust applied and for how long. Again... the conveyer belt is simulating the normal expected ground speed of a 747 taking off in relation to the amount of thrust and the duration of that thrust and thus moving the belt in the opposite direction to match that ground speed. If that's the case the 747 isn't going anywhere.

Would you "it will fly" believers agree to that much?

This thread proving once and for all that the average person is dumber than average. Lord.

I spent 10 years arguing with friends on the tailgate up, tailgate down theory, and after the masters over at mythbusters finally proved that I was correct- I won a beer over the deal and proved that my intelligence is indeed above average, which quite obviously is not saying much of anything at all, bringing me to my final thought.

If I am dating a girl that talks mach fast and doesn't allow me to get a word in edgewise, and so I talk faster, causing her to jabber even faster, causing me to match her rate of speech and raise her a vowel, will she eventually look at me lustfully, spread her wings, reveal her landing strip, and ask me to play "lets see who can be quiet the longest?"

Or am I better of just listening in amazement?

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Originally Posted by flabango

"tracks the planes speed" is where people are hung up. How do you measure the speed if it's not moving?

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Reread the question, there is nothing said about "keeping the plane stationary" or "plane not moving"

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Originally Posted by Lloyd Christmas

Its good to know I'm not the only one wondering about KITT. I actually asked my girlfriend that exact question last week... she didn't know either.

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So what.....you were sitting around watching Knight Rider DVDs or something?

It is a decent question, but I just dont sitting around thinking about Hasselhoff that often.

No- every time I see an empty car-transport 18 wheeler, I think to myself- I wonder if I could drive up on there like KITT did back in the day. I guess I'll never know the answer.

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Originally Posted by Dantheman

Reread the question, there is nothing said about "keeping the plane stationary" or "plane not moving"

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Uhhhmmmm... ya right. Excuse me while I go bash my head in with a hammer. Then maybe I'll understand. :rolleyes2

Well worded explanation of why the plane would take off here...
http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/

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Originally Posted by dbp

Well worded explanation of why the plane would take off here...
http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/

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he's got one thing right

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Gah… people are freakin’ stupid

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An airplane doesn't take off from forward thrust.It takes off when the airflow over the wings generates enough lift to raise it off the ground.

I'm a physicist. Reading this thread is hilarious. I starting thinking Beaver was an idiot, but actually he's mostly right. The confusion is because the statement of the problem is incorrect. If the plane stayed stationary relative to the ground, indeed it would not take off. But it won't stay stationary relative to the ground (assuming the wheels spin without friction). The plane will move relative to the treadmill, no matter how fast the treadmill moves. So it will merely fall off the front of the treadmill. Or, if the treadmill is very long, it will speed up relative to both the ground and the air, and then take off.

Think of it with respect to the wheelchair. You're sitting in a wheelchair on a treadmill. The wheelchair wheels turn w/o friction. I stand behind you and hold you in place. The treadmill starts to move. I just hold you there, and the wheels spin. The treadmill moves faster. I don't have to hold you with any more force, the wheels just spin faster. Work is done by the treadmill on the wheels, but there is no force whatsoever on the wheelchair from the treadmill. Its only function is to spin the wheels faster. Now say I walk alongside the treadmill and push you along. The speed you move forward is completely independent of the speed with which the treadmill turns.

This is basically the same thing that's happening to the plane, because as Beaver points out the engines are pushing you relative to the air, not the ground.

Edit: reread the original statement of the problem, and it's actually not incorrect, just intentionally confusing.

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Originally Posted by bircheater

The plane will move relative to the treadmill, no matter how fast the treadmill moves.

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But as stated the treadmill moves at exactly the same velocity in the opposite direction as the airplane instantaneously no matter how fast the airplane is moving! The airplane thrust will keep getting converted into acceleration, and it will infinitely accelerate, matched all the while by the treadmill. The problem defined is highly unphysical.

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Originally Posted by cj001f

he's got one thing right


An airplane doesn't take off from forward thrust.It takes off when the airflow over the wings generates enough lift to raise it off the ground.

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Everybody knows that. The argument has been (for some time) whether the plane moves forward (on the ground) or remains stationary (on the ground). Everyone who thinks the plane will take off believes that the treadmill can never make the plane remain stationary. They think the plane will move forward regardless of the treadmill and thus there will be airflow...

Forget about taking off. "Won't takeoff" people need to argue how the treadmill can prevent the plane from moving forward. Actually, instead of a plane, think of some other vehicle with free spinning wheels (say a car out of gear... and a jet strapped to the back). Will the treadmill keep it stationary?

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Originally Posted by cj001f

But as stated the treadmill moves at exactly the same velocity in the opposite direction as the airplane instantaneously no matter how fast the airplane is moving! The airplane thrust will keep getting converted into acceleration, and it will infinitely accelerate, matched all the while by the treadmill. The problem defined is highly unphysical.

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The treadmill could move in the opposite direction with twice the velocity. It still can't keep the plane from moving forward - the wheels are free spinning. That is the key to the problem.