A jet plane on a large treadmill

[Dantheman]

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.

[Cornholio]

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.

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

[Tippster]

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

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?

[Cornholio]

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?

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.

[cj001f]

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

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.

[Dantheman]

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

[DJSapp]

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.

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.

Didn't I say this somewhere around page 2?

[cj001f]

Didn't I say this somewhere around page 2?

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

[Dirty Hoar]

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).

"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?

[uglymoney]

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?

[Dantheman]

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

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

[Huckwheat]

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.

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.

[smmokan]

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.

[Dirty Hoar]

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

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

[dbp]

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

[cj001f]

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

he's got one thing right

Gah… people are freakin’ stupid

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.

[bircheater]

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.

[cj001f]

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

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.

[dbp]

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.

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?

[dbp]

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.

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.

[Dirty Hoar]

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.

Ya but.... ok... wow... it makes sense. Ok, I'll be the first to admit, I was wrong. I guess bashing my head in with a hammer worked. I feel much better now.

But I still think Beaver should stick to porn.

[cj001f]

It still can't keep the plane from moving forward.

Except by definition it is always moving at the same speed.

[mcsquared]

The speed of the treadmill doesn't make any difference. Imagine an ice treadmill and the plane is on skis. The plane will just start skipping over the treadmill. And sure the treadmill will go faster and faster but there isn't much force to couteract the forward thrust of the engines.

[Dantheman]

Except by definition it is always moving at the same speed.

You're not getting it. Because the (stationary) air is providing all of the thrust and the plane's wheel are free spinning the conveyor is essentially irrelevant to the problem. Remeber those word problems in junior high math where they gave you lots of extra information and you had to figure out what information was actually relevant to solving the problem? Same thing.

[DJSapp]

CJ, there are only four forces acting on an airplane

1. Thrust (from engines)
2. Lift (from air moving over wings)
3. Drag (from aerodynamics and wheels)
4. Gravity

All the treadmill is doing is increasing drag on the wheels by making them spin faster. This force is hardly significant compared to the thrust of four 747 engines. Unlike people or cars, airplanes to not require the tires to provide power to push the plane. Jets pull air into the turbine and push it out at great speed to move itself. The wheels have nothing to do with propulsion of an aircraft. Go back to page 2 and reread my example of the guy on a treadmill on rollerskates + a rocket pack. The friction of the treadmill will not overcome the thrust of the rocketpack no matter how fast the treadmill is going. The same applies for an airplane.