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vicaphit
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Post by vicaphit » Tue Apr 17, 2007 6:30 pm

alright, I retract my previous answer... the thrust is coming from the jet engines not the wheels, so the conveyor can go as fast as it wants, but since the wheel isnt driven by an engine, the plane still moves forward while the wheels just spin faster and faster with the plane's speed... the plane would be moving forward, so therefore it would achieve lift and a takeoff.

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Post by TristanM » Tue Apr 17, 2007 6:33 pm

bubba wrote:I not saying that it won't take off, I'm saying that both instances can happen.

Instance 1. Wheel speed and conveyor speed same, witch means that there is just enough thrust to keep the plane in one spot, like holding the skate in place with your finger, but instead of the finger its enough thrust to act as the finger.

Instance 2. plane speed is increased to match the conveyor, witch means the wheels are no longer the determining factor and you get forward movement of the plane and it will take off. Like pushing the skate with your finger.

I for one would like to see this on the Mythbusters, I think it would make for a good show. I want to see the rigs they would come up with to test it, and I think they would be split just like everyone I have talked to about this.
Well, that would mean that you basically agree that the plane would take off - the finger holding the skate is only a small amount of force right? No matter how fast the conveyer belt goes this amount of force won't change significantly, so any additional force will cause the plane to move forward. So the thrust necessary to take off from a moving conveyer belt would only be very slightly more than necessary on a normal runway - the only difference is that at the moment the airplane takes off the wheels will be spinning faster.

Another thought that came to me - If a a plane was already in flight with the landing gear down, and somehow you managed to put a moving treadmill under the wheels so they were just touching (but the treadmill is not connected to the earth etc.) would the plane slow down? No. The wheels are totally irrelevent to the equation. Or even better, a guy hangs out the door of the plane and spins the wheels with his hands - will that slow the plane down? Will it speed the plane up if he spins them in the other direction? Can he cause the plane to stop mid-flight by spinning the wheels? Again, no. The forces involved (for the most part) ignore the wheels and the treadmmill.

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Post by bubba » Tue Apr 17, 2007 7:11 pm

TristanM wrote: Well, that would mean that you basically agree that the plane would take off
Yup, but I'm also saying that it's not the only answer :mrgreen:

and I'm with rest of the net on this its :beatdeadhorse:
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Post by DMB2000uk » Tue Apr 17, 2007 7:18 pm

Well if we are taking the wording literally in this particular question, its a trick question!
A plane is standing on a runway ... Will the plane be able to take off?
No as the plane isn't moving :P

But seriously, if we're assuming that the treadmill is matching the speed that the wheels move at consistently, the wheels and treadmill speed increase will be something like exponential, meaning that the wheels and conveyor belt quickly shoot of to infinity miles per hour, at which point they both explode. So maybe this isnt recreate-able and hence why its never been settled properly.

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Post by yurchie » Tue Apr 17, 2007 7:53 pm

These topics are fun until they become frustrating, but I agree with Tristan.

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Post by vicaphit » Tue Apr 17, 2007 8:20 pm

ok... lets walk through this step by step.

1. plane is sitting on the conveyor... nothing is moving.
2. Plane applies thrust from the jet engines. (thrust = moving)
3. conveyor starts to move since plane is moving.
4. wheels start to spin as conveyor tries to apply opposite force.
5. wheels spin 2 times that of the conveyor since the plane is moving in the opposte direction at the same speed as the conveyor.
6. assuming wheels are frictionless with respect to the plane (just like a car in neutral gear will roll on its own downhill), the planes thrust will be able to move the plane in a forward motion allowing air to travel across the wing.
7. plane flies away, and crashes into the 2 guys gaurding the doors to the strip clubs allowing me to check for the female strip club before entering.

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Post by Alathald » Tue Apr 17, 2007 9:50 pm

TristanM wrote:Nice one Alathald. I have seen this question a few times, and it always ends in a massive thread with people on one side or the other. Then there is always a group that claims the question is worded incorrectly etc. so on, and sor forth ad nauseum. Try typing "airplane on a treadmill" into Google and see how many forums come up - unbelievable. The problem is no-one has managed to show an exact, scientific proof based in either mathematics or empirical evidence - at least not that I can find. This debate has started to bother me so much that I spent a few hours searching the internet for some properly executed experiment to answer the question once and for all - but if it's there, it's so lost amongst the arguiing that I can't find it. So back to the original point - nice one, thats the closest to the math that I've seen so far. I'll see if any of the math/physics guys come up with anything more specific...
Thank you, thank you very much... :)

I'll try to write up a more definitive "proof" for this one later...but Bubba's logic is OK; to a point. Depending on how loosely you interperate the wording of the question (and the Laws of Physics :shock: ), you can logically come to the conculsion that the plane has no airspeed because if the speedometer (speed of the wheels) is equal to the speed of the belt, the net velocity of the plane would be zero. As a Physicist (OK Physics Major :P ), I'm bound to point out that this POV measures the speed from two different reference frames, therefore, it is not valid.

The proper way to compare two speeds (which is what we are doing) is to first make sure they are both in the same reference frame, which is what I keep trying to point out.

BTW, if you use 'thrust' 'force' 'friction' etc., you need to reread the question, these terms have nothing to do with the question or the model for that matter, the only thing mentioned is velocity or (more loosely) speed, which is the magnitude of the velocity.
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Post by Illuminati » Wed Apr 18, 2007 7:00 am

Alathald, I'm still of the view point that in 0mph wind conditions, it would be impossible for the plane to stay in one point with a constant (however small) amount of thrust. Because as you have sort of indicated about measuring speed/velocity in the same frame, you have to measure the speed of a plane with respect to the air, even if the plane is touching the ground.

Whereas with a car, you have to measure speed with respect to the ground.

So with a plane, the conveyor belt is irrelevant, but with a car, it is possible for the conveyor belt to prevent the car from moving forward.

I think that in order for the plane to not go anywhere with a small amount of force that Bubba refers to, the wind speed would have to be the same speed and direction as the conveyor belt.

I think the wording of the question is probably incorrect and can not be taken literally from a scientific point of view as the question was probably first stated by some drunk friends in a bar! :) :drinkers: I'm answering the principal of the question, and probably not the question as if it were written in a physics text book.
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Post by Dragon_Cooler » Wed Apr 18, 2007 7:51 am

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Post by TristanM » Wed Apr 18, 2007 8:23 am

Someone should stick a model aeroplane on that thing...

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Post by smack323 » Wed Apr 18, 2007 9:17 am

i am reading the points given but just dont see how it can take off... if the treadmil is matching the thrust of the plane. more thrust=faster treadmill. the the plane wouldnt move forward thus no air for lift.. it needs a surface to take off from that is slower than what the thrust can produce for lift.
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Post by Alathald » Wed Apr 18, 2007 10:37 am

Illuminati wrote:Alathald, I'm still of the view point that in 0mph wind conditions, it would be impossible for the plane to stay in one point with a constant (however small) amount of thrust. Because as you have sort of indicated about measuring speed/velocity in the same frame, you have to measure the speed of a plane with respect to the air, even if the plane is touching the ground.

Whereas with a car, you have to measure speed with respect to the ground.

So with a plane, the conveyor belt is irrelevant, but with a car, it is possible for the conveyor belt to prevent the car from moving forward.

I think that in order for the plane to not go anywhere with a small amount of force that Bubba refers to, the wind speed would have to be the same speed and direction as the conveyor belt.

I think the wording of the question is probably incorrect and can not be taken literally from a scientific point of view as the question was probably first stated by some drunk friends in a bar! :) :drinkers: I'm answering the principal of the question, and probably not the question as if it were written in a physics text book.
I completely agree with you on that, I'm just pointing out that Bubba's logic isn't completely...um....illogical (in a physics classroom setting it would be, but by laymens logic, his logic is OK).

As for measuring the speed, I assume windspeed is 0knots so windspeed equals groundspeed. This is actually a good theoretical physics question because you have to pay special attn to the terms used. In that setting, the answer would be that the plane flys.

EDIT: also if you measure the car speed with respect to what it's wheels are on (the belt) you must also measure the speed of the belt with respect to the belt, see above argument for why this won't work...it only makes sense for an outside observer to measure the speed (say a police office with a radar gun) of both the car and the belt. That way everything is measure from the same reference frame and the appropraite reference frame.
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Post by vicaphit » Wed Apr 18, 2007 1:28 pm

anyone have a treadmill?

Here is the situation... get a toy car, or a skateboard, or something with wheels.

place the car on the treadmill and hold it in place with your hand while the treadmill is running. this simulates an engine in the car spinning the wheels to matching the speed of the treadmill.

Now, push the car from behind with your finger. this simulates "thrust" as it would occur from a jet engine.

Since the wheels on the car roll freely without friction (without much friction atleast) thrust from the car pushes the car in a forward momentum regardless of the movement of the treadmill.

has anyone ever seen a plane that had an engine that made the wheels turn to drive it down the runway? I rest my case

TRY IT!!!

EDIT: thrust from a jet engine is different from thrust from the rotation of a wheel. A plane that is driven by it's wheels would get enough speed for liftoff, but then would not be able to maintain lift because the turn of the wheels depends on a solid structure for friction. a wheel turning in mid air will provide no thrust.

so... a plane depends on thrust from a jet engine to maintain speed while in the air. the type of thrust does matter in the situation

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Post by stopthekilling77 » Wed Apr 18, 2007 2:25 pm

i concur.

i kept thinking in my head "aren't the wheels just there to provide a buffer for movement and not the source of motion?"

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Post by TristanM » Wed Apr 18, 2007 4:00 pm

Oh sure, said and done when he says it. I said and done that ages ago... I see how you guys are. I'm taking my ball and going home... :P

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Post by Alathald » Wed Apr 18, 2007 5:12 pm

oh man, I'm done...you guys all are getting caught up with friction and force and thrust, WHICH HAS NOTHING TO DO WITH THE PROBLEM...only the velocity means anything here. I'm done for now...
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Post by smack323 » Thu Apr 19, 2007 3:23 pm

the more i think about this the more it makes my head hurt.. its like i am looking at a minds eye picture.. for a second my brain is like yeah uh duh of course it will take off.. then i loose it and the plane stops because of all the damn snakes...
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