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skye, if you really think that fail pic was warranted, you should post one for yourself...

...and then take algebra again.

The only way to keep the plane from moving forward is for the rolling resistance to equal the thrust of the engine, which would make the air speed of the plane zero. In order to accomplish this, you must assume that the plane and the conveyor are both indestructible...which is impossible. This is necessary because while your sensors and feedback attempt to make x=x+c for c!=0 the conveyor will accelerate infinitely until it spins so fast that the rolling ********* of the plane equals the thrust of the plane.

Theoretically it is possible...but in reality, where things break and have limits, it is not.

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My point was that it doesn't make one damn bit of difference what speed the conveyor is moving at.

[This message has been edited by Red Skye (edited 1/31/2008 5:05p).]

How is this thread still going?

page 50

ttt for tcamilli99

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How is this thread still going?

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We're very talented here at TexAgs.

The plane will take off because the propulsion does not come from the wheels but rather from the engines/propellers.

The wheels are essentially ball bearings to reduce friction/increase mobility for the airplane and the speed at which they rotate have nothing to do with the plane's propulsion.

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Every person at work that I have talked to all assumed the plane being stationary was a given in the myth. Because, of #@$#@ing course the plane would take off if it was moving at take off speed in relation to the ground.

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This is what I understood as the question as well. Why all the hate for those that read the question differently?

I had posted on this earlier. I thought that this was basically a theoretical question that could never really be tested and thus never answered. I'm obviously not the only one.

I may not be able to state the question exactly as I understood it but let me give it a try.

I thought that somehow the treadmill (conveyor) had a system that could judge the speed of the airplane going forward and would counteract that speed to keep the plane in the exact same spot. I thought that the question was (and looking at some previous posts they thought the same thing) whether or not the fact that the airplane's engine was a full thrust but it was not moving forward and generating lift, had any impact on whether it would take off.

Perhaps we were just reading too much into the question. I had gone back and forth on the answer under the way I thought it was questioned and so did a lot of other people. But in the end I concluded that it would not take off. However, if I would have know the question as you guys state it, I could have told of course it would take off.

You all throw out the hate at us for having a different interpretation of the question and call us stupid - I would say that anyone who had any doubt that the plane would take off under the majority reading of the question was stupid.

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I thought that somehow the treadmill (conveyor) had a system that could judge the speed of the airplane going forward and would counteract that speed to keep the plane in the exact same spot. I thought that the question was (and looking at some previous posts they thought the same thing) whether or not the fact that the airplane's engine was a full thrust but it was not moving forward and generating lift, had any impact on whether it would take off.

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Yes, if you lived in some other dimension with physics laws completely different than our own, this could be possible. Otherwise, it's not.

Texaggie7nine...think of it this way....the wheels of the plane rotate freely around the axle, and are not connected to a drive shaft like a car. Therefore, the conveyor could go 1 meelion mph, and the wheels would spin very very fast, but the amount of thrust needed to hold the position is miniscule, just enough the overcome the friction of the wheel bearings and wheel on the road.

Then 25 mph more of thrust will cause forward motion and takeoff, while the wheels spin at 1,000,025 mph.

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My point was that it doesn't make one damn bit of difference what speed the conveyor is moving at.

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on paper where anything is possible, it does.

When the conveyor is traveling at somewhere around 6 lyps [exaggeration] the rolling resistance becomes so great that it counteracts the 248,400 lbs of thrust (assuming the plane is a 747-400 with GE turbines running at full power)

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We're very talented here at TexAgs.

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Talent is moot at TexAgs

Rebel, I think the problem here is the simple minds of those trying to describe the myth. They leave out too many unknowns.

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on paper where anything is possible, it does.

When the conveyor is traveling at somewhere around 6 lyps [exaggeration] the rolling resistance becomes so great that it counteracts the 248,400 lbs of thrust (assuming the plane is a 747-400 with GE turbines running at full power)

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Of course. But this is realistically impossible.

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I thought that somehow the treadmill (conveyor) had a system that could judge the speed of the airplane going forward and would counteract that speed to keep the plane in the exact same spot.

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But this is impossible if you interpret it that way.

I believe everyone agrees that if there is no movement of wind over the wings, the plane will not take off.

Unless it can generate enough thrust to go ballistic, but even that is flawed since the engines would have to be facing up.


Then there are the people who cannot grasp that wheels on the plane are not connected to the jet engines or propeller, and keep thinking of it like a car on treadmill.


Even if the treadmill is started before the plane starts its engines, it will eventually gain enough power to start moving forward (assuming the treadmill is long enough).

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Rebel, I think the problem here is the simple minds of those trying to describe the myth. They leave out too many unknowns.

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No, there was nothing wrong with the way the question was stated.

yes, so either way the plane takes off...

either forward, or backward when the landing gear breaks and the conveyor launches it backward.

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They leave out too many unknowns.

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Like what?

Look Red Sky and others - I am not an engineer and I am not a physicist. I always thought that this was a theoretical question. I thought that that if the conveyor belt moved at the same speed the airplane was moving, the wheels would simply spin and the plane would stay in the same place. I think this is what a lot of people believe also.

If this was not the case, then why did the question even put the part in about the belt moving at the SAME speed in reverse as the plane was going forward. The question seemed to make a big point about this factor. Instead it could have stated that the conveyor belt was moving in the opposite direction at twice or 1 and a 1/2 the speed of the plane.

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Texaggie7nine...think of it this way....the wheels of the plane rotate freely around the axle, and are not connected to a drive shaft like a car.

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Of course, that's not true for most airplanes...

True but the effect would be minimal.

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Look Red Sky and others - I am not an engineer and I am not a physicist. I always thought that this was a theoretical question. I thought that that if the conveyor belt moved at the same speed the airplane was moving, the wheels would simply spin and the plane would stay in the same place. I think this is what a lot of people believe also.

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And what they believe is wrong.

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If this was not the case, then why did the question even put the part in about the belt moving at the SAME speed in reverse as the plane was going forward. The question seemed to make a big point about this factor. Instead it could have stated that the conveyor belt was moving in the opposite direction at twice or 1 and a 1/2 the speed of the plane.

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To stump non-engineers and non-physicists.

dp

[This message has been edited by Texaggie7nine (edited 1/31/2008 5:27p).]

Like what the "speed" is in relation to.

This who deal is similar to asking this question -

Ok...

If you stood at the top of the empire state building and dropped a feather and an anvil which one would hit the ground first?

The physisist would say they would hit at the same time. The layperson would say the anvil. The only problem is that the physicist wouldnt reveal until afterwards that the experiment was taking place in a vacuum.

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Like what the "speed" is in relation to.

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Which does not make one damned bit of difference.

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Which does not make one damned bit of difference

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Of course it does. The plane is moving at a different speed in relation to the ground, than it is in relation to the belt's motion.

If the plane's velocity is 50 mph in one direction (relative to the ground) and the conveyor belt velocity is 50 mph in the opposite direction (relative to the ground), what prevents forward movement of the airplane?

I give up. The conveyor speed has 0 impact on the plane. Period.

The way you are trying to twist the wording would be similar to saying "If a glass of water was below 32 degrees in a hot closet, would it freeze?"

Well the fact that in that statement you say it's already under 32 degrees, then that's all the information needed for the question.

So with the myth "the plane is moving at take off speed" if that is in relation to the ground, then it takes off, regardless of what else you throw into the equation unless you are going to bring up wind movement, or a different force of gravity.

Txag7nine..Maybe this will help you wrap your mind around it[/shrug]

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First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.

A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.

As you point out, one problem here is the wording of the question. Your version straightforwardly states that the conveyor moves backward at the same rate that the plane moves forward. If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. Assuming you've got Indy-car-quality tires and wheel bearings, no problem. However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off.

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http://www.straightdope.com/columns/060203.html

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So with the myth "the plane is moving at take off speed" if that is in relation to the ground, then it takes off, regardless of what else you throw into the equation unless you are going to bring up wind movement, or a different force of gravity.

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*sigh* This is the only possible outcome. The plane's takeoff speed will always be relative to the ground. It is not possbile for the plane's speed to be relative to the conveyor belt's speed.

[This message has been edited by Red Skye (edited 1/31/2008 5:44p).]

I was actually wrong when I said it doesn't make a difference. The plane's speed can ONLY be relative to the ground.

[This message has been edited by Red Skye (edited 1/31/2008 5:46p).]

We could still make it to 51.

the whole time i was watching the show i was hoping they would simply say 'the conveyor belt is moot'