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Author Topic: Curiously depressing
GrumpySteen

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Icon 1 posted December 12, 2006 06:31      Profile for GrumpySteen     Send New Private Message       Edit/Delete Post   Reply With Quote 
From David Pogue's airplane post:
“Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?"

Read through some of the comments. It's surprising and a bit sad to see how many people don't grasp the basic concept of jet propulsion.

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Spiderman

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Icon 1 posted December 12, 2006 07:29      Profile for Spiderman     Send New Private Message       Edit/Delete Post   Reply With Quote 
Those comments are just amazing, where amazing == depressing indeed. Just...wow. [tired]

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Math problems? Call 1-800-[(10x)(13i)^2]-[sin(xy)/2.362x]

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Sxeptomaniac

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Icon 1 posted December 12, 2006 09:17      Profile for Sxeptomaniac   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
The question (and many of the answers) reminds me of this guy I knew in high school. He'd come up with goofy "what if" questions on a regular basis, in which one or more of the premises were just absurd. If you pointed out the absurdity of something, he'd come up with an even stranger "what if" as an attempt to explain it.

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Let's pray that the human race never escapes from Earth to spread its iniquity elsewhere. - C. S. Lewis

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garlicguy

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Icon 1 posted December 12, 2006 09:59      Profile for garlicguy   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Personally, I find the answer to the question is anything but depressing - it's uplifting. [Big Grin]

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I don't know what I was thinking... it seemed like a good idea at the time.

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Ashitaka

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Icon 1 posted December 12, 2006 11:07      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
This is why aircraft carriers launch thier planes against the wind and not with.

Not that I think the plane would take off mind you.

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TheMoMan
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Icon 1 posted December 12, 2006 11:41      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Steen, Spiderman, Sxeptomaniac, garlicguy, and Ashitaka_____________________________Think of it this way how hard would it be to land a piper cub on an Aircraft Carrior that is turned into the wind. Ship speed twenty knots, wind speed ten or more knots that means if the piper was showing sixty knots air speed it would only be going thirty knots faster than the ship, Now chop the throttle and watch the breeze blow it off the stern. This could get interesting.

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The Famous Druid

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Icon 1 posted December 12, 2006 12:00      Profile for The Famous Druid     Send New Private Message       Edit/Delete Post   Reply With Quote 
Appalling ignorance of basic physics.

It's not even a 'trick question' - the true physics of it is pointed out, and people still won't believe.

I've long believed that people should have to pass a test in basic physics to get a drivers license. The idea that clueless gits with no idea of Newtons Laws are allowed to be in control of a couple of tonnes of high-speed steel is really scary.

[edit]
It may be a 'trick question' after all, depending on how you interpret "the speed of the wheel" ...
Assuming the wheels aren't skidding, the speed of the tyre-surface relative to the runway is zero.

If you're talking about "rotational velocity" of the wheel, then on a normal runway, at the point of contact with the runway, the wheel's rotational velocity is equal to that of the plane, but in the opposite direction. So a treadmill "moving in the opposite direction" would be running in the same direction as the plane, at half the plane's velocity.

Not that any of this would matter to the plane, as the wheels are free-spinning, but it would make a world of difference to the treadmill designers.

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Sxeptomaniac

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Icon 1 posted December 12, 2006 12:27      Profile for Sxeptomaniac   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by TheMoMan:
Steen, Spiderman, Sxeptomaniac, garlicguy, and Ashitaka_____________________________Think of it this way how hard would it be to land a piper cub on an Aircraft Carrior that is turned into the wind. Ship speed twenty knots, wind speed ten or more knots that means if the piper was showing sixty knots air speed it would only be going thirty knots faster than the ship, Now chop the throttle and watch the breeze blow it off the stern. This could get interesting.

Right, but airspeed isn't part of the original question, only this hypothetical treadmill that supposedly keeps up with the plane's wheels. Since the wheels don't provide the push, it's up to the reader to fill in the information not supplied by the question in order to supply even a basic answer.

I kind of get a kick out of the people who talk about the friction from the wheels counteracting the push from the plane's engines. It almost makes me want to work out just how fast the hypothetical treadmill would have to be going in order for the friction to be high enough to provide equivalent opposing force to various types of plane engine configurations. Almost.

My point is that it's a question that depends on at least one impossible "what if" in order to be sufficiently answered. Whoever came up with the question was either an idiot (if he/she expected a real answer) or a genius (if they intended it to be impossible to answer and just wanted to mess with people's heads).

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Let's pray that the human race never escapes from Earth to spread its iniquity elsewhere. - C. S. Lewis

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GrumpySteen

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Icon 1 posted December 12, 2006 13:08      Profile for GrumpySteen     Send New Private Message       Edit/Delete Post   Reply With Quote 
For any who don't quite get it, here's the basic flaw with the original problem:

For the wheel to move at all, and thus for the treadmill to move, the plane has to move first. To cancel the movement the plane has already made, the treadmill would have to move further than the wheel in order to counteract the movement of the plane. The problem, however, defines the movement of the treadmill to be equal to that of the wheel, thus the movement of the plane is never cancelled out, thus the plane accelerates forward despite the other elements of the problem.

Then you get into the issues with the wheels and treadmill reaching infinite speeds instantaneously, but that's sort of beside the point.

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Ashitaka

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Icon 1 posted December 12, 2006 14:42      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
The way I read the problem is that the treadmill is there to conteract any movement forward of the plane. The question is basically asking will a plane be able to take off if the engines are at full throttle but the plane is not moving forward.

The answer is no, the plane cannot generate lift. (unless the plane you choose to imagine is a harrier.)

That being said, I agree that the question has flaws. With these semmantical arguments the problem becomes unsolveable, so don't bother trying to.

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littlefish
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Icon 1 posted December 12, 2006 15:24      Profile for littlefish   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
The way I read the problem is that the treadmill is there to conteract any movement forward of the plane. The question is basically asking will a plane be able to take off if the engines are at full throttle but the plane is not moving forward.
Makes me want to weep.
What provides thrust from a plane?

Answer: the plane will take off. Really. It does not matter what speed the wheels are going, or what speed the conveyor is going. The only thing that matters is the speed of air over the wing. A plane pulls (prop) or pushes (jet) against the air. Wheels mean nothing.

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Sxeptomaniac

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Icon 1 posted December 12, 2006 15:31      Profile for Sxeptomaniac   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Here's the closes thing I saw to a right answer, IMO [Big Grin]
quote:
#66. December 11th, 2006 9:03 pm

Nothing moves. Ever. Zeno proved it.

Posted by Mike

It was a slow day, so I read through the first hundred or so comments below David Pogues post. Comments #86 and #111 were pretty good.

Edit: #160 seems to be the first to get the paradox within the question itself.

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Metasquares
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Icon 1 posted December 12, 2006 18:06      Profile for Metasquares   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Zeno's paradox only makes sense on an infinitesimal scale... and when I say infinitesimal, I mean in the sense of the wacky variables that you can multiply by any real number and always get less than 1.
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garlicguy

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Icon 1 posted December 12, 2006 18:46      Profile for garlicguy   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by littlefish:
It does not matter what speed the wheels are going, or what speed the conveyor is going. The only thing that matters is the speed of air over the wing. A plane pulls (prop) or pushes (jet) against the air. Wheels mean nothing.

Bingo! Give this man a cigar. Except for one thing.

Fixed-wing aircraft:

A jet relies entirely on groundspeed and airspeed of the wing moving through the air to develop lift. Since the theoretical conveyor negates the jet ever developing groundspeed, the jet will run out of fuel right where it started. Same is true for a push-prop plane or canard.

A turbo-prop or conventional prop plane, if the engines had sufficient power, would eventually create enough lift to break gravitational pull and rise vertically in place, at which time it would continue to climb.

A rotary-winged plane, aka helicopter, would behave normally and rise within a few moments after engaging the rotor transmission.

The problem said "plane". Didn't say, fixed or rotary wing, jet, prop, turbo-prop, pusher or conventional. So the basics are: What creates the lift in the problem. No plane flies without sufficient lift.

That's my story as a former Naval Aviator, and I'm stickin' to it. [Big Grin]

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I don't know what I was thinking... it seemed like a good idea at the time.

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CommanderShroom
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Icon 1 posted December 12, 2006 19:02      Profile for CommanderShroom     Send New Private Message       Edit/Delete Post   Reply With Quote 
Nothing to see here.

There are no black helicoptors here.

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CommanderShroom
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Icon 1 posted December 12, 2006 19:06      Profile for CommanderShroom     Send New Private Message       Edit/Delete Post   Reply With Quote 
Sorry. In order to negate the turn of the wheels, the conveyor belt/treadmill would have to propel the jet forward.

And thus whether or not the wheels move, the jet will. Since the wheels are not the source of the thrust the belt can negate the spin of the wheels not the forward motion of the jet itself.

Take a can or something like it, place it on a sheet of paper and then go through the motions. You will get a better view on how the belt/treadmill has to move to negate motion.

--------------------
Does he know our big secret?
Has one of us confessed?
'Bout the wires circuits and motors
Buried in our chest

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garlicguy

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Icon 1 posted December 12, 2006 19:56      Profile for garlicguy   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by CommanderShroom:


There are no black helicoptors here.

How 'bout one of these?

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I don't know what I was thinking... it seemed like a good idea at the time.

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CommanderShroom
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Icon 1 posted December 12, 2006 20:47      Profile for CommanderShroom     Send New Private Message       Edit/Delete Post   Reply With Quote 
OK I did mis-read the sentence.

I was under the assumption that the treamill stopped the wheels instead of moving with them.

But the wheels have nothing to do with the thrust of the engines. So even if the wheels could move at an infinite rate as could the treadmill, the jet would still take off. Because the engine, which is what gives the thrust, will still propel the jet forward. Breaking contact with the treadmill sooner or later.

Of course either the wheels or the treadmill would fail spectacularly at some point if this was the real world.

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Does he know our big secret?
Has one of us confessed?
'Bout the wires circuits and motors
Buried in our chest

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Spiderman

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Icon 1 posted December 12, 2006 20:49      Profile for Spiderman     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by littlefish:
quote:
The way I read the problem is that the treadmill is there to conteract any movement forward of the plane. The question is basically asking will a plane be able to take off if the engines are at full throttle but the plane is not moving forward.
Makes me want to weep.
What provides thrust from a plane?

Answer: the plane will take off. Really. It does not matter what speed the wheels are going, or what speed the conveyor is going. The only thing that matters is the speed of air over the wing. A plane pulls (prop) or pushes (jet) against the air. Wheels mean nothing.

Sigh. Thank you littlefish. I wanted to post something to this effect earlier, but was too busy at the office...I've had heated discussions with three people today trying to explain thrust.

/me cries [tired]

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Math problems? Call 1-800-[(10x)(13i)^2]-[sin(xy)/2.362x]

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Sxeptomaniac

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Icon 1 posted December 12, 2006 22:59      Profile for Sxeptomaniac   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Metasquares:
Zeno's paradox only makes sense on an infinitesimal scale... and when I say infinitesimal, I mean in the sense of the wacky variables that you can multiply by any real number and always get less than 1.

Right, but the point is that this is a paradoxical question along the line of Zeno's scenarios. Supposedly the treadmill moves to match the rolling of the wheels, but that's logically impossible, since propulsion has nothing to do with the wheels.

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Let's pray that the human race never escapes from Earth to spread its iniquity elsewhere. - C. S. Lewis

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Ashitaka

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Icon 1 posted December 12, 2006 23:21      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by littlefish:
quote:
The way I read the problem is that the treadmill is there to conteract any movement forward of the plane. The question is basically asking will a plane be able to take off if the engines are at full throttle but the plane is not moving forward.
Makes me want to weep.
What provides thrust from a plane?

Answer: the plane will take off. Really. It does not matter what speed the wheels are going, or what speed the conveyor is going. The only thing that matters is the speed of air over the wing. A plane pulls (prop) or pushes (jet) against the air. Wheels mean nothing.

EDIT:
I retract all previous statements. the plane willtake off with the wheels spinning twice as fast as they normally would have.

I think the problem is that the guy meant to askwhat would happen if there was an equal and opposite force to the thrust of the jet engines, but he didn't. I took this magical treadmill to mean equal and opposite force on first reading. Upon going back and rereading I realize that is not what is written.

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-Assif Mandvi

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Callipygous
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Icon 1 posted December 13, 2006 00:23      Profile for Callipygous     Send New Private Message       Edit/Delete Post   Reply With Quote 
No Ash as the treadmill moves in the opposite direction to the 'plane the wheels on the 'plane accelerate rather than slow down.

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TheMoMan
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Icon 1 posted December 13, 2006 03:45      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Hi All_______________________Wilber & Orville flew from skids, Then they landed with skids. On the U2 the wing tip wheels are on sort of pogo sticks that fall away at take off, the fun part is at landing when the ground grews match the planes speed with a pickup truck and grab the wing tips.

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Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.


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Spiderman

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Icon 1 posted December 13, 2006 05:00      Profile for Spiderman     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Callipygous:
No Ash as the treadmill moves in the opposite direction to the 'plane the wheels on the 'plane accelerate rather than slow down.

Correct. The moment the wheels begin to move, acceleration begins. The wheels will continue to accelerate until the plane has gained lift off.

Though as Steen mentioned, there's the issue of the wheels gaining instantaneous and infinite speed of rotation, which is what *should* happen if the treadmill could actually match the speed of the wheels as the problem states.

I should also mention that the acceleration of the wheels is not dependent on the speed of the plane, but is rather a race condition of sorts triggered by the first movement of the plane. If only enough thrust was applied to move the plane at 1MPH, the wheels would still accelerate infinitely.

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Math problems? Call 1-800-[(10x)(13i)^2]-[sin(xy)/2.362x]

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GrumpySteen

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Icon 1 posted December 13, 2006 05:44      Profile for GrumpySteen     Send New Private Message       Edit/Delete Post   Reply With Quote 
Spiderman wrote:
Though as Steen mentioned, there's the issue of the wheels gaining instantaneous and infinite speed of rotation, which is what *should* happen if the treadmill could actually match the speed of the wheels as the problem states.

... and it would require an infinite amount of energy to produce such acceleration. Since energy can theoretically be converted to matter, that means it could produce an infinite amount of matter which would have to expand and... OMG! The big bang was the result of a badly written brain teaser!!!

I shouldn't be allowed to use my brain this early in the morning [Big Grin]

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