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Title: Brainteaser: There's an airplane on the runway..
Source:
Elsewhere
URL Source: http://www.someplaceelse.com
Published: Dec 1, 2005
Author: I have no idea
Post Date: 2005-12-01 01:10:02 by Jhoffa_
Keywords: Brainteaser:, airplane, runway..
Views: 1023
Comments: 202
Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation. There is no wind. Can the plane take off?
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#85. To: Dakmar (#44)
Nope. The question is complete.
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." wrong, thrust moves it forward and the plane lifts off once air speed is sufficient give it up, it's starting to hurt
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." wrong
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." tires were thinking as you all know, tires don't think until they reach terminal velocity
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." good image, gets to the point, thanks
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." sad, very sad
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." oh, my God
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." nice, clean, clear
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." think of the poor souls who will agree with this
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." that sells, I'm with ya
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." what patience and restraint you show, I'm impressed
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." I'm going back to repetition. Making fun isn't fun anymore. The plane will fly because it pushes against air in order to reach lift speed in contrast to a car which would push against the conveyer belt which never allows it to move forward. The car spins it's wheels in place and the plane moves forward. Best analogy... a bike and a bottle rocket. No matter how fast you peddle the bike the conveyer belt negates the forward motion. Yet, the bottle rock on wheels or not, is out of here.
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." I like it. You can't argue with it or... I've been keeping score and some of you folks are sad, real sad. Just goes to show you how forums are not a good place to persuade. No matter how much explanation, logic, patience and repetition of same, folks stick to their guns. Did anyone here change their mind? No progress was made. Yet, there is a correct answer to the question. The plane does fly. No doubt, absolutely a fact. It will fly. It's not a matter of opinion. It's a fact. Lesson to be learned: You may have the right to your opinion but, isn't it more important that your opinion be right? Isn't truth more important than ego? Isn't right more important than wrong? Now, let's go one more round and work towards a consensus. Their coming to take me away, ha ha
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." Forgive me. They're coming to take me away.
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." Well, we could hang a "Mission Accomplished" banner from the control tower. I'm not helping our case, am I? Sorry about that. ;)
When prosperity comes, do not use all of it. - Confucious Clever. Mission Accomplished fits, though. It's a lie that will send most to another issue.
Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." Interestingly, the place I first saw this question posted did not have a answer. Actually, that's part of the reason I posted it... because there was such a debate over it. IMO, Yes.. The plane will fly. As markm0722 pointed out above, it's generating thrust. If you make 100,000lbs of thrust, where does it go? You can't destroy it, that's impossible. You're not expending it on the runway, because the wheels don't drive. You're not burning it up with friction, either.. as that's not mentioned. Perfect hypotheticals here. It's obviously going into forward motion. The wheels rotating (forcing the conveyor to match speed) is proof that the plane is indeed moving forward and regardless of what the conveyor is doing.
Dubya to the serfs: "It's Raining!"
Therein lies a problem made by many: confusion and/or equating the behavior/actions of solid fuel or liquid fuel rocket engines WITH turbofan gas turbines. Jets have the latter. Two extremely different propulsion systems-- two totally different engines.
Why should we hear about body bags and deaths. Oh, I mean, it's not relevant. So why should I waste my beautiful mind on something like that? -- Big Mama Bush
The wheels rotating (forcing the conveyor to match speed) is proof that the plane is indeed transferring engine thrust to the conveyor belt while the plane remains stationary. Forward movement in a jet on the ground is created by friction between the ground and the tires. Remove that friction and no forward movement. Again, jet planes DO NOT have rocket engines. High bypass turbofans DO NOT behave like rocket engines.
Why should we hear about body bags and deaths. Oh, I mean, it's not relevant. So why should I waste my beautiful mind on something like that? -- Big Mama Bush
But it could be a window fan, so far as that goes. It's making thrust, that's all that matters.
Dubya to the serfs: "It's Raining!"
No it isn't, it is created by thrust from the jets. There are no drive motors for the wheels.
And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush
Forward movement in a jet on the ground is impeded by friction between the ground and the tires. I does not matter in the least whether the craft in question is a jet turbine, a turbofan, a pulse jet or a rocket. Wheels or skids or any other gear that serve to support an aircraft while it approaches takeoff speed only burn up energy or momentum. They do not transmit it. They ain't no driveshaft on a 777.
Of course. Not helpless, just inefficent.
oh, my God Adolf Hitler... "What luck for rulers that men do not think." John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable." wakeup posted on 2005-12-02 02:10:32 ET Reply Trace Private Reply Care to elaborate? Remember, we are talking about an air breathing engine here. By the way, I am using "Areospace Propulsion" by Dennis Sheppard, 1972, Cornell University, as my source.
I'm back to my original conclusion that the plane in this scenario could not fly since it would not be moving. If the conveyor belt is turning at exactly the same speed as the wheels, then by definition the plane is standing still and thus could not get airborn. Of course this could never happen, but that's the way the question is phrased.
And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush
The thrust from the engines will move the plane forward unless there is sufficient friction on the ground to offset the force. There is no indication in the question that this is the case. Thus, the plane will move forward. Assuming the wheels maintain contact with the belt and the belt is moving in the opposite direction, the rate of rotation of the wheels is a function of the speed of the plane + the speed of the belt. Therefore, there is a flaw in the assumptions because it is impossible for the conveyor belt to move as fast as the wheels once the plane starts moving forward. So perhaps, the trick answer is that the belt, in attempting to match the wheel rotation speed accelerates to infinity causing the wheels to fall off and the plane to crash.
Moving forward generates the lift required to climb, of course. Since the plane is not thrusting against the belt, but against the gases being expelled by the engine, it would accelerate. On further thought, though, the only way the plane could move forward is if the wheels turned fasted than the conveyor belt, and the operation of the belt would make that impossible. Ergo, as soon as thrust was applied, both the belt and the wheels would accelerate to an infinite speed, at which time the wheels, and probably the belt would both instantly explode, killing everyone and making the question moot. At that point the question would need to be redefined.
Ya beat me to the punch.
LOL. Amazing how similar our thought processes were.
You are still making the incorrect assumption that an airplane moves forward the same way a car does, by turning the wheels against the ground. If I were standing on ice and had a jetpack on, it wouldn't matter if my feet couldn't get any traction. If my car had jato jets strapped to the trunk, I wouldn't even have to put it in gear to move forward. Airplane wheels don't produce thrust. They're just along for the ride.
Now, I turn the treadmill up to 1000 mph and I am running at 1000 mph with it. I still have no forward motion. I would still be stationary and there is ZERO relative wind speed on my face as I remain stationary. All the energy I am expending is between my feet and the treadmill. If I had wings in this situation, they would be useless. Fine. Now strap a jet engine on your back and fire it up. My guess is you'll start feeling that wind on your face as you body shoots off the treadmill like a rocket.
You must be seeing some very rare and unique airplanes if they have drive wheels. In all my years, I've never, ever seen one. Not even on TV or in books. There might be one out there, but personally I doubt it.
If a plane can't take off on a conveyor belt, then an airboat cannot go up river.
Don't be so sure about those planes not existing. All you would need is an propulsion system that produces enough thrust to overcome the friction of non-moving landing gear. Think ski-plane. Think sea-plane.
But the plane WOULD move forward. Lets break it down: 1) The plane starts at a standstill. The conveyor is not moving. 2) The pilot lays into the throttles and the plane starts to move forward due to the thrust created by those, oh so inefficient, jet engines. Let's say it moves at 1 mph. 3) The conveyor now matches by moving at 1 mph in the opposite direction. 4) Whoa! That causes the wheels to rotate at a speed equivalent to 2 mph. 5) Which in turn causes the conveyor to double its speed ... which speeds up the wheels ... which speeds up the conveyor... 6) Meanwhile the turbines are spinning up and generating more thrust which moves the plane forward a bit faster -- relative to the surrounding environment and thereby adding even more speed to the feedback loop between the tires and conveyor. 7) The only question now is how long it will take for the conveyor and wheels to reach the speed of infinite or major component failure first. 8) Gawd, I hope the plane takes off before those suckers blow up!
It doesn't say that the wheels/conveyor cannot reach infinite speed, so given the parameters of the question the plane cannot take off.
And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush
There's an airplane on the runway.. I think we need to change the title to .. There's a fly in your soup .. or some other such nonsense..
Click to see: Making a difference in Iraq
If I could walk that way I wouldn't need the talcum poweder.
And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush
Even better! Click to see: Making a difference in Iraq
Therein lies a problem made by many: confusion and/or equating the behavior/actions of solid fuel or liquid fuel rocket engines WITH turbofan gas turbines. Jets have the latter. Two extremely different propulsion systems-- two totally different engines. All of these engines generate thrust. What is going to counter that thrust? If you want my opinion, the problem lies with the "skidding". What's skidding? The wheels are allowed to freely rotate. Freely rotating tires do not stop planes. In fact, they are designed to hinder the plane as little as possible. If engineers had their way they'd be frictionless (at least in the direction the plane is moving) to ease takeoff. Time for another analogy I think. Let's say I am standing on a frictionless treadmill (conveyer belt). As I try to run the treadmill slides out from under me. Each step I take merely makes the treadmill spin. I'm not going anywhere no matter how hard I try. The reason is I am using my feet (wheels) to provide forward "thrust" and all that energy is lost. It cannot be transferred to the surface I am standing on because it is frictionless. I might just as well be trying to walk on ice with while wearing icy shoes. Perhaps this is why many seem to think the plane will not move. However, now let's say I mount a fan on my back (jet engine). I turn it on. I'm standing on a frictionless treadmill. Am I going to move forward? Most definitely. A force is being applied to me. What's going to stop me from moving? What is going to counter the thrust of that fan? My feet? Not a chance. I'm standing on the equivalent of ice! I can do anything I want with my feet and it isn't going to matter. I could try running backwards in place. I could try running forwards. My feet don't matter though. The more frantic I move them the more comedic the effect as I get pushed off the front of the treadmill. And believe me, if I was a coyote and mounted Acme's jet engine on my back, I'd be trying to maximize comedic effect. ;) It does not matter how efficient or inefficient its fan is. As long as ANY force is being applied which is not countered by an opposing force the plane will accelerate. There is no opposing force to counter it. The engine is not the key to understanding this problem in my opinion, but the realization that the wheels of the plane cannot counter the thrust of the engines. As a side note, on the off chance the wheels spin infinitely fast in this brainteaser (not saying they are, wasn't needed for a solution), there is one more issue with the plane taking off. It would have perfect gyroscopes mounted (Gyroscope). It would prevent the plane from performing "yaw" and "roll". Luckily only "pitch" is needed in takeoff (Pitch, Yaw and Roll). That could definitely pose a problem on landing though. There would be no circling back to the same airport, lol.
When prosperity comes, do not use all of it. - Confucious
Comments (125 - 202) not displayed.
Top • Page Up • Full Thread • Page Down • Bottom/LatestThere are too many unaccounted-for factors for this to be a classic brain teaser.
#86. To: wbales (#46)
he plane would have forward motion thanks to the thrust caused by the engines. That is, BTW, very true ONCE AND ONLY AFTER a plane is airborne.
#87. To: tom007 (#49)
if it used a high bypass turbo fan jet for it's propulsion, as is typical for jetliners, the answer is never,
#88. To: Dakmar (#50)
A jet would throttle up and not give a damn about what the tires were thinking.
#89. To: Dakmar (#52)
get some bottlerockets and visit your local supermarket
#90. To: tom007 (#53)
#91. To: tom007 (#54)
The bottle rockets would fly, no matter what the conveyor was doing. They are a self contained control system. The propellent contains an oxident. Thrust here is independent of external conditions.
#92. To: Dakmar (#55)
How would sitting on conveter belt with wheels spinning be different from idling on tarmac
#93. To: wbales (#56)
And anyway, back to the question: if this plane could take off on a moving conveyor belt, we shouldn't need runways at all.
#94. To: Dakmar (#57)
watch me take a airboat up a particularly wide escalator
#95. To: Dakmar (#58)
it all makes perfect sense
#96. To: Dakmar (#60)
I should have stopped at shooting bottle rockets at grocery clerks, but what do I know?
#97. To: markm0722 (#61)
There is a force applied by the thrust of the engines to propel the plane forward. Where is the opposite force to counter it and hold the plane motionless?
#98. To: All (#97)
#99. To: wakeup (#97)
Now, let's go one more round and work towards a consensus.
The nation is prosperous on the whole, but how much prosperity is there in a hole? - Will Rogers
There are 9,000 hedge funds out there. There aren't that many smart people in the world. - Michael Driscoll, a trader at Bear Stearns & Co. in New York
Some days you just want to pull out the Bonehead Stick and beat people senseless. - mirage
#100. To: markm0722 (#99)
#101. To: All (#0)
#102. To: RickyJ, tom007, Neil McIver, markm0722, wakeup (#66)
But if they had jet engines powerful enough and planes able to withstand the skidding, then it would take off.
#103. To: Jhoffa_, tom007 (#101)
The wheels rotating (forcing the conveyor to match speed) is proof that the plane is indeed moving forward and regardless of what the conveyor is doing.
#104. To: wbales (#103)
Again, jet planes DO NOT have rocket engines. High bypass turbofans DO NOT behave like rocket engines.
#105. To: wbales (#103)
Forward movement in a jet on the ground is created by friction between the ground and the tires.
#106. To: wbales (#103)
Forward movement in a jet on the ground is
created by friction between the ground and the tires.
#107. To: Dakmar (#55)
I know I've been on jets that were able to utilise their engines for taxiing, don't tell me they are helpless if they come to a complete stop, I know better.
#108. To: wakeup, wbales, jhoffa (#91)
The bottle rockets would fly, no matter what the conveyor was doing. They are a self contained control system. The propellent contains an oxident. Thrust here is independent of external conditions.
#109. To: tom007, wakeup, Jhoffa_ (#108)
#110. To: tom007 (#108)
#111. To: RickyJ (#66)
How does a multi-ton object become air born when it has no upward force being applied? The only way it could move forward is if it skidded down the runway and took off that way.
#112. To: Soren (#110)
#113. To: Neil McIver (#112)
#114. To: wbales (#41)
Think of running on ice--there is no traction--no friction to create forward movement despite a lot of thrust. Or think of a car wheel spinning on ice-- lots of thrust--no forward motion.
#115. To: wbales (#43)
Suppose I am on a treadmill going at at snails pace. I have no forward motion. There is ZERO relative wind speed on my face as I am stationary.
#116. To: wbales (#46)
That is, BTW, very true ONCE AND ONLY AFTER a plane is airborne.
#117. To: Dakmar (#57)
Bull, you should watch me take a airboat up a particularly wide escalator sometime. It's all about horsepower vs weight. Don't cheap out on landing gear too, I guess.
#118. To: RickyJ (#63)
Well if there are such planes then of course it would fly, but since there aren't, it's a mute point.
#119. To: Dakmar (#109)
I'm back to my original conclusion that the plane in this scenario could not fly since it would not be moving. If the conveyor belt is turning at exactly the same speed as the wheels, then by definition the plane is standing still and thus could not get airborn. Of course this could never happen, but that's the way the question is phrased.
#120. To: Mr Nuke Buzzcut (#119)
#121. To: Dakmar (#120)
#122. To: Zipporah (#121)
#123. To: Dakmar (#122)
#124. To: wbales, RickyJ, tom007, Neil McIver, markm0722, wakeup, Dakmar, Mr Nuke Buzzcut, Soren (#102)
But if they had jet engines powerful enough and planes able to withstand the skidding, then it would take off.
The nation is prosperous on the whole, but how much prosperity is there in a hole? - Will Rogers
There are 9,000 hedge funds out there. There aren't that many smart people in the world. - Michael Driscoll, a trader at Bear Stearns & Co. in New York
Some days you just want to pull out the Bonehead Stick and beat people senseless. - mirage
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