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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: 1184
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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#108. To: wakeup, wbales, jhoffa (#91)
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 Here's one for you. Your in a boat in a pond. You throw something overboard, it sinks. Does the level of the pond go up, go down or stay the same?
"Unthinking respect for authority is the greatest enemy of truth." - Albert Einstein
Pingasaurusreximusmaximus...... Yall
"Unthinking respect for authority is the greatest enemy of truth." - Albert Einstein
Does the level of the pond go up, go down or stay the same? This isn't a veiled analogy about the war and the presidential popularity polls is it? If not, then the pond's level stays the same. ;)
When prosperity comes, do not use all of it. - Confucious If it IS a veiled analogy about the war and the presidential popularity polls I'd have to use the president's own words to solve it. Make the pie higher. Now we just need to figure out what that means and how it applies here. ;)
When prosperity comes, do not use all of it. - Confucious It is not a political analogy, and yet I cannot respond with a confirmation due to others who might find a need to answer.
"Unthinking respect for authority is the greatest enemy of truth." - Albert Einstein
Does the level of the pond go up, go down or stay the same? "Unthinking respect for authority is the greatest enemy of truth." - Albert Einstein I'll bite. Because what you threw sinks, it is denser than water. The boat would rise upwards enough to equal the water weight of the rock (negitive displacement?) This would cause the water in the pond to become lower. BUT, the rock's volume would cause the water level to rise. As the rock is denser than the water (it sinks), the volume of water displaced by the rock is less than the volume of water need to float the rock when it was in the boat. So the boat rises AND the level of the water is lowered. Nice question.
Ding, ding, ding.....we have a winner. The simple fact that it sinks tells you that it is denser than water and would displace more water in the boat than it does when you let it fall to the bottom where it would rest it's weight.
"Thinking that women have a corner on caring is delusional" - Albert Einstein
No it's not and no it doesn't. I can't believe I got sucked into this again.
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." Somebody stop the madness.
"Thinking that women have a corner on caring is delusional" - A Man not thinking with his dick
Does the level of the pond go up, go down or stay the same? Was what was thrown overboard lighter or heavier than water?
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
Ding, ding, ding.....we have a winner. The simple fact that it sinks tells you that it is denser than water and would displace more water in the boat than it does when you let it fall to the bottom where it would rest it's weight. I stand corrected. I should have done the math. I got too caught up in taking a stab at political humor, lol. A 100 ton boat displaces 100 tons of water. If 99.999 tons of nearly infinite density material contained within a thimble is suddenly dumped over the side it will displace next to nothing, and neither will what's left of the boat. My bad. Well done.
When prosperity comes, do not use all of it. - Confucious It sinks.....
"Thinking that women have a corner on caring is delusional" - A Man not thinking with his dick
How can anyone think the wheels are driven by the plane engines?
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." Imagine a boat with a chunk of a star in it weighing hundreds of tons and it's the size of a thimble and you throw it overboard......
"Thinking that women have a corner on caring is delusional" - A Man not thinking with his dick
not true
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." no trick answer, no trick question
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 could see it considering I'm on a copious amount of wine. All is good and all is a little fuzzy...:P
"Thinking that women have a corner on caring is delusional" - A Man not thinking with his dick
nope, the wheels and conveyer belt are none issues
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." both of you are mistaken
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." In the real world a powerful plane would take off after wasting precious fuel and wear & tear countering a conveyor belt some idiot is running in the wrong direction. Steam catapults currently in use seem much more sensible for that task. This is why I said there isn't enough detail, like are the wheels skidding. It seems clear (to me, anyway) that we are to assume that the tires of the airplane are in perfect contact with convbelt, so once again, no, the plane could not get airborn because an unknown force is causing the conveyer belt to negate any forward motion.
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
That is the way I see it. Also many do not realize the difference between rockets and air breathing jet engines.Understandable as some of the jargon is not strictly accurate. A jato (jet assist take off) for example is a rocket, though its name implies it is a jet.
I actually used a bit of a neutron star in my mental picture (give or take), so you aren't far off! ;)
When prosperity comes, do not use all of it. - Confucious One of the brighter of the minions.....congratulations
"Thinking that women have a corner on caring is delusional" - A Man not thinking with his dick
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Top • Page Up • Full Thread • Page Down • Bottom/LatestThe 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
#125. To: Jhoffa_ (#0)
#126. To: All, lodwick, Zipporah, wbales, who knows what evil, Soda Pop, Red Jones, christine, robin, wakeup, Diana, YertleTurtle, Elliott Jackalope, TommyTheMadArtist, BTP Holdings (#125)
#127. To: timetobuildaboat (#125)
Your in a boat in a pond. You throw something overboard, it sinks.
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
#128. To: markm0722 (#127)
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
#129. To: markm0722 (#127)
If not, then the pond's level stays the same. ;)
#130. To: timetobuildaboat (#125)
it sinks.
#131. To: tom007 (#130)
So the boat rises AND the level of the water is lowered.
#132. To: wbales (#103)
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.
#133. To: wakeup (#132)
I can't believe I got sucked into this again.
#134. To: timetobuildaboat (#125)
Your in a boat in a pond. You throw something overboard, it sinks.
#135. To: timetobuildaboat (#131)
So the boat rises AND the level of the water is lowered.
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
#136. To: Dakmar (#134)
Was what was thrown overboard lighter or heavier than water?
#137. To: randge (#106)
They ain't no driveshaft on a 777.
#138. To: markm0722 (#135)
#139. To: Dakmar (#109)
If the conveyor belt is turning at exactly the same speed as the wheels, then by definition the plane is standing still...
#140. To: Soren (#110)
perhaps, the trick answer is that the belt...
#141. To: wakeup (#140)
no trick answer, no trick question
#142. To: Neil McIver (#111)
the only way the plane could move forward is if the wheels turned fasted than the conveyor belt
#143. To: Soren (#113)
LOL. Amazing how similar our thought processes were.
#144. To: wakeup, wbales, markm0722 (#132)
#145. To: Dakmar (#109)
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.
#146. To: timetobuildaboat (#138)
Imagine a boat with a chunk of a star in it weighing hundreds of tons and it's the size of a thimble and you throw it overboard......
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
#147. To: markm0722 (#146)
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