Conveyor-belt runway

Anybody get into this debate within the last few weeks? My trumpet forum is having a real field day with this brain teaser. see below........

Conveyor-belt runway


trumpetherald.com Forum Index -> The Lounge
Location: Atlanta, GA
Posted: 13 Dec 2005 04:52 Post subject: Conveyor-belt runway

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OK! I've got another debate for you wanna-be engineers! Any other Boortz fans (or haters) will recognize this from last week. (Actually, the debate spilled over to this week, even!)
Sorry about the wording as I find a bit ambiguous but I'm going to faithfully reproduce the "riddle" as I found it:
Quote:
On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?

Enjoy!

Oh, let me be the first to say it will fly! (I think)

Absolutely it will fly.

The apparant ground speed (with respect to the moving runway) will be twice the forward speed (with respect to the still air) but it isn't ground speed that makes lift.

The plane will take off at exactly the same distance from the start of the roll that it would if the ground were at rest.

.
.
The key to this riddle is to realize that a planes forward motion isn't created by the wheels driving the plane forward (If it was, then in this instance the plane would stand still), but instead comes from a source that pushes against the air and thus can't be countered by the motion of the ground.
_________________
The only problem with trouble-shooting is that sometimes trouble shoots back.


Location: Clinton, MS
Posted: 13 Dec 2005 05:59 Post subject:

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If the aircraft is sitting on a runway that is propelling it backwards at the same speed as the aircraft is going forward, how would it generate the necessary airspeed to provide lift from the wings? Interesting problem. Maybe I should have paid a little more attention in high school and college physics.

Location: Spokane/Everett WA
Posted: 13 Dec 2005 06:11 Post subject:

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No, the plane will not fly.... the lift is created by the air moving around the plane. The thrust from the engines just keeps the plane moving through the air fast enough for that air to hold the weight. If the plane is creating the thrust but the ground is moving opposite so that the plane isn't moving, the air is not moving to lift the plane. It's the same as if you put a plane's wheels on rollers. all that happens is the engines run and the wheels and rollers spin, there will be no gravity defying act.

Think about running on a treadmill. When you run outside, the forward movement causes wind resistance against you. If you were to harness that force and were shaped like a wing, you'd theoretically fly. But, as you run on a treadmill, you don't feel any wind resistance because you are not moving.

In the riddle, the conveyor belt does not move the air, it only moves along with the plane.
Location: Atlanta, GA
Posted: 13 Dec 2005 06:14 Post subject:

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Heh heh heh. ...and the fun begins! After the_lip's post, (nice job, btw), I thought it was going to be this thread that wasn't going to get off of the ground. But now we're gettin' somewhere!
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John Ford

We had the same debate over here in South Africa on a local forum, 15 pages long!

Yes the plane will fly.

The forces at play are in 2 independent systems.

The wheels and conveyor going backwards is one system, the thrust of the prop on the air and the moving air over the wings the other system.

The friction of the wheels on the conveyor is independent of speed. So no matter how fast the conveyor moves backwards, the friction force it generates is the same as per normal take off (Speed does not matter here)

This means the conveyor belt cannot keep the plane back, so the plane will accelerate forward, pulled by the prop thrust. If the plane is moving forward you get airflow over the wings, and hence the plane will take of when enough airflow generates enough lift.

Kind Regards
Rudi

Yah, what Rudi said...

Now, if you are travelling at the speed of light and you turn your lights ON... what happens?

Your lights come on!

CJ

Nope

The plane most certainly will not fly. With the exception of prop wash, there is zero airflow over the wings. The pitot tube will not see any air and the ASI will read zero.

ericwolf said:

The plane most certainly will not fly. With the exception of prop wash, there is zero airflow over the wings. The pitot tube will not see any air and the ASI will read zero.

Click to expand...

Over the last month or more, I've been involved with three different forums, where this question has generated hundreds of replies.

At first, I too, figured the wings would not move. But in the end, the wheels just turn at a higher speed, as the prop pulls the airplane through the un-effected air. In reality, the wheel friction remains rather stable, no matter what the speed of the conveyor is.

L.Adamson

speed of light

For Capitan John

How about this, if you are traveling faster than the speed of light and you turn on your lights, are they back up lights? (he he).

I cant believe some folks think the airplane will fly. They can't be serious.

RV9-A N51HS
in process

L.Adamson said:

At first, I too, figured the wings would not move. But in the end, the wheels just turn at a higher speed, as the prop pulls the airplane through the un-effected air. In reality, the wheel friction remains rather stable, no matter what the speed of the conveyor is.

L.Adamson

Click to expand...

Ah, but since the conveyor increases speed to match the speed of the wheels, the airplane doesn't move. If the airplane doesn't move, there's no airflow over the wings.

Huh?

since the conveyor increases speed to match the speed of the wheels, the airplane doesn't move

Click to expand...

If the airplane isn't moving, why are the wheels turning faster?

What about ice?

Wouldn't sheer ice on the runway be the same thing as the conveyor belt? The moving belt is simply acting as a frictionless surface, same as ice. Will a plane takeoff from ice? I think so.

Gotta love it...

Ahhh... the stuff debates are made of...

As usual, its all in the interpretation

Oh no! Not here too!!! (This is the 3rd message board I've seen this brought up and am amazed at the truly convincing arguements on both sides.)

I for one have no doubt you'd be able to take off with no problem. The only difference is your wheels would be turning twice as fast as they normally would be at liftoff. (forward speed + speed of the conveyor going backwards.)

AVWeb had an article on this a few weeks ago... Click Here

--Ken

Not fly

Put yourself in the pilots seat-------and look out the window-------the forward speed of the a/c is countered by the speed of the belt, and therefore the resultant speed is zero----as you look out the side window, any ground based referance will be stationary. the airflow over the wings will be propwash only----------the man-on-treadmill analogy is correct.

Would be nice if it could fly, just think of the possabilities of having a "runway" at home that is only 25' long.

Bottom line, wont work.

Mike

All depends on airflow over/under the wings. Now if there were hurricane force headwinds....

Mike S said:

Put yourself in the pilots seat-------and look out the window-------the forward speed of the a/c is countered by the speed of the belt, and therefore the resultant speed is zero----as you look out the side window, any ground based referance will be stationary. the airflow over the wings will be propwash only----------the man-on-treadmill analogy is correct.

Would be nice if it could fly, just think of the possabilities of having a "runway" at home that is only 25' long.

Bottom line, wont work.

Mike

Click to expand...

The man-on-treadmill analogy is unfortunately incorrect.

Where does a man get his "drive" from? The ground resists his legs motion and propels the rest of his body forward. So in that case, if a conveyor (or treadmill) was travelling backwards at the same rate as his legs were driving him forward, zero relative motion results.

Now, where does an airplane get it's "drive" from? The air resists the propellers act of pushing on the air, independent of the ground on which the airplane is sitting. The propeller pushing air backward causes the airplane to move forward. Looking out the window would have the ground rushing by at twice the indicated ground speed.

Similar to the ice idea above, what would happen if an airplane had "magic" wheels that didn't actually touch the ground? They allowed it to hover 6" above the ground, but never touch it. (Imagine the floating skateboards in Back to the Future II) Would you be able to move the plane with only the prop?

--Ken

Of Course...

... it would fly. It's all about relativity and has been argued in many shapes and forms over the years. As has been stated, nothing has changed with regards to the relationship between the aircraft and it's environment. The engine will still create thrust and Newton's third law tells us that the plane will move forward unless an opposing force overcomes that thrust. The only thing that changes in this hypothetical scenario is that there is an additional (although miniscule) opposing force being applied which must be overcome. This additional force is miniscule because the near frictionless wheelbearings allow very little of this opposing force to act upon the airframe. Theoretically speaking it would take a very small additional amount of thrust to move the plane forward. From a practical standpoint it makes absolutely no difference.

I don't buy the treadmill analogy as walking on a treadmill depends upon friction between the feet and belt to convert energy in the muscles into motion. The prop on the plane creates thrust independent of the friction between the plane and ground.

Replace the plane with a car or a person and you have a different ballgame.

Mike S said:

The forward speed of the a/c is countered by the speed of the belt, and therefore the resultant speed is zero----as you look out the side window, any ground based referance will be stationary. the airflow over the wings will be propwash only----------the man-on-treadmill analogy is correct.
Bottom line, wont work.

Click to expand...

Except that the prop wasn't told about the conveyor belt and accelerates the plane forward just as it normally would. This is typical multiple axis system dynamics problem. One axis system is relevant to the problem (air fixed), and the other is entirely irrelevant (conveyor fixed).

mark manda said:

The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?

Click to expand...

Here's the folly with the arguments so far. The control system obviously measures vehicle position relative to a ground location and attempts to keep that position constant.
Since there is no substantial relationship connecting the airplanes thrust to the ground position (other than wheel bearing friction (thrust connects the plane to airmass position)), as soon as the plane begins to move down runway and away from the control position setpoint, the runway conveyor motor will begin to overspeed.

As the conveyor runs faster and faster trying unsucessfully to reset the plane to its initial position, it reaches light speed and bursts into flame from atmospheric heating, catching the plane on fire and melting off the wings.

The plane will not fly in this condition.

(although it may be violently propelled backwards as a ballistic projectile due to the extremely high velocity local wind created by the lightspeed conveyor)



-mike

Oops

I stand corrected-------------upon further thought, and reading, it will fly, but the wheel bearings will take a beating-------------

Mike

As the conveyor runs faster and faster trying unsucessfully to reset the plane to its initial position, it reaches light speed and bursts into flame from atmospheric heating, catching the plane on fire and melting off the wings.

Click to expand...

Best. Answer. Yet!

If you put instant coffee in the microwave, do you go backwards in time?

More misc rambelings

O.K., I think I really hate the internet----------my poor brain wont let go of things like this.

Way I see it is we are asking the wheels/bearings to do two opposite things at the same time.

1. Impart backward motion from the belt to the airframe to counter the prop thrust motion.
2. Allow the plane to move forward, so the sensor can read the plane motion so the belt/runway can counter it.

Now if we add an automatic device to apply and release the brakes at an infinate switching rate----------------

mike

Bottom line

The moving conveyor belt runway (in the opposite direction of flight) will not be able to stop the forward movement of the the aircraft thru the air. The original premise is faults; regardless of how fast the belt is moving the plane will move forward thru the air and take off; The moving belt will NOT be able to stop the plane from accelerating; albeit there with be a slight increase in distance due to the slight increase of friction in the wheel bearings. George

Finally

George, been wondering when you would chime in on this-----------

Mike

Mike S said:

O.K., I think I really hate the internet----------my poor brain wont let go of things like this.

Way I see it is we are asking the wheels/bearings to do two opposite things at the same time.

1. Impart backward motion from the belt to the airframe to counter the prop thrust motion.
2. Allow the plane to move forward, so the sensor can read the plane motion so the belt/runway can counter it.

Now if we add an automatic device to apply and release the brakes at an infinate switching rate----------------

mike

Click to expand...

Here's my take on it:

If the conveyor is closed loop on the wheel RPM, then the plan will always fly and the wheels will simply turn at 2x the aircraft's velocity. This assumes that the wheels and bearings can roll that fast without blowing out.

If the conveyor is closed loop on the aircraft velocity the situation is not so clear. In this case, the conveyor will simply go faster and faster to counteract any forward motion of the aircraft. More thrust just makes the conveyor turn even faster. The caveat here is that the force vector due to friction (kinetic friction, that is) does NOT go up appreciably with increased velocity. In fact, it can go down....or it can go down and then back up...it's not an obvious situtation. However, in this case the conveyor always wins regardless because it can always go fast enough to blow up your tires.

In a perfect world, i.e. one where bearings don't overheat and tires don't blowup and conveyors can move as fast as they want, the plane always flys, but it's not obvious why. The aircraft will immediately get the conveyor turning blisteringly fast in an attempt to maintain aircraft position (which it can't do). This will allow the aircraft to take off in ground effect and scoot merrily on it's way.

Now, the REAL question is: "What's better for takeoff's on a conveyor belt? Taildragger or nosedragger? Tipup or slider? What's the best primer to use for conveyor belt takeoffs? Should I use steam gauges or go glass pan........."

Snakes!!!!

Ever heard of a snake called Ouroborus?????????

He flys off a treadmill, in a taildragger.

Merry Christmas to all, I am going to go clean the hanger now.

Mike

For those that think the plane will not fly, the error is in the assumption that the conveyor belt will keep the plane in the exact same spot where it started. In this case there would be no relative motion of the wing to the "clam" air. However, since a plane does not depend on ground traction for forward movement, the conveyor would never be able to stop the forward motion of the plane.

The problem with this question and the debate that follows, is that when the counteracting treadmill is mentioned, it immediately causes some to think that the plane would have no forward motion. Discussion of wheels, bearings and ground speed only add to this incorrect assumption. Since the prop pulls on the air and not the ground, a treadmill runway would have no effect.

Now if there was a big fan behind the plane that would counteract the relative wind of the prop pulling the plane through the air....

Reminds me of the trains and bird problem

This reminds me of the classic problem of two approaching trains with a bird flying betweeen them. Here's the setup:

There are two trains on a track that start 100 miles apart. They travel toward one another, each at 20 mph. There is a bird that starts at one train and flys at 40 mph toward the other train. When it gets to that train it turns around (instantaneously) and heads back, again at 40 mph. It repeats this process until the two trains collide and it gets squished between them.

How far has the bird flown?

Alex said:

This reminds me of the classic problem of two approaching trains with a bird flying betweeen them. Here's the setup:

There are two trains on a track that start 100 miles apart. They travel toward one another, each at 20 mph. There is a bird that starts at one train and flys at 40 mph toward the other train. When it gets to that train it turns around (instantaneously) and heads back, again at 40 mph. It repeats this process until the two trains collide and it gets squished between them.

How far has the bird flown?

Click to expand...

Two (easy) ways of doing it:

1) Trains going towards each other @ 20, therefore their relative velocitys are 40mph. 100/40mph = 2.5 hours until they colide. Birds travelling at 40mph, so 2.5 * 40mph = bird travels 100 miles

2) (This is how I initially thought of it...I think this is more intuitive). The trains will cover 50 miles @ 20 mph therefore the bird must cover 100miles @ 40 mph.

The only interesting part of this thread is that it contains (I think) George's shortest post ever.

jcoloccia said:

Two (easy) ways of doing it:

1) Trains going towards each other @ 20, therefore their relative velocitys are 40mph. 100/40mph = 2.5 hours until they colide. Birds travelling at 40mph, so 2.5 * 40mph = bird travels 100 miles

2) (This is how I initially thought of it...I think this is more intuitive). The trains will cover 50 miles @ 20 mph therefore the bird must cover 100miles @ 40 mph.

Click to expand...

Yup. The conveyor belt problem just reminded me of this because it's another one of those problems where people get focused on something entirely irrelevant to the solution. In the case of the bird problem people start worrying about the fact that the bird will make an infinite number of trips in a finite amount of time and think they have to solve the infinite series that results.

The plane's power is completely independent of the conveyer and it does not matter how fast the conveyer moves.

The conveyer will make the wheels turn faster (than the plane) as the propeller drives it forward until it takes flight. The wheels will turn at the speed of the planes forward speed plus the conveyers backward speed, unless the pilot hits the brakes to stop the wobbling.

This is not a big deal. The original question says that the plane's speed is matched by the belt, implying that the plane has speed. If this speed is measured relative to the fixed ground, then the plane has forward motion and hence airflow. The fact that the belt is whirring around underneath the tires makes no difference.

The plane will fly and lift off at the same distance as you say. The only difference, is that the wheels will be spinning at a much higher speed, since the only thing the conveyor will do is turn the wheels. Since the wheels are normally used to a stationary ground instead of a moving one, their rotation will be much higher. Hopefully, the bearings won't overheat and sieze, which will put a very different spin (no pun intended) on the equation. The conveyor would then throw the airplane off the back of the conveyor, assuming thrust cannot overcome the friction. Think I'll stop at this point.

OK, so what about an airplane sitting on the end of the runway, ready for takeoff, immediately after applying full power, the winds change to a steady 10 mph above Va directly on the nose?

Or, what if we applied the same treadmill scenario to an airplane that was landing? Ground speed higher than airpeed, like a tailwind.

Airplanes do not use their wheels to produce thrust.

Obviously, the wheel friction has an effect, but it will only slighly lengthen takeoff distance (depending on tire roll resistance/inflation). The prop will continue to accelerate air at it's normal rate and thrust will act on the relative position of the airframe as normal. Ever wonder why you never see a positive traction option on aircraft?

Now, put an automobile on this treadmill and see if it ever takes off?

First Post needs clarification

Since the first post says that this is for engineer-wanabees, I shouldn't be replying to this since I am a real engineer. This post is, however, driving me nuts.

I think the reason that a seemingly simple problem is producing such varied opinions is because the original post is not clear. I think that there are two ways of interpreting this as it was written:

1) After power is applied, the belt?s speed relative to the ground increases to match the speed of the plane relative to the belt.
2) After power is applied, the belt?s speed relative to the ground increases to match the speed of the plane relative to the ground.

I?ll explain each one in more detail:

1) In other words, if the speed of the belt relative to the ground is 100mph and the speed of the plane relative to the belt is 100mph (in the opposite direction), the resultant speed of the plane relative to the ground is zero. Zero relative wind and zero relative movement to the ground means that there is no air movement over the wings resulting in no lift and a plane that is not flying. As most would agree, the belt would have to accelerate and move very fast to hold back a plane, but there are no real-life limitations in these assumptions.

2) In this scenario, the plane does accelerate relative to the ground, but the wheels are moving at exactly 2X the speed of the plane relative to the ground. As many have mentioned before, the extra tire and bearing friction will result in a slightly increased takeoff distance (measured on the ground, not the belt), but it will definitely fly in this scenario.

If you look at my original reply, I originally thought that the creator of this ?riddle? was trying to describe scenario #1 and it appears to me that many of you thought he (or she) was trying to describe scenario #2. I now wonder if he (whoever it is) really knew what he meant to say in the first place.

I hope that everybody can see all speeds are relative and it is important to state what the speed is relative to, otherwise the statement can be interpreted multiple ways and we start arguing about something we probably mostly agree on.

Rabbit chasing

Eric, I think the wording is exactly what was desired---------make it ambigous so folks would misinterpet things so as to get folks confused and follow the wrong rabbit--------this is a typical ploy in word games/riddles.

Good analysis as to the two scenarios, however.

Mike

I have to go with the airplane will not fly crowd.

Ignore for a minute the though of the propeller pulling the airplane through the air. Bear with me here:

The conveyor belt SOMEHOW senses the aircraft speed, and moves opposite to the forward movement. It at that point, it doesn't matter WHAT is propelling the airplane forward, because no matter how much power the engine puts out, the airplane CAN NEVER get ANY forward speed through the air.

Physically it would be nearly, if not totally impossible to accomplish a conveyor with that instantanious reaction the the aircraft speed, which is why it is so hard to fathom what would happen. I myself am conflicted between two possible results:

1. The aircraft applys full power, and the conveyor speeds up to a point that the engine HP=the resistance of the wheel bearings, and then maintains that speed (How fast would that be?)

2. The start of the aircraft wheels turning gets the conveyor going, but the lack of any airspeed around the prop simply means that the conveyor turns extremely slowly (Whatever the initial power application provides).

Of these two, I think the first scenaro is the most likely. I understand how you can say the aircraft will take off, and agree that in reality, the entire setup is so impractical, that if the experiment were attempted, the conveyor could not accelerate fast enough to prevent the aircraft from taking off. However, in the theoretical sense, in a split second, your wheel rotation friction would equal the 180HP from your engine, and stationary you would stay.

That is untill your wheel bearings locked up and you were doing 5 billion mph backwards...

THIS engineer is staying as far away from this puzzle as I can....reminds me of the dinner-table brainteasers my fatehr ( a mathematician) and I (a budding engineer) used to have. I generally solved the specific problems quicker, but he usually had a general solution!



Paul

Contrary to popular believe, the friction for wheels & bearings is independent of the speed at which the conveyor travels at.

http://hypertextbook.com/physics/mechanics/friction/

Friction: The force between surfaces in contact that resists their relative sliding motion. Types: static & kinetic

Classical Approximations
* independent of

o surface area,
o speed (except when v = 0), and
o temperature
​

Click to expand...

This mean the wheel/bearing friction will not keep the plane back enough to overcome the thrust of the prop, no matter what speed the conveyor travels...Unless they break or lock up.

The orginal question does not say the plane is stationary. This means the plane will move forward, and get airflow over the wings, and generate lift and take off!

No more debate for me, on this one!

I've been in both the will fly and won't fly camp - but I think I have an analogy that solves it. You can even perform this experiment at home.

First off we need a wheeled vehicle - such as a Hot Wheels car - to simulate our airplane. A marble or small ball would do.

Next we need a runway - such as a long board - but light enough to move.

Finally we need a way to simulate our prop thrust. We can do this simply by inclining the board and letting gravity do the trick.

Slightly incline the board so that the car/marble/ball will start moving. Now start pulling the board back (along the incline) to try and hold it in the same spot.

I find I can keep the car in the same place, meaning the conveyor can also and thus the plane can't take off.

Mike

BTW - Santa came early - my RV-9A empennage kit and Avery tool kit both arrived this week!!!

Bad analogy

MTBehnke said:

First off we need a wheeled vehicle - such as a Hot Wheels car - to simulate our airplane. A marble or small ball would do.

Click to expand...

Right of the bat your analogy break down. The wheeled vehicle you are using to simulate the plane gets its forward motion from its wheels and traction on the ground, an airplane does not. Since ground traction does not matter, the plane will move.

Try this, put a plane and a car on a frictionless sheet of ice, which one will move?

And if you tilt the sheet of ice???????????

MTBehnke said:

Now start pulling the board back (along the incline) to try and hold it in the same spot.

I find I can keep the car in the same place, meaning the conveyor can also and thus the plane can't take off.

Click to expand...

If you're keeping the car in one spot, then it has no speed. If it has no speed, then the conveyor should also have no speed (according to problem description). But if you stop the conveyor, then the car will move forward again and have speed, so you'll have to get the conveyor moving again...

Not a good experiment

MTBehnke said:

Slightly incline the board so that the car/marble/ball will start moving. Now start pulling the board back (along the incline) to try and hold it in the same spot.QUOTE]

Mike there are a couple of problems with your experiment.

1) You are using gravity to simulate thrust but by tipping the board just enough to get the car moving. At this point gravity is exerting just enough force to overcome axel friction and wheel to board friction. This is not even close to the thrust to wheel bearing friction ration of an RV. To more accurately simulate an RV's thrust to wheel bearing friction ratio you would require a much steeper incline. At this point the force of gravity greatly exceeds the opposing forces of friction and there ain't no holding back the car no matter how fast you move the board in the opposite direction.

2) The friction in your hot wheels axle is much more than the friction in the RV wheel bearings. Therefore, as you pull the board back, you are relying on the bearing friction to supply the opposing force to gravity. Ain't going to happen in the real world in which the RV bearings are much more free. There isn't enough friction generated in the RV wheel bearings to balance the thrust of the engine.

Bottom line, the opposing force on the plane will be negligible and that puppy is going to move forward and take off in practically the same distance it would on a paved runway.

Click to expand...

Look Boss the Plane! The Plane!

Lets go to the free body diagram! The end result is the tire speed will be double the usual amount and plane will................. This puzzle obfuscates the fact there is airspeed, ground speed and "TIRE SPEED". G

airspeed....

only one thing will make a plane fly.....AIRSPEED!

WAIT! That is all WRONG!!!

In my mind's eye, I imagined the plane moving from the left to the right!

How does that affect the scenario!?!

CJ

Lack of blood in head

Captain_John said:

WAIT! That is all WRONG!!!

In my mind's eye, I imagined the plane moving from the left to the right!

How does that affect the scenario!?!

CJ

Click to expand...

HaHaHa Stand on your head, it will be much clearer, G

Proving my point

gmcjetpilot said:

Lets go to the free body diagram! The end result is the tire speed will be double the usual amount and plane will................. This puzzle obfuscates the fact there is airspeed, ground speed and "TIRE SPEED". G

Click to expand...

czicree said:

If you're keeping the car in one spot, then it has no speed. If it has no speed, then the conveyor should also have no speed (according to problem description). But if you stop the conveyor, then the car will move forward again and have speed, so you'll have to get the conveyor moving again...

Click to expand...

This just proves my point that you are assuming two different things and arguing when you probably actually agree (see previous ranting by me - post #35).

What I agree, no I disagree. There is only one solution

ericwolf said:

This just proves my point that you are assuming two different things and arguing when you probably actually agree (see previous ranting by me - post #35).

Click to expand...

Yes you are right and wrong.

No its not just interpretation, and yes the plane will fly. I'll take the stand that there is only one correct interpretation and solution.

I agree interpretation can be an issue in Puzzles. Some have no solution and are more philosophical than analytical (does a tree falling in a forest make a sound), but most riddles/puzzles have one intended solution, albeit with miss leading or non-relevant statements to "de-clarify", trying to lead you to the wrong conclusion.

I agree people interpret words differently and that's why there are lawyers, but I the real reason for disagreement is analytical.

The main reason some folks came to the "plane won't fly" conclusion is they did not understand forces involved with bodies in motion (balanced forces, free body equations, summation of forces ...... thrust, friction, lift, gravity, relative motion, degrees of freedom, reference systems and Newton's laws of motion Force = mass x acceleration).

Yes it's common to obfuscate and "de-clarify" to miss lead like:


Well of course GOLD IS heavier!!!!! Well the answer is neither is heaver. A pound of anything is a pound. Again yes one might be miss lead by intuition or lack of careful reading, but they are also miss lead by assumptions of weight, mass and density. That is why there are nerdy engineers. Some people no matter what you say will tell you a pound of feathers is lighter than a pound of gold.

Before anyone gets pissed and takes insult, please don't. The intent of riddles is fun. The whole moving runway scenario is not realistic or practical...... Puzzles and riddles and solving them is an acquired taste and you get better with practice.


If the plane was tied down or could not move, it would never takeoff despite the tire speed, thrust or conveyor speed. Of course this would be a trivial situation. The "puzzle" only said the conveyor moved inversely proportional to the aircraft (fwd) speed. People assume that means the plane can't move forward. Clearly the puzzle never SAID it can't move, but people ASSUME it can't if the runway moved opposite. THIS IS THE GOTCH-A.

A free body diagram (engineering tool to analyze the forces on an object) clears the issue up. If the plane has thrust and is NORMAL in every way (degrees freedom of motion, e.g. not tied down), except it's on a weird moving runway.

The OTHER INFORAMATION that they included to throw you off is the conveyor can SENSE the aircraft speed and move OPPOSITE at the SAME speed. Again like feathers vs. gold, you assume the WORDS opposite and same speed means the plane will not move forward.


The take off distance may be a few feet longer on the moving runway, but in the end excess thrust will overcome the small extra drag, which allows the plane to accelerate (until thrust = drag). The first point in time where the thrust and drag (gravity and lift) are all balanced after starting the "take-off roll" is after lift-off, established in a steady state speed climb.


A backwards moving runway will not stop the plane from moving forward.

So the question is clear. Will it take off or will it not?

The answer is yes IT will. G