Airplane on a Conveyor Belt
December 8, 2005
11:54 am
A riddle was proposed on the Neal Boortz show today:
If an airplane is on a large conveyor belt and is trying to take off by exerting the thrust needed to move it forward at 100 knots, and the conveyor belt starts moving backwards at 100 knots, will the plane be able to take off, or will it just sit stationary relative to the ground, with the backwards speed of the conveyor belt counteracting the forward thrust of the plane?
Astoundingly, Neal and the rest of his crew took the position that the plane would sit there stationary! Good God… this man is a pilot and has a law degree! I could understand a random high school dropout being fooled by this, but a pilot?
Then I googled the riddle, and found a thread on Airliners.net that has been raging on, with the vast majority of people taking Neal’s position… that the plane would not be able to take off.
Their argument is this, to quote one poster:
Thrust acts accordingly to Newtons Third Law of Motion - every action has an equal and opposite reaction. In the case of an aircraft, the reaction of the engines is that of forward motion, against whatever medium it is stationary. But the ground the aircraft is sitting on in this case is NOT stationary, its providing an exactly CANCELLING force pushing the aircraft back.
The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.
So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we’ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!
When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.
Gah… people are freakin’ stupid.
Update: There is a variation on this riddle that says that the conveyor belt matches the speed of the plane. It doesn’t matter… the plane still takes off. The conveyor belt could be going 5 times as fast as the plane, and the plane would still take off. You’d get into issues about tires blowing out, but assuming that the wheels can take the strain, the airplane would still take off.
Update: Well here we are more than two years later. The show “Mythbusters” attempted the experiment. And yes, the plane took off. The laws of physics still apply. Back to life as usual.
Ahh.. actually technically hes right by the way he worded it. Wouldnt time be relevant of GROUND SPEED. Hes saying the flying characteristics are the same flying into the headwind or with a tailwind. You just move faster or slower compared to the ground (i.e. ground speed).. you may want to rethink your statement at who put whos foot in mouth.
Hey guys… C’mon… if both forces (conveyor and plane) neutralize themselves, so, waht’s the point in having the belt or plane moving, try to figure it out while both static… the engines should be on a different angle… so that the plane would take off… Same theory applies to helicopters and that prototype jet that needs no area to take off… simply by rotating the engines…
Sookie Old Chum:
Uh no.
Did we read the same post? He said: “…tail winds and headwinds are not relevant to the flight abilities of the aircraft, it is only relevant to the GROUND SPEED.”. And you say he said “Hes saying the flying characteristics are the same flying into the headwind or with a tailwind. “.
I understand that you are reading INTO something he said but…that is NOT what he said.
Trust me - planes handle differently if flying with or against the wind. For example (one of many) ever try to make a gradual 180 while flying into the wind - and then against the wind. The overall concept is similar, but the may you handle the contraols is different if you want to try to duplicate the same turn radius or size. Even time needed is different.
But I’m sure you already knew that…
Yea, I’m pretty much sticking to my original supposition, your airway is constricted.
You see, I thought we were discussing the original question, whether or not an airplane would generate lift and take flight from a conveyor belt.
Perhaps you can seek justice here http://en.wikipedia.org/wiki/Levitation or http://en.wikipedia.org/wiki/Metaphysical_levitation
Jason is right.
if the speed of the conveyor belt cancels the speed of rotation in the wheels caused by the thrust, then why not think of this in the following way :
lets say that the wheels of the plane were bolted into the ground (since rotation and conveyor cancel each other, this is what you’ll get : zero force in either direction).
Now the question is, will the thrust generated by the plane into the air, exert pressure on the wheel bolts? ofcourse it would.
planes fly by moving air. same as rockets.
ever seen the baffle/wall behind the thrusters on an aircraft carrier?
think of it this way.. say youre on roller blades on a long treadmill or walkway like at an airport. say youre holding onto a rope or the rope is tied around your waist and on the other end it is being used by a wench. that belt/walkway can moves however fast it wants..but with every crank of the wench you will move forward with respect to a stationary object and will continue to move forward untill it gets to the end. the belt doesnt cancel the wheels
The crucial point is that the plane engines will accelerate the plane towards 100 knots relative to the surrounding air (not relative to the ground) - and that’s the only speed that matters here. The fact that the “ground” is going backwards is irrelevant - the wheels just spin twice as fast as they would normally and it makes no appreciable difference to anything.
The plane flies.
Your all fucking retarded.
Hmm, i have read many or the arguments and i have one simple solution.
The overall question is “would it take off?”
and since he already explained that the plane would move forward, even just a LITTLE bit
then sometime
eventually
even if its gaining speed REALLY slow ,
sometime it is gonna eventually its gonna take off(unless it runs out of gas)
so i say yes(unless it takes so long it runs out of gas)
that’s an answer that is as simple as it can get really
unless there is outside forces
but since this is hypothetical then we should leave that’s out cause you can always take the measurement of the fact that this will practically never happen
so….there you go an answer from someone who is as stupid as a 14-year-old (i know I’m stupid but i still have basic reasoning)
does anyone own a conveyor belt and a model airplane to test this out ?
anyone?
Simple experiment if you still dont get it.. when youre at the supermarket next time.. get a can of soup. Take it to the little conveyor belt.. now when its moving..lay it down on its side…now take your finger and push it from behind.. you can move your finger as fast or slow as you want but the can will still move forward regardless of the speed of the belt
At first I thought that the plane would lift off, but as stated methinks it won’t..
Think of it like this, The conveyor belt it’s quite long, let’s say a whole mile. And it’s turning slowly. If the plane has it’s engines turned off, the plane will move backwards. right? (since the weight of the plane, and all of this nature-it’s-not-a-schoolbook thingie kicks in). If you fire up the engines, the plane will move forward (or if you are a very, VERY good pilot, maybe you’ll be able yo keep it still.)
Now, why is that? because the engines are pushing the plane forward. In that little sentence lies the answer. It’s not the wheels that move the plane, but the engine (or propellers or slingshot, or whatever).
If the conveyor belt it’s quite long, then you must exert the same force on the engines either to move it forward all the way to the end of the belt at the same speed, or to keep it still. So, the forward movement of the plane is a function only of the force of the engines, again, not the force of the conveyor belt.
If you speed things up, everything should remain the same, except that the wheels will spin faster and faster, but the plane will either move forward or stay still. ( or move backward if you’re not such a good pilot
The original post states that if the plane’s engines push forward at 100 knots, and the belt moves backward at 100 knots, then the plane will fly. It will not, if you account for reality (friction and weight) but anything above 100 knots will move the plane forward. IANAP (I am not a pilot) but eventually you will gather enough airspeed to generate lift and fly.
If this whole thing would be done in a ideal world, then the wheels and conveyor get canceled out at any speed and the plane will be ideally suspended without friction and gravity so any force will move it forward.
So as stated the plane will not lift off, but ideally it will
or maybe I should have read the whole shebang of comments
i dont know what to tell you… your close but it does take off. if you look up a ways under one of my posts i actually did the physics on it using a 747.. The fact that the engines are capable to move it forward at any point means that it will take off. the belt doesnt act directly onto the plane so no matter what speed the belt is moving its still capable of moving forward.. basically you have thrust force vs the rolling resistance force.. rolling resistance isnt a resultant of speed. its the same at all speed. it is dependent on coefficient of friction or mu. in order to keep it at bay.. mu would have to be something like 20 times greater than what it currently is i believe iirc.
The reason you state that it moves backwards when the engines are off is because thrust now = 0 and you have static friction which is greater then rolling friction. so resistance > thrust thus net force is negative. again if you have questions you can ask..or else look at the physics of it somewhere up the page under the name sooks.
here it is for you.
its not hard… if thrust is > than drag it will accelerate and continue to till liftoff. if drag is > than thrust it will accelerate in the opposite direction (of decelerate depend on viewpoint) if thrust is = to drag it will be at a constant velocity at whatever speed it was at when they became equal. Now.. the forces of drag are wind resistnace which would be the same as on a normal runway. bearing resistance, which with aeronautical grade bearings can virtually be ignored. and rolling resistance, which its equation would be coefficient of rolling friction which = about .001 * weight. *SPEED DOESNT FACTOR IN THE EQUATION. so on a 747, thrust = 50k lbs per engine * 4 engines so 200k lbs thrust. its weight is 847k lbs * .001 and drag is roughly 10k lbs…. 200k > 10k.. it accelerates and would continue to till take off,
along with a seperate example
Here is another example if you dont think its true. Tilt the conveyor belt so its vertical. now take a matchbox car and attach a magnet or something similar so that its wheels will stay attaced to the belt. now place the car on the belt and drop it.. do you think that the belt will keep the car hovered in air because it is matching its speed??
and looking at the math..mu actually has to be 300x greater.
My god, it took me 30 seconds to think about this riddle and it’s a good one, since I tended to agree that the plane would not fly. But then I saw through the riddle and understood.. and you.. talk about this in 400 posts???????????
#404 sums it up quite well I think.
Roger
Holland
The force that counters the thrust is the force that accelerates the belt.
Beer, how do you propose this force is translated to the body of the plane, which the engines are acting on?
How so?? Tell me how it acts upon the plane.. where is it countering the thrust? by pushing tangential to the wheel? Doubt it. Do a free body diagram and show me how they act against it and what the forces are.
The wheel doesn’t matter. They just have to spin fast enough to cause a deceleration equal to the acceleration imparted by the thrust.
thrust resultant of wheel spinning (too fast)
Umm.. WHAT? Does anyone else get that? I cant figure out what youre trying to say.
The wheels have to spin fast enough to decelerate = to the acceleration created by thrust??
damn arrows missing…
thrust (plane) resultant force
put it this way… what if i place this entire set-up (plane + belt) on top of another belt that counters the acceleration? what if i reduce the 2 belts into 1 resultant?
If the conveyor is matching the speed of the plane.. then the wheels are just spinning twice as fast as the would on a normal runway.
I dont get quite what youre saying with the two belts.. My point is that the wheels dont matter to the planes movement. They are simply there to reduce friction. Otherwise how do sea planes take off?? or planes on snow.
The wheels could be revolving backwards and the plane would take off. What if it was on a conveyor belt that matched its speed in the same direction… Then the wheels would be stationary as it took off. The wheels are irrelevant.
If the speed of the belt has to be equal to the airspeed of the plane then there isn’t anything to argue about… we only get a discussion if the speed of the belt is adjusted proportional to the airspeed of the plane.
Nevermind it’s midnight here… I gotta sleep.
What do you mean adjusted proportional.. I look at it as the plane moves a foot.. the belt moves a foot. Or the speed as meausred from an object on the earth..or the speed youd see if a radar gun was used. But really you can use whatever speed you want as long as its the same for both the plane and the belt.. The only time ive seen arguements for the belt is when the either change constraints of the problem or use a different reference point for speed of belt than the plane.
One last comment
If the speed (of the belt and the plane, in opposite directions) has to be equal, then the plane would take off.
If the speed of the belt can be greater than that of the plane, then it is possible (with the help of friction) to keep the plane at the same spot.
Nights.
Agreed..kind of. Through basic physics the belt is irrelevant as speed isnt a factor in the equations of the friction. However.. obviously at some point you breech the limits of the bearings and wheels and theyd fail. If you look at it hypothetically though that they wont fail. I could see an argument that the wheels spin so fast that the increased heat raises teh coefficient of friction to a point where friction is greater than thrust. No idea what point that would be at though.
Okay, I can’t believe I’m jumping in here, but here goes:
In the cases where the conveyor rolls backwards at some fixed speed (e.g., 100 knots), even if we start with the plane stationary on the conveyor, moving backward with it at 100 knots relative to the earth, the plane eventually takes off (it just takes longer), because as many have rightly pointed out, the constant thrust pushes against the air, and the wheels will just be spinning 100 knots faster than normal when liftoff is achieved. The friction due to gravity on the plane pressing the wheels into the runway, and conveyed against the forward movement of the plane would presumably be, as many have pointed out, negligible in this case.
However, in the case where the conveyor matches the speed of the *wheels*, and does so *instantaneously*, the negligible force due to friction on the wheels, (unless you assume perfectly free-wheeling bearings) even tho small, would still, by definition in fact, be enough even to counter thousands of pounds of thrust. How so? Well, the conveyor may well soon approach infinite speed, but multiplied by infinite speed, even a negligible friction can counter infinite thrust.
Unless you can convince me that the countering force of friction on the plane, conveyed from the wheel, is in fact exactly zero, I don’t see how the plane can take off in the second case.
And if you really want to reach, the only other way I could see arguing that, in the second case (conveyor matches wheel speed), the plane theoretically takes off, is if at some hyper-fast wheel speed against the conveyor, some additional fancy physics comes into play that suddenly reduces friction to zero, at which point the plane breaks free and can begin accelerating forward toward eventual liftoff.
Of course, a real plane’s wheels would blow long before this, but that wouldn’t exactly do anything to help it’s friction problems, unless it was putting out so much thrust as to be able to “instantly” jump to liftoff airspeed before gravity had it crashing into the conveyor
My god people, this conversation has completely destroyed my estimation of the intelligence of my fellow man. Airplanes fly through the AIR. The engines on an airplane move AIR. Pilots use AIRspeed to calculate how fast they are going. An AIRcraft can only take off once it has reached a certain AIRspeed. It DOES NOT MATTER how fast the ground is going when you’re talking about how fast an AIRplane needs to go to fly. The only time a moving conveyor would affect an AIRplane is in how the friction is transferred to the wheels. I saw links from people who say they’re Navy pilots, people who say they’ve seen this tested with model airplanes… those people are either lying outright or just plain dumb. I’ve been flying all my life and this is really just the most rediculous aviation-related discussion I’ve ever seen. It’s a bunch of people who only have experience with boats and cars trying to apply their limited knowledge to the world of aviation which operates on a different set of rules. My God people, stay within your depth!
OK, so I think we finally figured it out. The conveyor weighs more than the airplane, which is what causes the airflow to find the direction. The propellers are making wind, noise, and they are causing all kinds of greenhouse gasses and scaring the endangered rooted fowl. When the vertices of acceleration exceed the desired friction that is when you finally achieve nirvanal summary. If you built this in simulation in solidworks, you could reverse the entire process and learn exactly butkiss. Fear not the re-hash for it follows that the wheels are actually spinning, the conveyor is moving the plane is moving, or not, and the thrust of the propellors is making the plane fly, or not.
When you finally realize that the airplane is made of aluminum, and that the delorian is made of stainless steel, of course you need 21 jigawatts of power to attain flight in this manner. Forego the presupposition of inarticulate transmission and you can trancend the thread, which is pulling on the plane, see some f*Cker photoshopped the thread out a long time ago and it just resurfaces as an ongoing anomoly.
I respectfully submit that there is ony one way to solve this, and that is to somehow destroy all evidence in the database, in the meantime,
CALL MYTHBUSTERS.
To the people who still think that a speed-matching (but in the opposite direction) conveyor belt will prevent the plane from taking off, consider this diagram, which represents that scenario.
As you can see, the belt will move backward at exactly the same speed as the airplane. Obviously, I had to make a few assumptions, like weightlessness of the conveyor belt, but once those assumptions have been made, this is the speed-matching scenario.
Still think it doesn’t take off?
“…with the backwards speed of the conveyor belt counteracting the forward thrust of the plane?”
So if the wheel bearings don’t have friction then how can the conveyor belt counteract the forward thrust of the plane? Your riddle should be trying to counteract the forward thrust of the plane. For the conveyor belt to actually be counteracting the forward thrust it has to provide rearward thrust. This is where your riddle is set up to make people assume that the plane sits still, ie: forward thrust + counteracting reverse thrust = no movement.
Okay, I have read this entire ridiculous thread. Let me “try” to help…
Facts we know:
Plane is THRUSTING at 100MPH (I know changed for ease sake)
Conveyor is moving at 100MPH constant
Assumptions we can make:
-Brakes arent applied
-No other wind resistance (cross wind/head wind/tail wind) is affecting the plane
-The wheels/tires failing is not an issue
-Plane is capable of flying at 100mph and just under that (lets say minimum of 60mph to take off)
-Conveyor belt length is just a little bit longer than the plane normally needs to take off(due to the fact that the increased rolling resistance will lower the speed slightly - probably several more feet)
Forces at play:
-Force needed to make a plane go 100mph - lets say this is a small sized plane and needs 5000lbs of thrust to reach 100mph ignoring wind
-Rolling resistance of the conveyor belt speed on wheels
There are no other forces at play here with some simple assumptions. Some people here are led to believe that the rolling resistance of a airplane wheel is going to keep the plane from reaching 100mph. My prediction is the plane would reach somewhere between 95mph and 100mph and still gain flight.
But in the riddle, what is the only alternative, that the plane would remain stationary relative to the ground? Do you really think that? I mean honestly, lets say the large belt is moving at exactly 100mph and the plane is ONLY exerting ONLY the force it needs to reach 100mph on a STATIONARY runway. Think about that. Really think. If a plane has a certain rolling resistance at 100mph and you can define that as something, lets just say 500lbs or rolling resistance. And your arguement also might be that rolling resistance INCREASES with speed (which is true). Wouldnt the conveyor at exactly 100mph exhert MORE rolling resistance than the plane normally would get at 100mph. AND IN THE RIDDLE, it specifically says “by exerting the thrust needed to move it forward at 100 knots”. If you only exert the amount of force to go 100knots EXACTLY and you oppose the force with what some here thinks is 100Knots the opposite direction. And in the incorrect mind feel they are canceling each other out, wouldnt the plane at the wheels now be moving 200mph(knots) and thus have MORE rolling resistance than it takes at 100mph(knots) and thus the plane in THAT mindset should be traveling BACKWARDS?
Trust me, that is what those that say it wouldnt fly are saying.
Now… On to the correct thinking…
You have a plane backed against a stationary wall. You move a conveyor belt under it at 100mph constant. Plane of course stays put, the wall is holding it in place. You are telling me that the amount of force needed to keep the plane from going through the wall is equal to the amount of force needed to get a plane up to 100mph, and thus keep the plane in place relative to the ground (not moving). In this scenario, if the engine was then turned on to produce 5000lbs of thrust for instance, it would start moving away from the wall. It HAS to. There is not enough force to keep it from doing otherwise….
You can also substitute a person.
Put a person on roller skates on a treadmill moving 100mph. A little scary but no big deal. Give them a rocketpack that would normally send them going down the street at 100mph. I assure you, they will speed off the treadmill at roughly under the speed of 100mph. Dont believe me, try it at lower speeds, same result… If you have a treadmill you really can try this, although I am not responsible…
Get some skates on. Have someone push you down the street at 10 miles per hour or even close to that, doesnt have to be exact. Then with the skates on, get on your treadmill and set it to 10 miles an hour. Now have those same people who pushed you earlier, give you the same force of push or even close. Get ready to taste treadmill handle bar/control station…. You will hit the front of the treadmill quite easily. The concept is the same. Rolling resistance on modern wheeled vehicles or people are fairly constant and considered negligible compared to other forces like wind resistance which increases exponentially.
Do some reading on land speed record forums. They are much more concerned with wind resistance and aerodynamics than rolling resistance of the tires which on a virtually flat surface is nill…
THE PLANE WILL TAKE OFF….
I like waht your saying there ^^^ Just one thing to add though.. rolling friction DOES NOT increase with speed. Look at the formulat coefficient of rolling frition * w. Nothing about speed is there. The only way it will slightly go up is with increased speed your creating much more heat causing the rubber to “stick” more to the asphalt.. but tires also expand at speed less surface area..so whatevr. but speed isnt a factor. IT may be in bearings.. but not rolling friction.
As a secondary problem, how fast will the wheels rotate for any given speed the plane is going? I can picture the wheels spinning the same speed as the plane on a regular surface. But on the speed-matching conveyor belt? Will the wheels end up going simply twice as fast, as some have figured?
(This question is for the people who have grasped the concept that the plane will move. Do not answer otherwise.)
/// Never mind that. I got confused between rotational speed and airspeed again. The wheels will move at 200 knots.
Oh for the love of God!!!!!!…this question was posted on an automotive forum I’m in awhile back, and of course it went on for many pages. It’s really not that hard of a concept to imagine, and if those who stubbornly stick to the idea that it wont fly cannot grasp the fact that the wheels are freerolling and the thrust acts independently of the wheels, then no amount of detailed physics equations and frictional coefficients or whatever is going to help them!
As I said it’s not that difficult. With the exception of high school, I’ve never taken a physics course in my life, nor have I ever flown a plane. Everyone can(or should) agree on the following:
The wheels are freerolling
Thrust from the engines acts on the airframe(body)..not the wheels!
Now take the damn airplane, sit it on the conveyor, turn everything off so both are stationary. Attach a cable from the plane to one of those plane-puller trucks(I’m sure that’s the technical term) that is not on the conveyor. Now, pull the plane forward with the truck.
What happens?: The plane begins to move forward.
How Fast? The same speed as the truck.
What are the wheels and conveyor doing? Spinning.
How fast?: It doesn’t frickin matter!!!!!!!
The plane is moving forward at the same speed as the truck that is towing it, just like it would under normal circumstances. Sure, the wheels are spinning faster, but this has no effect on the plane’s forward movement whatsoever. The truck acts in the same way as the engine thrust: exerts force on the airframe independent of the wheels. The damn plane would take off exactly like normal, except the wheels would be spinning faster…they are just along for the ride.
To argue otherwise would be to say that the truck would also magically be held still by the conveyor that it is not on.
One interesting aspect of this riddle is that most people who argue for take-off put a heavy emphasis on the wheels of an airplane being freerolling. Since friction is so low to be largely irrelevant, the belt has no way to stop the plane from accelerating relative to the ground.
While this is certainly true, it isn’t even necessary: even if you had a plane that accelerated by using its wheels, it would still be able to move relative to the ground.
The riddle here says the plane moves forward at 100 knots, as does the belt. Now, forward relative to what ? If we assume relative to the ground, the plane accelerates to 100 knots in one direction, and the belt in the other direction. Combine the two, and the plane moves at 200 knots relative to the belt. Nothing is impossible here, and as said, you don’t even need free-rolling wheels.
(Off course once this bizarre plane is in the air it needs a method of propulsion other than its wheels to keep flying. If just doesn’t need it to accelerate to take-off speed.)
This is getting ridiculous.
Apparently this has gotten so convoluted that people are missing the question posed in this riddle which is simply: “will the plane be able to take off, or will it just sit stationary relative to the ground?”.
Forget wheel friction and other concepts which “drag” this into oblivion. It doesn’t matter.
The IDEA of this is that, at the MOMENT the plane exerts the thrust necessary to counteact the treadmill and be stationary…will it be able to fly? Not hours or mintes later - not eventually if the plane gathers up speed - THEN. AT THAT MOMENT WHEN IT IS CHUGGING AWAY AND MOTIONLESS IN RELATION TO THE GROUND. Read the riddle - don’t add variables to it that don’t exist. Can it fly THEN?
Also, remember, it says it is exerting the thrust needed to move it forward - it does NOT say that it IS moving forward. What is does say is that it is NOT moving at that instant.
As a pilot who has performed more than my share of short-field take-offs and landings when the local landing strip was being rebuilt, my answer to the riddle is a qualified and resounding NO. Why? To make a short-field take-off you rev up the plane to its maximum (which is the same as providing the thrust mentioned in the riddle) while the brakes are applied to the plane so it does not move forward (which is the same scenario as being still on the treadmill). THEN the brakes are released and you STILL need to build up some actual speed prior to lift-off…you can’t immediately fly - which is what the riddle asks.
‘Nuff said.
Hahaha. What you’re saying is the equivalent of asking: “An airplane is sitting on a runway, its engines exerting enough thrust to accelerate to take-off speed. Will it take off ?”, and answering that it won’t. Off course it won’t just jump into the air and start flying at 200 knots or whatever, that’s insane. It will accelerate to take-off speed, and then take off.
If you interpret the question as “assuming engines that can instantly spool up to maximum trust, will an airplane that it sitting still and revs up its engine in the aforementioned zero seconds instantly jump up in the air and fly ?”, then I submit that you’re willfully ignorant.
Wow man, that was one of the poorest responses ive seen. You talk about not adding variables.. where in the question does it state that the plane has to fly from a standstill or from a limited distance?? thats a variable you completely added yourself. What the question is truely asking is if the plane is capable of moving forward and gaining acceleration to the point where it can fly.. or will the belt prevent its forward motion. The question your asking is ridiculous..obviously no plane (save vstol aircrafts) will take off from a standstill the belt isnt even an issue there..
Its not saying will it standstill and take off. The fact is the belt is incapable of stopping its forward motion.. Picture this, Say you are on a treadmill and wearing roller blades… You are also holding onto a rope, at the other end of the rope there is me across the room (not on the treadmill) holding teh rope. Now if i pull on the rope would you not move forward?? That belt could be going 1000 mph and youd still move forward.. Its the same concept, the belt and thrust from the engine are two seperate systems that dont act upon each other. The plane will move forward, gain acceleration to the point where it can lift off and fly with the only difference is the wheels are spinning twice as fast as they would under normal circumstances.
By the way, the riddle as posed here literally says: “.. by exerting the thrust needed to move it forward at 100 knots”.
“the thrust needed to move it forward”
“move it forward”
“MOVE IT FORWARD”
MOOOOOVE
Not “be stationary”. Moving forward is not equal to being stationary.
Scroll up for about 100 people who think that.
There are 3 kinds of people:
1) People who think wheel friction can stop a plane from moving. If you assume a regular plane that can overcome wheel friction to start moving on stationary ground, they’re wrong because wheel friction doesn’t increase with speed. There’s a subclass of people here who desperately want to keep the airplane in place by letting the belt move at something approaching the speed of light. They’re doubly wrong, first because wheel friction will still not increase no matter what, and second, because the riddle literally states that the belt only moves at the speed of the plane. Increasing the belt speed to something insane while the airplane only needs 150kts to take off is clearly violating the premise.
2) People who translate “the plane moves at a speed fast enough for take-off, and the belt at the same speed in the opposite direction” as “the plane tries to move at a speed fast enough for take-off, and the belt counteracts the plane’s efforts”. This is just changing what the question says. For some reason these people insist in saying that “moves in the opposite direction” actually means “counteracts the plane’s movement”, when it does nothing of the kind.
3) People who have the right anwser.
Sigh.
Sooky old fart - here it is again. I added no variables (I believe that YOU are the one doing that with rollerblade comparisons):
IT SAYS:
“If an airplane is on a large conveyor belt and is trying to take off by exerting the thrust needed to move it forward at 100 knots, and the conveyor belt starts moving backwards at 100 knots, will the plane be able to take off, or will it just sit stationary relative to the ground, with the backwards speed of the conveyor belt counteracting the forward thrust of the plane?”
It says TRYING to take off by exerting thrust needed to move it forward - NOT that it IS moving forward!
It asks if it will “just sit stationary” (which pretty much qualifies that it is NOT moving forward at that time)…
Egads, man - what more do you want? The idea is that it is sitting there not moving, motor revved up, and then will it AT THAT MOMENT be able to fly?
Interesting analogy that I read and that may help understand it:
Instead of a plane on a conveyor belt, take a car in a wind tunnel. The wind tunnel has sensors that detect how fast the car is going and blows wind through the tunnel at the same speed in the opposite direction. The driver puts the car in gear and starts accelerating. Will the increased air resistance keep the car from driving through the tunnel ?
The answer is obvious, even though air resistance actually does increase with speed, as opposed to rolling friction which doesn’t.
And in essence, friction and even free-rolling wheels are still irrelevant if you take the logical assumption that the riddle is asked from a neutral point of view, from a stationary point next to the conveyor belt. The planes moves to the right, the belt to the left. The wheels of the plane have a rotational velocity equal to the sum of the speed of both the airplane and the belt relative to a neutral observer.
This would even work with a car: viewed from the outside, the car would travel at a speed X, the belt would turn in the opposite direction, also at the speed X. The car’s speedometer would indicate a speed of 2 times X. The car’s engine wouldn’t even have to work much harder than a car on a normal road since most of the energy in driving it is used to overcome drag from the air; rotational friction is a much smaller force and, as said before, does not increase with speed.
Off course in the case of a car, it wouldn’t lift off but fall of the belt when it reaches the end. Unless it was a special car with wings and a rocket engine that kicks in the instant it loses contact with the ground.
Sttork: the question is not “is the plane taking off, or is it stationary relative to the ground” ?
The question is: “will the plane be able to take off, or will it just sit stationary relative to the ground”. “Will” here indicates the future, as in “what _will_ happen”, not “what _is_ currently the situation”.
If you twist the question, as you did, then off course the plane isn’t instantly jumping into the air. If you don’t twist the question, the plane _will_ accelerate, and _will_ take off. (Notice the “will”.)
No.
I heard the original Boortz show when this first aired and the idea is will it IMMEDIATELY be able to fly from a still position since enough air is passing on the wings to theoretically enable lift. Plain and simple.
Hmm, that’s the first time I’ve heard it like that. If a plane is sitting still on the ground, with a windspeed of zero, than off course it won’t take off. Unless it’s a helicopter or other VTOL aircraft. The conveyor belt below it is even more useless then than in the regular version of this riddle, which is quite a feat since it’s equally useless there.
On the other hand, if on the show they started from the original premise of the riddle (found all over the internets), and assumed or concluded that the airplane would sit still relative to the ground, they were wrong.
Stork, that absolutely makes no sense and clearly it looks like youre in the wrong here as theres already been about 4-5 people that sya that isnt even what the question is asking.. If that was the case, why is the belt even in the question.. why doenst it just say can a plane take off from a standstill?? why all that extra crap? I mean there is no even thinking to the question.. why would it be posed.. I suggest you look at the question again..specifically this part.
“will the plane be able to take off, OR will it just sit stationary relative to the ground”
Notice the OR in there.. meaning not just sit stationary relative to the ground
Sookie You Old Fart:
You folks are freakin driving me crazy! I HEARD the Boorts show - which is where this problem originated in this long listing of comments. The CONCEPT on the show (whick folks here have apparently diverted from) is that the treadmill is there to make the wheels spin so it (the plane) will neither go forward of backwards thanks to the force of the props. Once an equilibrium is reached where the plane in a position where it is basically sitting there and going neither forwards or backwards THE QUESTION WAS RAISED AS TO WHETHER THE WINGS WERE GETTING ENOUGH “LIFT” THAT THE PLANE COULD SIMPLY START FLYING AT THIS POINT. That was the original argument - plain and simple.
The original poster of this question tossed in some confusing and unnecessary red herrings like saying it is a LONG runway, and the original question on the show made it quite clear it was just sitting there.
Why you keep calling me an old fart.. im 24. haha.
Anyways.. i dont THINK it originated on that show.. this question has been around for quite awhile, im assuming they picked up this question and proceeded to try to answer it. And i was actually debating this question prior to dec. 8 2005 if that is when this question aired according to the top of this thread.
Now, back to what you said… You said the reason for the belt being there is to stop the plane.. THAT IS EXACTLY WHAT WERE TALKING ABOUT… The belt CANNOT stop the plane and THAT IS why my rope example does work.. they are two independent systems that dont counteract each other…
If i was swimming underwater (in no current) pushing a wheelbarrow that had a wheel on the lake bottom. On the lake bottom was our conveyor that matched the wheelbarrow speed. Could i not be able to still push the wheelbarrow forward??
Secondly.. Why woudl the wheels be spinning or the converyor belt be moving if the plane hasnt actually moved..it violates the question.
There is one red herring..ill give you that.. its the conveyor belt.
Sooks you Young Fart (heh heh):
I imagine that this has been around prior to the Boortz Show, but the original question pertained to it and that is the basis I have been using.
Of course there is one brief time when the plan won’t be moving forward or backwards on the treadmill - that is the point of reference they use for this question.
And of course the wheels will be moving on the plane at this time as long as the treadmill is moving - why would you think otherwise?
Well what i am saying why is the treadbelt moving if the plane is not.. the belt is matching the speed of the plane.. if the planes speed is 0 why is the belt moving.
Why is this being analyised so deeply, the question does not ask you to consider thrust, momentum, frictionless wheels or what ever else you want to throw in there, just simple velocity. If that velocity is relative to the belt it wont take off as to do so it would have to move faster than the belt to actually be moving relative to the “real world”, if the velocity is relative to the “real world” then it will and the belt will cause minimal effect on the plane. Its all about what the velocity of the plane is relative to, the question has no definite answer as it is not specific enough so you are all mostly right except for the odd idiots who really have no concept of physics.
owen,
True, it is relative to what youre comparing speeds to… whether it be wheel speed or ground speed. Ground speed is the most logical. that is the scenario that i consider and with that the plane will move forward like normal with belt having minimal effect on the plane and the wheels rotating at twice their normal rate. if you choose to compare speeds relative to the wheel it doesnt really work. it basically creates a paradox. the plane engines fire.. the mass of the plane moves, thus the movement causes the wheels to begin to turn.. say 1 mph. the belt then senses this movement and tries to match it by tuning to 1 mph.. As it does this, it actually accelerates the wheels.. the wheels then turn to 2 mph, the belt compensates and speeds up and so on and so forth. i.e. they basically accelerate to infinity instantly.. but the belt is always playing catch up. it doesnt really work.
Sooks:
C’mon now man - get with it. You ask WHY is the treadmill moving in the first place? Well, gosh, let’s me think - could it POSSIBLY be because that is a PART of the riddle?
It says the belt is moving enough to counteract the plane’s thrust so the plane does not move at that one point.
Also:
You say to Owen that the belt is always playing catch-up…I don’t see how you can have a problem with two equal forces cancelling each other out so that there is no movement. 100% push VS 100% pull = ZERO movement. Try walking on a real treadmill - you can’t conceive that you can do so without advancing? Same concept.
Sttork, it’s a misconception to compare this situation with that of a person walking on a treadmill.
A plane’s engines do not interact with the treadmills surface like a person’s feet do. Locomotion is provided by thrust against the air itself, NOT the belt.
Imagine a person on rollerskates standing on a treadmill. Now, turn on the treadmill and simulate the effects of a jet engine by pushing the person forward. What happens?
The person will move forward, and the wheels on his rollerskates will simply turn faster, nullifying any effect the treadmill has.
Stork,
That is incorrect, but explains your viewpoint. The question DOES NOT state that the belt counteracts the plans THRUST. it counteracts the planes SPEED. that is a huge difference. counteracting force would infact stop the plane.. speed does not. matching speeds doesnt equal matching forces.
It isnt part of the riddle. the riddle states that thebelt matches the speed of the plane. if the plane isnt moving i.e. it has 0 speed, then the belt cannot be moving.
i do believe you can have a point where the plane can be held steady.. that is the point where the friction that is transfered is equal to thrust. However, that is a very small amount of friction. See my post up aways.. around 167 i believe to see how the thrust and friction relate. But when those two are equal then it would be held steady, but like i said, same speed doesnt equal same force. they are two seperate systems that the belt can only impart a fraction of its force to the plane
Kevin:
I only mentioned walking on a treadmill when Sooks couldn’t understand why wheels could turn in an earlier post. Of course it is 2 different things.
Sooks:
Whoever wrote the original question here look liberty with the original conversation on the Boortz Show and embellished. The original question and discussion on the show assumed the given that a plane was on a treadmill, not moving (thanks to the counteraction of the treadmill in the opposite direction), motors turning enough to cause plenty of lift around the wings for take-off (which the poster embellished with the 200/thrust mention) - and the question was could it simply rise off the treadmill and take off and start flying AT THAT MOMENT since it was getting enough lift on the wings. That was it. And Boortz was correct in his answer of NO.
i still say that youre not interpreting what the question is asking or what the show was conveying correctly. i didnt see the show but i highly dount that is what they were trying to accomplish. That question makes absolutely no sense. in that question the belt serves no purpose, and youre asking can a plane take off from a standstill.. no obviously. if thats what theyre trying to do, why dont they just say.. fire up the engines with the brakes on and release the brakes will it jump in the air and fly? I mean, cant you see how ridiculous that is.. i cant see how they could even ponder the question on a tv show.
Sookie, you need to get out more. First, it wasn’t a TV show - it was talk radio. They started out by discussing planes and why they fly (lift, etc) and then brought up the question of could a plane fly under those conditions since, they assumed, that all if needed to fly was the lift the air rushing over the wings provided.
Now that isn’t that hard for you to understand, is it?
Sooks,
Im afraid it does work, if the planes speed is relative to the belt, it would simply act like a “closed loop” system where by the belt takes reference of the speed and acts accordingly. Also as far as we know the belt is capable of travelling at infinity. You seem to want to convince me & every one your perspective of the question is right but as you admitted when you said the word “logical” you are making an assumption. Im sure you know how the saying goes… Assumption is the mother of all f*** up’s. Sorry if anyone takes that as offensive but it is a genuine saying. Anyway my point is you cannot insist there is only one answer when you are assuming certain factors.
Consider this -
Using everything it says in the question, the speed can only be relevant to be belt as the question makes no reference to, or at any point say the anything about the world outside the belt, so therefore as far as we are concerned it does not exist therefore the pre mentioned is correct. I do believe that would be the most logical answer since logic is the basis of your methodology!
I know you will argue because it hurts your pride to consider this as a valid arguement. I do beleive though, anyone who is rational about this will agree the points I have made are fair and I will say again it is all about perception of the question.
Owen,
It has nothing to do with my pride.. I strive to seek the correct answer and if someone can actually prove to me that it doesnt work i am more than willing to accept it. I at first did not tihnk the plane would take off actually till i thought about it more and listened to other peoples viewpoints. i have seen many many people who thought it wouldnt change their viewpoint to taking off..but never the other way around and i have been involved in debates on it for close to a year now.
With that being said, i appreciate your viewpoint on it but there is a flaw in the rationale. What your saying works if youre using the speed relative to the belt, but its logically incorrect. if youre measuring the planes speed relative to the belt.. what is the belts speed relative to? the flaw in your thinking is that youre using two different reference frames for comparing speeds which is incorrect. You must keep your reference frames the same, so if you measure the planes speed relative to the belt, then the belts speed must be relative to the belt as well. (i.e. the belts speed would be 0.) and the reason two reference frames dont work is shown here.
Say im driving down the freeway at 75 mph relative to a point on the earth. And i pass a police man who is doing 65mph relative to the same point. Now say the speed limit is 65mph. Now the police man would surely pull me over for doing 10mph over the limit and prob give me a ticket. but what if he was using a different reference frame to measure my speed. He measured my speed in comparison to him. now compared to him im only doing 10mph.. (while i still may get a ticket for going to slow..) but you can clearly see how using multiple reference frames doesnt work. you dont get equal speeds when changing reference frames. so to compare the speed of two objects they must remain in the same reference frame.
Perhaps I am visualizing this wrong but here it goes. When a friend posed this question to me, my first thought was that it would not fly. My rationale was this: as the question was posed to me, the belt would counteract the forward motion of the plane. Since the forward motion of the plane causes the airflow over the wings which in turn causes lift which then causes flight, there could be no flight without forward motion.
I base this assumtion on the fact that the conveyor completely counteracts the forward motion of the plane, should the plane have any forward acceleration it would eventually pick up enough speed to fly.
After reading post 464 another perspective was made clear. In my previous thoughts I pictured a 747 type aircraft, using turbines mounted behind the wings. If it is assumed that a propeller aircraft can move enough air over the wings to generate lift in a stationary postition relative to the ground, then the plane would take off, moving forward through the air immediately after it lost contact with the conveyor which was negating its ground speed.
I would love critiques on my point of view, but please guys, its really a scientific discussion. There isn’t any reason for some of the hostility I am seeing in the posts.
Correction on my last post, I was actually reading post 465 by sttork
A prop driven aircraft will never force air over the wings and allow flight.
The plane will take off, prop or jet driven, because the belt cannot counteract the forward motion of the airplane. The belt is acting on the wheels.
Uh, I didn’t have time to read all 400 blathering posts about whether the speed of the belt is equal to the speed of the plane moving forward. The question you have to ask yourself is ‘Are the wings moving forward or is there enough air movement across the top and bottom of the wings to generate lift and take off?’ It’s called Bernoulli’s Law and it has very little to do with speed. What is has to do with is the speed of the air across the wings, not necessarily the speed of the plane. This is why pilots like to take off and land into the wind, it gives the greater lift with the same airspeed of the plane. Speed is only a minor part of it. Think of wind tunnels, people. They do the same thing as your conveyor belt BUT WITH AIR. The question was could it generate lift and take off. The definitive and only correct answer is NO. (unless there is a headwind of equal speed as the required speed to take off)
Dumb Physics Guy, the airplane is moving forward with respect to the air. The wings are generating lift because the plane is moving. The belt does not keep the plane stationary because it acts on the wheels alone.
Geez - disregard any items below the undercarraige of the plane. The wheels, landing gear, ground, conveyor belt, baggage carts, etc have no impact on the plane. The plane could care less if it were on skis, wheels, or even its own underbody (although friction would be a bit higher).
The thrust from the jet engines makes the wheels inconsequential to the equation.
The jet engines are acting on the AIR around it, not the ground. Newton’s 3rd Law is applied to the AIR, not the ground.
If the conveyor belt is moving the same speed as the airplane (x) then the speed of the wheels will be 2x when the plane takes off and the pilot probably won’t even notice - except when taxiing.
take a remote control airplane, put it’s power on full blast and hold it in air, then drop it, if it takes off without ever dropping, then the answer is yes. if it drops a little before taking off, then the answer is no, and not enough lift is provided to actually take off without and forward velocity.
now i know none of you have tried it yet, but i’ll give you a hint, the remote control plane will fall straight down, and crash. why?
Lift is caused by a creating a high pressure system under the wings vs the low pressure system above the wings. This is caused by speeding up the wind velocity “relative” to your plane above the wing and slowing it down below the wing. When the GROUND VELOCITY=The WHEEL velocity, the plane is stationary, therefore the RELATIVE velocity between the wind and the wings is 0, therefore no lift will occur. Now, if instead of a conveyor belt, you are in a wind tunnel with massive air blowing against the plane, while the plane stays stationary at full thrust, the RELATIVE VELOCITY is >>0, now, the plane can generate lift.
Matt, the plane’s velocity relative to the air is not zero.
If the plane is “stuck” in it’s spot, it cannot lift off. If the plane can move forward relative to the normal ground (not belt), then it can eventually take off, but even then probably not in the same distance as the runway, because it wouldn’t have the velocity needed-as the acceleration would be dampened. So basically, the only way the plane’s taking off is if the wheels slip and become frictionless- and the plane (for a lack of words in my vocabulary) hovers, building up the same final velocity it would on a normal runway. You need a velocity of X (with the safety factor taken into account) to take off, which gets reached before the end of the runway. To get that velocity, your acceleration is Y. If your accelerations is
it cut me off!
if your accelerations is
WTF, the end of my messages arent getting thru, last try
if your acc is less than Y, your vel at the end will be less than X, thus the relative velocity needed will not be reached for a safe takeoff. Therefore the whole question is whether or not the belt has ANY effect on a moving plane.
Yes, the question is whether the belt has any effect on the plane. And the answer to that question is: very little. The wheel bearings make sure there is very, very little force being translated to the plane itself. This means the wheels can do anything they want….they can spin faster, that can spin slower….and it makes virtually no difference to the plane at all.
With the belt matching the speed of the plane, events would progress this way: the plane moves forward relative to the ground and the belt immediately moves backward at the exact same speed. This causes the wheels to spin at twice the plane’s forward velocity…that is, if the plane is moving forward at a speed of 10 MPH, the belt is moving backward at a speed of 10 MPH and the wheels will moves as if the plane were doing 20 MPH on a standard runway.