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.
Lol, well firstly i didnt bring wheels into it. The plane speed is the speed of the body of the plane moving across the earth.. i.e. if a cop was there and had a radar detector, it would read howver fast it was going.
if you are to consider comparing the wheels… or wheel and belt.. its the distance they cover over time..
I don’t know what you’re talking about harry. There is never any paradox in this situation. The only affect that the belt has on the problem is the rotational speed of the wheels. It cannot by design apply a force to counter the linear motion and therefore linear speed of the wheels unless the plane has it’ brakes on. remember the linear speed of the wheels is the same as the linear speed of the plane as they are fixed to it.
Therefore if the question states that the belt matched the linear speed of the “wheels” but in the opposite direction. It doesn’t change anything. The belt can still only cause the wheels to spin it cannot stop the linear motion unless the plane’s brake are on.
Plane moves forwards at 100 knots. wheels move forward linearly at 100 knots. Belt moves backwards at 100knots. Wheels spin faster. There is no paradox. Wheels linear speed is always the same as the plane.
If you’re thinking “FORCES” and then stating that the conveyor belt’s applied force cancels out the forward force of the plane exerting on the wheel you would have to state how that is possible because in the current setup the force exerted by the belt can only cause the wheels to spin.
So if you’re assuming that this is not the case then you are putting the entire situation in a world where the normal principles of physics do no exist and therefore the entire question is moot.(that is the only way to get a paradox and therefore you should take up philosophy…not physics :P)
If you want to get REAL picky, remember that the question states: “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, ”
Please note that the way it is phrased the airplane is ALREADY exerting the thrust BEFORE the conveyor belt even starts - hense it can eb argued that it is already moving forward when the belt begins…
Exerting thrust and moving are not the same thing. It needs to exert thrust first to overcome rolling resistance then when the thrust is greater than the rolling resistance it will start to accelerate. SO what you said is technically incorrect. the plane will be at rest then start exerting thrust then start moving…in that sequence. But it still doesn’t change the fact that the conveyor belt has no impact on the plane’s forward motion.
Anyways I think this question has been explored enough…I would recommend closing this post as it’s already filling up my inbox with responses and I don’t think anyone else has anything intelligent to add that has not already been posted somewhere on the last 7 pages of comments.
This question makes no sense any more. They keep on changing it from one thing to another. But I read somewhere that someone asked why the Navy doesn’t use this on their carries instead of thier slingshots, the answers is because a conveyor belt does not affect the motion of the plane great enough to effect where the slingshot is actually connected to the body.
But the answer to this question is the plane will take off but will need to use more thrust to overcome the small friction force from the rotating wheels. And as the speed of the wheels increases the force of friction will increase.
You said: ” And as the speed of the wheels increases the force of friction will increase.” Well, kinda. WHICH friction - the wheels touching the ground (which should actually DECREASE as the plane builds up speed) or the frichion on the bearings in the wheel (which increase UNTIL it gets hot)…
Yeah, ive gone back on forth if it actually increases with the speed of which the wheels spin. Rolling friction, speed isnt actually a factor… however, the coefficient of friction could change, depending on heat of tires, at different speeds etc… either way, its nothing significant
If the treadmill matches the rotational speed of the wheels, the plane will not take off. All the physics degrees in the world cannot overcome common sense. All the comments about “it’s not a car”, forces exerting every which way, etc., will not change the fact that the plane will not take off. People get off on all kinds of tangents, talking about planes on ice, pushing a car with their finger that is sitting stationary on a spinning treadmill, etc.
The wheels are part of the plane. When a plane takes off on a normal tarmac, it moves forward by rolling on it’s wheels. If that forward movement is instantanteously counter-acted by a tarmac (treadmill) rearward movement of the wheel circumference, the plane will not have moved at all relative to the air around it. The plane attempts to move forward one inch by rolling on its wheel, and at the same time the conveyor moves backwards one inch, resulting in a net gain of 0 inches of movement.
Capice?
Robert; The problem with common sense is that it just isn’t that common.
As I said before, the argument that you’re espousing only has merit if you can explain how the conveyor is applying more force on the plane than the propeller is. The wheels are free-spinning, and apply very little resistance to movement in either direction. That’s what they are there for — to allow the force of the prop to move the plane.
The conveyor would need to spin close to the failure speed of the wheels, axles and bearings in order for the wheels, axles and bearings to overheat and get resistive enough to stop the plane rolling forward.
Use common sense: The wheels only turn as the plane begins to move. If the plane is moving forward at 1 mph, then the wheels turn at 1 mph, which makes the conveyor turn at 1 mph. The wheels are free rolling and clearly don’t provide enough resistance at 1 mph to have any effect on the planes motion, so the plane CONTINUES TO ROLL FORWARD and actually increases its speed. The key is that the wheels don’t make the plane move (like in a car), the plane makes the wheels move.
The wheels are now moving forward at 1 mph, and rotating at 2 mph. If you’re among the minority who believe the conveyor goes by rotational speed, then the conveyor speeds to 2 mph, the wheels are now going 3 mph, the conveyor speeds up to 4 mph, the wheels to 5, etc. until the wheels fail and the plane crashes. Of course nothing in the problem statement should leave you to believe that is the case.
In the more “common sense” case, the plane now reaches 2 mph. The conveyor speeds up to 2 mph, the wheels are now rotating at 4 mph. Planes goes to 3, conveyor 3, wheels 6.
planeconveyorwheels
5510
6612
101020
5050100
100100200
and the plane lifts off!
The ONLY way the plane doesn’t take off is if the conveyor spins so fast that the wheels fail, or the bearings get so hot that the axle can barely turn. A force MUST act in an opposite direction to the propeller’s thrust in order to prevent the plane from moving.
Capice?
My table got mangled in the last post. Maybe this will look better:
plane conveyor wheels
5 5 10
6 6 12
10 10 20
50 50 100
100 100 200
No preview mode sucks!
“When a plane takes off on a normal tarmac, it moves forward by rolling on it’s wheels.”
@Robert the above is false. The plane moves forward because the propellers move it forward not the wheels.
That’s why they are called propellers (because they propel the plane).
The wheels do not move the plane forward in real life they actually very slightly impedede the forward motion.
Waht part of the treadmill cannot match the rotational wheel speed didnt you get? The speed of the wheel rotating is derived in part from the speed of the belt… The speed of the wheel is the planes speed forward + the belts speed backward.. So if the plane is moving forward at 10mph and the belt backwards at 5mph, the wheels are turning at 15mph.
Capice??
Its impossible/paradox because if it tries to match wheel speed it goes like this… The plane starts to move, which with friction from ground, the wheel starts to turn, the belt senses the wheel turn and in turn starts to move itself. This movement causes teh wheel to spin faster, which the belt then compenstates for by going faster, which makes the wheel spin more… etc etc.. and so on so basically instantly both the wheel and belt would be going infinity mph…
Yet once again, this wasnt the question asked, it matches plane speed not rotational wheel speed
Ok if the plane is on a conveyor belt, and that belt is spinning to match the speed of the airplane, then you cannot take off. The tires absolutly play a part in this one. If the conveyor is matching the speed of the tires rotating, the plane cannot be moving relative to the actual ground; therefore the air moveing over the wings = 0 mph on a windless day. You must have the airflow over the wings to create an upwards lift. There are no outside forces acting upon the plane itself, only the tires which are spinning on their bearings. An object at rest stays at rest unless acted upon by an outside force. So this plane is staying on the ground.
That is also not true.
If you are talking about a prop plane the propeller is acting on the air and moves the plane forward.
If you are talking about a jet plane the jet engine is acting on the air.
Real life planes do not move forward because of wheels.
You know in the movie Armageddon where they were in a huge hanger and the air was pumped out? What would happen to a plane put in there, would it be able to move forward? Or would it stay in one spot?
Answer me this, why did old prop planes and very big prop planes use wheel blocks untill they wanted to move forward?
Ok I’m really getting sick of people posting the same stupid comments over and over again. Obviously they have not read the thread or any of the preceeding postings.
for any NEW POSTERS it is very simple the PLANE WILL TAKE OFFbecause the PLANE WILL MOVE FORWARD!!!
If you want to know how then read all the previous posts. If you don’t comprehend what those posts have said then don’t bother posting because you will never understand.
We cannot dumb this down for you anymore than we have.
If you still don’t think the plane will take off then you don’t understand physics enough to grasp the problem.
Take solace in knowing that the people who actually design your planes and cars and bridges and buildings, those of us called “engineers”, have that understanding. That way those planes and cars and bridges will contnue to function and you can go on your live none the wiser.
Wow, Mattman, I didn’t realize how important AND smart you engineers are. I guess this is because I’m not an engineer.
I’m sure the families of those who were injured or died when that bridge collapsed recently in Minnesota are busy this holiday season taking solace in the fact that it was designed by engineers and therefore it was working exactly as it was supposed to.
Likewise, the wives, husbands and children of the passengers of the TWA jet that exploded off the coast of Long Island about 10 years ago must have been solaced when they found out that it was designed by engineers.
And let’s not forget the families of all those killed and injured by Ford Explorer rollovers, okay? They’re probably thrilled right now to know that their SUV was designed by people who have that “understanding” as you so aptly put it.
I could go on and list many more high profile examples of people who should take solace in the fact that engineers were involved in their lives such as those involved in the Kansas City hotel skyway collapse, those living around the breached levies in New Orleans, those who owned the Ford Pinto, etc.
Instead, I’ll just say that calling oneself an engineer doesn’t mean your right and it doesn’t mean you’re wrong. It doesn’t mean you’re smart either, but that last comment you posted will certainly be evidence against you if anyone should want to start a thread debating that topic.
And, as Forrest Gump (who was neither smart nor an engineer) would say, that’s all I have to say about that.
Well ill say for one, i am NOT an engineer… i started out my schooling as one, but couldnt see myself doing it for a career.. But i think, i have shown in MANY different ways how and why the plane will take off so you dont need to be an engineer to figure it out, as well as i know several engineers that get it wrong.. The question is somewhat a trick on the mind and thats why it is so perplexing, i for one didnt see it correctly either right away…
However, i do agree with the concept that mattman is putting forth in that is extremely annoying to have people come in, not think about the problem, not read ANY of the responses and put an idea out there thats ridiculous in the fact its already been covered.
I would like to add one thing to what harry said.. I for one live in minnesota only about 15 minutes away from the bridge collapse, and USED the bridge somewhat frequently and dated a girl that lived right next to it so it hits very close to home for me. But that instance in my mind was not a fault of poor engineering. The bridge was old, and probably needed repairs. As well the bridge recently had passed its inspection. The media liked to use the term “Deficient” and how could we let a bridge still being used that is deficient. When in fact about 70% of the bridges in this country fall under that category (Note: i dont remember the exact %’ge, but it is somewhere in that range) Deficient just meant that it needed some work and it was being worked on at the time of the collapse. The fact that where the bridge failed was that it was determined not worthy of full upgrades due to funding and its current status, so blame our inspection process as well as how the gov’t is allocating its funding for roadways.. But its not the engineers fault. But imo, it was a freak thing that probably had many factors come into play.
How about the thousands of bridges you drive over everyday to get to work…or the subways you take the planes you fly and lets not forget that each of those failure you mention was not related to design but were errors in maintenance and manufacture based on decisions of small minded business people trying to save a buck at the expense of human lives.
we in the engineering profession no full well the extent and responsibility we have to the public we know full well the consequences of a mistake or error. That’s reason I wear an iron ring it serves as a reminder to all in my profession and why we take considerable care in what we do.
Without us there would be no society, no progress no safety regulations either. we are a profession that learns from our mistakes when they happen But we also take considerable care in what we do to ensure that the rest of society continues unaware of the real skill it takes to accomplish what we do.
If my work is done correctly you will never know it and you will only notice it when we make a mistake. Maybe you should think about what holds the golden gate bridge up next time you drive across it or why you don’t drown when going through the chunnel. Those feats exist because of our skill and ability and it is society that reaps the rewards for our hard work.
I probably know more engineering mistakes and disater than you could possibly comprehend and I know how to avoid repeating them. Can your profession say the same? I doubt it.
Maybe you should also check some of your facts before making another stupid post and proving my point about your ouwn ignorance
“How about the thousands of bridges you drive over everyday to get to work…or the subways you take the planes you fly and lets not forget that each of those failure you mention was not related to design but were errors in maintenance and manufacture based on decisions of small minded business people trying to save a buck at the expense of human lives.”
How about the use of commas and apostrophes, the improper compounding of words or the eliminations of run on sentences?
“we in the engineering profession no full well the extent and responsibility we have to the public we know full well the consequences of a mistake or error.”
Maybe, but apparently you’ve never learned about capitalization or the concepts of proper spelling (in case you can’t figure it out, it’s “know”).
“Without us there would be no society, no progress no safety regulations either.”
And no F’s in 8th grade english class either.
“Those feats exist because of our skill and ability and it is society that reaps the rewards for our hard work.”
You mean engineers work for free? And by the way, society reaps the reqards OF your hard work. We also reap the rewards of the hard work of teachers, or at least some of us do.
“I probably know more engineering mistakes and disater than you could possibly comprehend and I know how to avoid repeating them.”
So why don’t you share your semi-omniscience (sorry for using 5 syllables and a hyphen in the same word) with the rest of the world and eliminate engineering mistakes completely. If you truly know more engineering mistakes than I can “possibly comprehend” you must be pretty smart. However, you seem to know more grammatical errors than I can possibly comprehend as well. Is that Yin and Yang at work?
“Can your profession say the same? I doubt it.”
How can you possibly claim to be smart and make that statement? You don’t even know what my profession is? What could possibly lead you to a conclusion like that other than your own hubris (go ahead and look that one up, I’ll wait).
“Maybe you should also check some of your facts before making another stupid post and proving my point about your ouwn ignorance”
As for the facts, engineers don’t only design. They also maintain and manufacture. Engineers are called upon to come up with appropriate maintenance regimens and to insure manufacturing processes are free of defect. If there are problems in either of these areas, thank an engineer.
The Ford Explorer was negligently designed such that it was more likely to roll over at low to medium speeds when one of the rear tires went flat. The problems with the vehicle may have been contributed to by poorly manufactured tires, but no vehicle should roll when driven straight at 40 mph on a level surface when one of the rear tires goes flat. The problem was remedied substantially when the center of gravity was lowered in subsequent model years. Design, certainly not maintenance or manufacture.
The Pinto was also clearly a design issue, and not a maintenence or manufacture issue.
As for the Kansas City Hilton Skyway collapse, the consensus of opinion for over 20 years has been that the engineer who designed the walkway cut corners in order to make a profit on his low bid and allowed inferior construction techniques to be employed. The hotel was practically new and maintenance was not an issue.
Ultimately, you missed my point entirely, which is that you were making an argument from authority. This is a category of logical fallacy in which someone attempts to carry a debate by saying I’m a (fill in the blank) and therefore I’m right and you’re wrong.
You’re own response confirms my point. Engineers, just like everyone else, make mistakes all the time. You, who “probably know more engineering mistakes and disater than you could possibly comprehend” should be as aware or more aware of that anyone else.
I may be wrong about some or all of the things I’ve thought about this interesting puzzle, but please don’t throw that “I’m an engineer so I must be right” crap at me. Would you tolerate that from your doctor or your auto mechanic or your lawn guy?
Thanks for pointing out all those spelling mistakes, Harry. I don’t know what I would have done without you as with many others in my profession, engineers don’t spend as much time focusing on our grammar and spelling as we do with the maths and sciences. It’s a funny thing that correct punctuation doesn’t seem to keep planes in the air.
I did not mean to imply that people, such as yourself, who do not have a strong understanding of physics are stupid. I simply stated that based upon the repeated posts people are either not reading the previous comments or do not understand what has been said. Doe that mean they are stupid? no. It just means they are not as knowledgeable in physics as others may be. I’m sorry you find that insulting.
However, it is quite immature of you to insult the engineering profession as a whole. When you do that I must defend the profession I love. We engineers often work hard without thanks or consideration from others. Our successes often go unnoticed while, as you stated, our failures and mistake have huge consequences. The engineering profession is one of the few self regulating professions(Medicine and law would be the others). Here in Canada our engineering council and regulating bodies have taken strong measures to reduce the potential for error so that those such as you can go about your daily affairs without worrying if the landing gear on your plane will collapse or if the brakes on your cars will fail.
I would never insult another’s profession as there are many we depend on. By your statements you claim to be in a position to criticize the engineering profession but by your completely uninformed statements you have no clue about our profession or what we do. I would not make statements about doctors or lawyers or teachers. I can only comment on my profession.
If you feel the need to insult myself go for it. I’m an asshole most of the time anyways, but to insult my profession is to insult those who have studied hard and work harder to make you’re own life easier. You obviously missed the whole point of my original statement and have proven you’re own level of extreme ignorance by attacking my grammar (the last refuge of the internet ignoramus [hope that's not too big of a word for you])
BTW, it is pretty hypocritical (you may need to look that one up) to insult the engineering profession by posting such an insult using a computer designed by electrical and computer engineers.
We have a similar situation with mother earth. If you take off to the east, you are going into the earths spin. Going west by your lines of reasoning you going against the spin, but somehow I have taken off both east and west, spinning wheels have nothing to do with it.
What is WRONG with you people?! The ONLY question as far as a plane taking off is AIRSPEED.
Airplane thrust providing forward motion which is counteracted by a conveyor belt equals ZERO AIRSPEED.
Wheels have NOTHING to do with it. It woudl be the same if the plane were on pontoons on afast-flowing river. It’s all about how fast AIR is flowing over the wings.
zero airspeed = zero lift … and no take off.
Jeez. Gov’t schools should all be imploded.
A barn has a roof that goes east-west.
A rooster lays an egg on top, which way does the egg roll off the roof?
Answer Roosters don’t lay eggs.
And conveyor belts cannot counteract an airplane’s forward motion.
But unlike the “rooster lays an egg” question this one doesn’t even say the conveyor counteracts the planes forward motion. It’s only implied but in any case it’s impossible.
Unless the no it won’t take off people want to say the rooster had a sex change operation.
My bad. It’s true; the plae will take off. However, its wheels will be spining twice as fast as in a normal takeoff. The conveyor belt cannot impede the plane’s forward motion through the air.
Now, if a giant fan blew from the behind the plane …
My bad. It’s true; the plane will take off. However, its wheels will be spining twice as fast as in a normal takeoff. A conveyor belt cannot impede a plane’s forward motion through the air.
Now, if a giant fan blew from the behind the plane …
There are two variations of the question. One states that the conveyor matches the speed of the wheels, and the other the speed of the plane.
If the rotational speed of the wheel matches the speed of the conveyor belt, the plane will not take off. Thrust is a moot point. The conveyor instantaneously matches the rotational speed of the wheel. If the plane cannot move forward relative to the air around it, then it cannot attain lift, and therefore cannot take off.
If the speed of the plane matches the speed of the conveyor, then the plane will take off. The rotational speed of the wheels will be greater than the speed of the conveyor, thereby allowing the plane to move in the opposite direction relative to the air around it.
The problems are pretty basic and don’t require any F=MA type physics; they require only basic logic and the understanding of relativity.
So Robert, in my question you say the egg rolls “down” because the question “says” the rooster lays an egg?
My take the on airplane question is;
(Question) a plane is on a conveyor blah blah blah. Can the plane take off?
(Answer) conveyors can’t stop the plane’s forward motion anymore than roosters can lay eggs.
tex ash…Its impossible to match the speed of the wheels..its an infinite loop in that case.. the wheel speed is = the planes speed + the belts speed… So the belt will always be chasing the wheel speed…and almost instantly both the wheels and the belt will be going infinity mph…
I hear that all of the super intellegent people that believe that you need only move the area of ground that an aircraft is sitting on rearward, at a speed equal to or exceeding its takeoff speed to make it takeoff, have succeeded in having the dumbass physicists employed by the Navy, design an aircraft carrier 150ft long thus saving over 850ft on the current aircraft carrier length of over 1000ft. Turns out after all of this time that YOU DON’T NEED THE LIFT developed by the wings forward motion through the air to have it takeoff at all. All you need do, is move the ground underneath it REAL fast and the lift is magically created under the wing to create liftoff by fairies, congered up by the speed of a conveyor belt and some commercially available fairy dust sprinkled on the conveyor belt. Talk about saving some money.
Fastchip
Well thats actually NOT what were saying…were saying it will move forward like normal and take off in the about the same distance as it normally would (maybe just slightly longer) and gain speed and take off…. So…youre…dumb
Skooks - you said that: “So the belt will always be chasing the wheel speed…and almost instantly both the wheels and the belt will be going infinity mph…” and “Its impossible to match the speed of the wheels”.
Uhhh, since it is a given in the questions that the belt is moving at a constant 100 knots - how can be the belt ever increase in speed - let alone “chase” the wheel speed?
Sigh…
Stork,
My response was in reply to the hypothetical question, “What if the belt was matching the wheel speed?”… as robert talked about in his post a few above mine and not to what was actually being asked in the first post.
…Sigh
Okay Mark Jaquith, the author of this article, is a COMPLETE IDIOT.
The airplane will NOT take off.
Why? Because the airplane will remain stationary. An airplane that is not moving forward will not take off because it is not creating LIFT. If the airplane is creating forward thrust of 200mph and the conveyor belt is moving at the same speed, THE AIRPLANE WILL REMAIN IN THE SAME PLACE. HOW HARD IS THAT TO UNDERSTAND? The airplane remains at the same place because the forces cancel each other out. A plane that is not moving forward CANNOT create lift and therefore remains stationary. Mark Jaquith is a fucking idiot.
JChapmanSC, you’re mistaken. I’m not judging here; I was wrong, too.
The plane WILL take off because the conveyor belt cannot hold it back, no matter how fast it goes. That is because the plane’s wheels are not pushing against the ground for thrust, as in a car. The plane’s propeller is pulling the plane through the wind.
As long as the wheels can turn as freely as they are designed to, the conveyor belt offers no influence on the propeller’s ability to pull the plane thru the air and create the lift necessary for takeoff.
JChapmanSC, you’re going to be quite embarrassed when you realize how wrong you are. I’d suggest you tone it down for your sake.
I think this causes so much angst because the question is invalid.
If the question is taken literally, as it is stated at the top of the thread, then the planes wheels would be ripped off when that conveyor instantly goes from zero to 100 knots. If the question is considered to be “If a magical conveyor belt under a plane prevents the plane from moving forward, by moving backward at a speed equal to the speed the plane would be moving based on the thrust of the plane’s engine, will it fly?” The answer is not YES or NO. It is that it depends on the type of plane. If the plane’s engine can create enough thrust to overcome gravity the conveyor belt doesn’t matter, but if it can’t the plane can’t move forward enough to create airspeed over the wings, therefore no lift and no fly. The plane moves forward because of thrust. The wheels turn because the thrust is acting on the plane AND gravity is acting on the plane, pushing it against the ground. When the ground moves under the wheels, ie the belt, no forward movement is created relative to the air surrounding the plane. (This ignores the ground effect of the air current created by the belt but once again whether that would be enough to create lift depends on the construction of the airplane.) Therefore an annoying question!
Place your bets now because Mythbusters airs its examination of this question Wed. Jan. 30, 2008 on the Discovery channel.
Episode 97: Airplane on a Conveyor Belt Premieres 01/30/08
Take the line breaks out of this URL and paste it into your browser:
http://mythbusters-wiki.discovery.com/page/Airplane+on+a+Conveyor+Belt+-+Get+Exclusive+Photos%21
in case any of you still doubt it
http://uk.youtube.com/watch?v=4owlyCOzDiE
Any aircraft is able to achieve lift and become airborne through lift. Lift on a fixed wing aircraft is due to the movement of air over the wing and control surfaces. If the airplane is not moving forward, relative to the air around it, then it will not achieve lift.
thank you for defining how an airplane takes off Rob, now do you care to enlighten us about the question that is asked.
If we’re talking about about a prop driven airplane, then it will take off, due to the the prop wash over the wings generating lift. The speed of the wheels and/or the conveyor belt has zero affect. If we’re talking about a jet powered aircraft, then there will be no prop wash, and no movement of air over the wings/control surfaces, and it will not achieve lift. Sound about right?
No… If that was the case, why use a runway at all, a plane could take off from a standstill… The questions isnt really stating that the plane cant move, or can it take off when it cant move… the question is esesentially asking if the plane can move down the runway or not… if it can, then it will take off..if it cant, then it wont.. Were saying the plane will still move down the runway and will accelerate and get to a speed where it has enough wind going over/under the wings and create lift and fly away..
Read some of the other posts.
I see that there are two ways of looking at the question. I think the important point though is drag. If the airplane is putting out enough thrust to move it forward at 100 knots, what stops it from going faster? The drag induced by friction on the airframe is most of it (although there is a small amount of friction lost in the tires/wheels).
So if the airplane begins to move forward, and the conveyor begins to move backwards, the wheels will just spin faster as the airplance moves forward, until the drag from the friction on the airframe equals the thrust of the propellor.
Assuming that the thrust is *just* enough to reach 100 knots (under normal, non-conveyor conditions), and assuming that the take off speed is exactly 100 knots, the airplane will probably not take off, since the drag induced by the friction in the tires/wheels, (which now must spin much faster) will be greater than normal, so the airplane will not quite reach take off speed for that thrust setting.
Mike, youre actually right on with your thinking.. but the friction is minute compared to the amount of thrust…so it still moves forward like normal..
I concur, Skooks. The drag caused by the tires and wheels is minute compared to the drag caused by the airframe.
I added that last statement only for those who wish to pick nits.
and thats where i come in :p
Busted, even that pilot on Mythbusters didn’t think he could fly, but I question whether or not he had nothing more than an ultra light or sport pilots licnese, no real knowlege of flight.
Goo day
Dude I been flying airplanes for thrityfive years, and have jumped out of them for 10 years, when are we all going to learn here.
Good Day
Sigh. So much confusion.
As a pilot, a plane with normally working easily spinning wheels WILL be able to accerate THROUGH THE SURROUNDING AIR MASS pretty normally once full take-off engine power is applied and will proceed to take off with no problem. (need a conveyor belt as long as a runway because the plane WILL move forward as shown on the show). Only effect of the belt being there is that the wheels are spinning much faster than normal.
In the theoretical statement of the problem the belt would have had to accerate backwards MUCH MUCH faster than the planes normal ground speeds to try to get the rolling wheel friction to keep matching forward propeller thrust. Would take a belt speed of 100’s mph — and would make even a tiny plane need high speed jet tires. I suspect that even with the belt going at Indy car speeds it’s a lost cause because the engine/propeller would still provide MUCH more forward thrust than any reverse drag from wheel friction. (Even a car going 150 mph straight is expending 95% of it’s energy pushing air out of the way rather than wheel/road friction. Even more so for a plane.)
On the other hand…
A perhaps simple way I can think of to demonstrate the ‘failure to fly’ situation is to put OVERSIZE FLAT tires on the plane so that rolling resistance is VERY LARGE. At that point the tires rolling on the belt could possibly be made to matching the forward thrust provided by propeller. So THEN maybe you can force a situation where plane won’t accelerate physically forward on this moving belt.
(And the belt acts differently than low/flat tires on a runway. On a normal runway the plane’s slowly gathering forward speed would slowly lift weight off the flat tires and allow an actual takeoff to occur (but with a REALLY BAD landing to follow). With high enough rolling resistance an increasing belt speed could probably keep up with propeller thrust and prevent any lift and lessening of weight on the wheels — and plane would stay rolling (poorly) on the belt - even up to full engine/propeller thrust.
That’s my two cents on this whole thing.
I can’t imagine a pilot willing to risk a real plane in such a setup with such broken/flat wheels. So I suspect this will stay unproven for a long time.
Frack!
Forget wheel friction, lift, thrust, etc. It’s all moot. The question is basic logic and not one requiring deep knowledge in the math of supersymmetry.
The specific question was whether the plane on a conveyor could take off, with the conveyor instanteously matching the forward speed of the plane. There is not such thing as instanteously matching, so bringing bearing friction into the debate makes no sense as this is a theoretical question that has no practicality.
The plane will no move forward at all if it doesn’t roll on its wheels. That’s why the conveyor matching the rotational speed of the wheel will cause the plane to stand still because the conveyor always moves back as much as the plane moves forward on its wheels.
But the question states that the conveyor matches the speed of the plane. In this case, the wheels will spin with a rotational speed exceeding the backward speed of the conveyor (and the forward speed of the plane). Because this occurs, the plane will roll forward on its wheels.
It’s clear that everyone’s hangup with this is they think the plane is stationary when it is not. T
Maybe if you think of it this way. Airplane wheels freely spin. They aren’t like a cars, and they aren’t used as propulsion. Do NOT think of this like the plane is running on a treadmill. When the conveyer belt moves underneath a stationary plane with it’s engine off, (ignoring friction right now) all that will happen is it’s wheels will spin. The plane will not move back with the conveyer belt. Now, of course, in the real world, we have some friction, so the Plane wouldn’t stay absolutely still, but it wouldn’t only be moving back at a small fraction of the speed of the Conveyer belt. The Wheels would just keep spinning.
Now, turn the jet engines or propeller on. Theoretically (again, no friction), you would need just as much thrust as you would regularly need to get yourself to takeoff speed, even if the conveyer belt was going 5000 MPH. If the takeoff speed was 200 MPH, the Plane would travel 200 MPH, Wheels spin at 5200 MPH. The plane is not stationary. Now, of course, again, in the real world, there’s friction, so we’d need a bit more thrust to reach the desired takeoff speed, but it wouldn’t be a problem. Plane would still be going forward, only difference is the speed the wheels spin at.
Johnny, that sounds logical to me.
The source of the confusion is the way the problem was worded (and it was worded slightly different than above on the Mythbusters episode). I think that most everyone would agree that if the plane is traveling at TWICE takeoff speed WITH REPSECT TO the conveyor belt, it will take off, similarly, if the plane is traveling AT EXACTLY takeoff speed WITH RESPECT TO the conveyor belt, the plane will remain stationary with respect to an observation camera and NOT takeoff. Thus the confusion is due to NOT people’s lack of brains, but rather different pictures in their brains about which scenario the problem is describing.
Ahh, but heres the thing that could be lack of brains… (not that its not easily confused :P).. Sure, often people want to compare the speed of the plane to the speed of the belt. But you CANNOT do that.. because then you are talking about two frames of reference, and anyone with any physics knowledge knows you cannot compare two speeds from two different frames of reference and consider them equal.
The plane will never take off if there is no lift force, simple. If the airplane is not moving forward fast enough to gain the lift force greater than airplane’s weight generate by wings, the plane will not take off.
youre dumb..go read the rest of..or at least the last few posts.
The plane will also not take off if there is no atmosphere.
Both of which are irrelevent to the question at hand as the conveyor will not keep the plane stationary relative to the ground outside the conveyor nor relative to the air.
The plane moves forward as normal despite the conveyor.
For what it’s worth (which isn’t much), contary to what the opening comment states, Neal Boortz always said the airplane WOULD fly.
Ok, I see one assumption that seems to be tripping everyone up. Both sides are right depending on this assumption. Is the conveyor belt moving at a speed equal to the planes air speed or the planes speed relative to the conveyor belt itself? If its moving relative to the planes are speed as many assumed then the plane will indeed take off because when the plane is moving forward at 4mph the belt is moving backwards at 4mph and the wheels simply spin at 8mph and the plane continues moving forward. However, if the belt matches the planes speed relative to itself then the planes forward movement is limited entirely by the delay in which the belt matches its speed. If we assume no limit to the top speed of the belt and an almost instant increase in belt speed then the plane will never take off because an infinite belt speed equals an infinite amount of friction on the axles and eventually the frictions negative force will equal the engines positive force and it will remain motionless. Even with a non instant increase in belt speed the plane will only move forward slightly before the belt reaches an insanely high speed and the plane becomes motionless. (There could be problem with the plane tipping forward depending on the weight of the plane and how high its engines are off the ground but that still results in no liftoff)
Think of it this way: The planes wings are meaningless until it reaches a certain forward speed relative to the air. So it might as well be a car until it reaches that speed. If a car were on a conveyor belt that matched the cars speed relative to itself then a fast enough reacting belt would match the cars speed after is has moved a distance limited only by the belts reaction time. Theoretically the belt could match the cars max speed in the amount of time it takes the car to shift gears.
I think the question was intended to imply that the belt matches the planes speed relative to itself and therefore prevents forward movement. The catch that was supposed to trip people up was that the plane does not take off relative to the speed of the ground below it but rather relative to the speed of the air around it.
If you think the thrust of the plane will move it forward regardless of the amount of friction on the planes axles(which increases with speed without an upper limit other than the axle melting) then imagine an amount of friction equal to a brake. Enough friction at the wheel will prevent forward movement and there is no limit to that friction if there is no limit to the belt speed.
Some people compare it to a rocket but that is completely flawed because a rocket doesn’t take off due to lift around the wings but rather creates enough thrust to push off of the air behind it. A rocket doesn’t need wings or momentum to leave the ground. If you had infinitely powerful engines then yes, the plane could take off because they could pull the plane into the air even without lift but I don’t think that was the intention of original question.
If the belt matches speed fast enough the plane will not reach take off speed relative to the air before the tires blow out. You can not achieve lift off unless you have a belt that adjusts speed too slowly, frictionless tires, or engines that achieve enough thrust to push the plane forward without lift(like a rocket). Its all about the belts reaction time.
Draticus what you said does not make any sense. The conveyor belt cannot match the speed of the airplane relative to the conveyor belt. Well it can but only when both he conveyor belt and the airplane are at rest.
Consider the case where the aircraft has just begun to move through the air at, say, 1 m/s. The conveyor belt is then moving backwards at 1 m/s and therefore the speed of the aircraft relative to the conveyor is 2 m/s (the airplane covers a distance of 2 m of conveyor belt per second). If a 2 m/s rolling speed created enough friction to overcome the engines, then no aircraft could ever accelerate beyond this speed on a regular runway.
While rolling resistance does increase with velocity, the speed at which the rolling resistance combine with the aircraft’s aerodynamic drag would counter the thrust of the aircraft would be significantly higher than the takeoff velocity of the plane making it a moot point.
Bottom line is the drag forces will not equal thr thrust forces before the aircraft reaches takeoff speed.
As well, you cannot compare the planes speed to the conveyor belt, and then consider the speed of the conveyor belt to the ground equal. Those are two different things.. And must be considered from the same reference point.
Yeah, but think of it this way: everything is being compared to the ground speed; in this case the ground (in regards to the plane) would BE the conveyor belt…
If any of you would bother to watch TV you would see that episode 97 of MYTHBUSTERS (first aired 1-30-08) attacks this myth in a rather unorthodox manner…
No, you CANNOT compare the speed of the plane to the belt.. AND then compare the speed of the belt to the ground, Thats two different reference points, thus the speeds are not equal. IF you compare the speed of the plane to the belt, then the belt must also be compared to the belt. which obviously would be 0.
heres an example. Say im driving down the road at 100mph. I pass a police officer who is doing 60mph, and we are in a 60mph speed limit zone. Should the officer not pull me over because from his frame of reference im only doing 40mph? NO, you have to compare both of our speeds to the ground to get the actual representation,
Yes, i watched teh episode.
What makes you think it was unortodox? it was done perfectly fine aside from using a tarp type thing as a conveyor belt..
Your cars at 60mph makes no sense as you BOTH are on the same (ground) and have the same point of reference: the ground.
Youre totally missing the concept of my example. Sure you can use different reference points to measure a speed. My point is, if youre COMPARING speeds, they have to be within the same reference frame otherwise they are NOT equal.
True - everything is compared to ground speed but notalways directly…
I dont get what youre saying?
The plane has multiple speeds… The plane has the ground speed, airspeed, the speed over the belt, the speed in the orbit, speed compared to the sun, speed compared to the bird flying overhead… Same with the belt. In order to actually compare the speed, they HAVE TO, MUST, ABSOLUTELY ONLY be within the same frame.
You can pick whatever frame you want… but both speeds must be used from that frame.
Skooks - just wondering - are you a pilot?
Nope, why?
one more example.. Take a penny and take a 5lb dumbbell. Now obviously you would say that these are not of equal weight.. But if i take that penny and measure its weight in jupiter, it weighs the same amount as the 5lb dumbbell does on earth (now, they prob dont actually weigh the same, i dont really know what it weighs on jupiter, but the point is, the penny weighs much much more on jupiter) So in your theory, we have two equal weighted objects, but its not, its just two different reference frames..they have to be within the same reference frame
I just want to thank you all for the Posted Comments. I have not laughed so hard by just reading something about Airplanes, lift, thrust, and a Conveyer belt in a long time. Just to let the Author know People still read this topic even as far away as Switzerland, like me. ïŠ To keep it amusing how about if there is a strong 100 knots Wind against the plane? Will the plane take off and how is it affected by the Conveyer belt or the wind?
A skateboarder is standing on a hill that can move (some sort of band conveyer). The skateboarder rolls downhill, while the conveyer moves uphill. This conveyer has a control system that tracks the skateboarder’s downhill speed and tunes the speed of the conveyer to be exactly the same (but uphill). Will the skateboarder reach the bottom of the hill?
Everyone misses the original intent to the question: will the plane be able to fly AT THAT POINT WHEN IT AND THE TREADMISS ARE AT EQUILIBRIUM and not moving (and powered on full force with the wind hitting the wings strong LIKE it is moving forward and taking off).
Um no Sttork the original intent of the question is:
Will the conveyor belt prevent the plane from moving forward.
the treadmill and the plane will never be at equilibrium because the treadmill does not apply an opposite force to the plane and it never will.
If you’ve read all the posts, understand the math watched the myth busters episode, the numerous videos on youtube and still don’t understand that then go buy a very thick physics text book and beat yourself over the head with it.
the only way the treadmill and plane can be in equilibrium is in a world where the laws of physics aren’t applied….this would then make the entire question moot.
Take some roller skates to and escalator, one with a significant horizontal area before the sections break up. Stand on the escalator and hold, not on to the moving rubber hand rails but to the metal or glass paneling that doesn’t move. The escalator will roll under and you will hold still, no?
Now imagine how a prop works, it pushes on air and the air pushes on the prop which in turn pulls a plane along. say the metal or glass paneling is the air that you are pushing on (much like a prop does) and you hands are pushed on by the paneling (much like air on a prop does). Now try to pull you self alone and you will find it is quite easy regardless of the speed of the escelator.
The plane takes off, I get it.
I’m sure this has been mentioned before, but I just want to confirm my suspicions:
Let’s assume the conveyor belt CAN move at an infinite speed, and DOES actively counteract the forward force of the plane. The plane doesn’t take off?
I don’t want a flame war. Just a “Yes” or a “No” from some educated people.
seraphrevan, yes, at a high enough speed (assuming the airplane isn’t also increasing its thrust), a conveyor belt could stop the plane from taking off. But it’d take a lot of speed, and the tires and wheels would likely fail before you even got close to that speed.
What’s the relationship between the acceleration of the conveyor belt and the rolling resistance force of the wheels?
None of the formulas for rolling resistance shed any light on this, but my hunch is that rolling resistance is increased by the acceleration of the belt.
If this is true, then, if the wheels are indestructible, and the conveyor belt can accelerate many many times faster than the acceleration provided by the jet thrust, then it would be possible to keep the plane from taking off. I’m curious how much greater the acceleration of the conveyor belt would have to be, and how much energy would be required. . .
PS–I assume the surface area of the wheels is directly proportional to the resistance force, so if the wheels were really really BIG. . .