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.
PPS– Wait, I just realized–I believe we also have to take into account the friction of the wheel bearings. . . if the wheel could spin on the axle without friction, then no amount of rolling resistance from the wheel’s contact with the ground would oppose the thrust force of the jets. . .
The ORIGINAL IDEA of the Conveyor belt question, when asked on that talk show (yes, I heard it!), was to have the spinning wheels match the conveyor belt (so the plane stayed in place) - and would the plane, motors and prop running at full throttle, be able to take off AT THAT INSTANT.
Since then so many bastardized versions of this question (including the incorrectly rephrased question atop this long-going post), and a lot of whacky tangents, have been added to prove points either way - but that is the original question…and the answer is naturally NO.
Physforum.com has a thread that started six months before that TV episode.
And you do realize that the spinning wheels matching the belt speed would not stop the plane from moving forward don’t you?
How do people think that planes accelerate after takeoff when their wheels no longer touch the runway?
Yes, I realize the nature of the original question. And I realize what the result would be.
Thinking about this got me interested in another question (if the conveyor belt speed is unbounded and the wheels are indestructible, how would one compute the necessary belt acceleration and the energy required to drive it?).
This question, I believe requires accounting for some or all of three forces opposed to the thrust: the rolling resistance of the wheels on the ground, the friction of the wheel bearings, and the wheels’ rotational inertia.
sttork give it up arleady.
if we assume what you say to be true (whih I doubt as it sounds like you’re pulling this outta your ass)
that the original question said as you say:
“The ORIGINAL IDEA of the Conveyor belt question, when asked on that talk show (yes, I heard it!), was to have the spinning wheels match the conveyor belt (so the plane stayed in place) - and would the plane, motors and prop running at full throttle, be able to take off AT THAT INSTANT.”
Then we would have to assume that we live in a world where the normal laws of physics don’t exist for the simple matter that the velocity of the wheels are a sum of both the forward velocity of the aircraft plus the speed of the conveyor.
And therefor if we live in a world where physics don’t exist we can assum a whole lot of things like the plane can spontaneously levitate at will and gravity works in reverse.
YOu can sit here and complain about the wording like a little bitch all you want but the fact still remains you are wrong.
I’m wrong? - you say it CAN simply start flying immediately from the moment it is at a total standstill? You need therapy.
You are wrong about the original intent of the question.
The plane moves forward.
The TV show was NOT where the question started.
If the original intent is to see if people understand that the wheels don’t make planes move forward then it’s a nice simple good question.
If the original intent is as you say it just becomes a big mess.
Neal Boortz says it takes off!!!
It’s a little below the middle of the page.
http://boortz.com/nuze/200801/01152008.html
Yes Sttork you are 100% absolutely wrong.
Because once the wheels are moving there is no point at whicch the aircraft will be at standstill.
If the wheels are spinning rolloing resistance has been overcome and the aircraft is moving forward, as there is no longer any counter force to oppose the aircraft’s forward thrust.
The only point where the conveyor belt and the wheels would be moving at the same speed would be when neither is moving. Yes that’s right when the aircraft is sitting on the runway not trying to take off. When it’s parked. And at this point it wouldn’t matter if the aircraft was on a conveyorbelt, tarmac, ice, or chocolate pudding.
Now I will conceed that there is a brief period of time (very very brief as in less then a fraction of a second) when the aircraft will apply forward thrust and not move. This is because the force at that brief point in time is less than the rolling resistance on the wheels. This occurs in all vehicles, however, regardless of running surface and no aircraft will take off at that point. But if the question was seriously asking if an aircraft takes off at that point there would be no reason to mention a conveyor belt at all as this is just the natural delay caused by the vehicle tranferring power to it’s propulsion system from rest.
Frankly sttork you just sound like another pathetic loser trying in vain to reword the question to make himself sound right because you can’t accept that you were incorrect in your evaluation of the question.
If I were you I would read the posts and try to expand my understanding of things that I didn’t understand before. However if you wanna just keep posting like a whiny bitch be my guest.
Something seems to have a lot of you confused, and that is what makes an airplane capable of flight. The simple answer is lift produced by the wing and that is a function of relative wind over the wing and angle of attack (and a couple of hundred other more minor factors). Without the necessary relative wind and angle of attack the plane will not fly. That minimum wind speed (airspeed) is just a little less than the stall speed for the plane in a landing configuration due to ground effect. Gear speed has absolutely nothing to do with it.
With that said, if you state the question so that engine thrust is only allowed to overcome the reverse speed of the conveyor, ie, the plane remains stationary with respect to the runway - it can’t fly. The wing sees no relative wind. If the engine is allowed to run normally, trust will easily overcome the reverse movement of the conveyor, airspeed (relative wind) will increase to takeoff speed and the plane will fly. The wheels will turn at airspeed plus conveyor speed and I don’t recommend doing that very often, but it will work.
By the way - I’m a commercial pilot, CFI, CFII, MEI and Ground Instructor.
We get that. What has you confused is the question says nothing about the engine only overcoming the resistance of the conveyor.
There is nothing in the question to stop the plane from moving forward unless you twist the question to make it say that.
We get that the plane must move forward, you don’t get that the plane does move forward.
If someone thinks that the plane moves forward the same way as the car then they will think the conveyor will stop it. But the original question has nothing to stop the plane from moving forward.
True - IF the belt is long enough. What bothers me, though, is that folks miss The Big Picture: the only purpose of the belt is to allow the plane to rev up at full throttle w/o initial forward movement (which is the point at which this puzzle peaks, so to speak). How about THIS thought: what if the plane was held by whatever means necessary to the ground so it could get to full throttle w/o forward movement and, at that instant, it was released AND the ground was removed (which would be even LESS friction than a treadmill). You think it would fly then? Nope.
Which..is pretty much the same as the original premise as the ORIGINAL question as asked on the Boorts Show asks if it could fly AT THAT TIME…not eventually. Which is again a big NOPE.
To the person who says this thread started months before the Neil Boortz show - dumb statement. Why? Because the original QUESTION that started this thread REFERENCES the show as where the question was asked.
Unless, I suppose, you heard a later rerun.
The purpose of the conveyor is to TRICK people into mistakenly thinking the plane does not move forward.
Then some people would not admit to themselves they were tricked and pervert the question into one where the plane does not move forward.
I didn’t say this THREAD started months before the show, the QUESTION started months before this show. And Boortz now says the plane moves forward and takes off.
http://www.physforum.com/index.php?showtopic=2417
It is a nice simple question when the plane is allowed to move forward and turns into a stupid question if the point is the plane does not move forward.
thank god Mythbusters solved this. Basically, the prop provides thrust through the AIR. The ground means nothing. Let’s take for example hurricane force winds across the airport. If my takeoff speed in (indicated airspeed) is 70, and the wind is 70, the plane WILL take off. this will happen even if the plane is sitting still in reference to the ground. The prop is moving the plane through air not ground, and therefore, when the airspeed is sufficient to fly, it will fly.