Mythbusters - airplane on a conveyor belt

keystone light knight wrote:

Terminator X wrote:

Really? If a plane sitting on a (stationary) runway with it's wheels locked slowly cranked up it's engines to full blast, you think the plane would just sit there? I'm pretty sure the force created by the engines would overcome the friction at some point. I could be wrong, I suppose it would depend on the mass of the plane and on the power of the engines, but I still think any commercially made/mass produced plane would be able too.

true, but with the wheels sliding rather than rolling down the runway, i'm pretty sure the friction would melt/shred/otherwise destroy the wheels and landing gear before takeoff

Taking off shouldn't be a problem, but landing is going to be a bitch.

It is very difficult to overpower the thrust that a jet engine creates.

Kevin Rosenberg wrote:

Hi, I am a physicist.

The conveyor belt is irrelevant to the question. If I were teaching an introductory physics course, we would say that the wheels in this problem are "frictionless." However, the small amount of friction is still irrelevant to the huge amount of thrust produced by a jet engine. The jet engine of a plane is "pulling" on the air, which is irrelevant to the ground. Engineers design the wheels of a plane to have as little friction with the axle as possible, so that its main purpose is just to keep the plane up in the air. That being the case, the plane will simply turn its jet engines on, the huge thrust produced would quickly overcome the relatively minuscule amount of friction of the wheels, and the plane will take off normally.

K.R.

Ph.D. Physics, MIT, 1999

Sorry Kevin, but not true. If the hypothetical treadmill can keep up with the huge amount of thrust there is still something called gravity keeping the plane on the ground and in contact with the treadmill and in turn motionless. You will have to remove gravity to infer frictionlessness (is that a word).

.................... wrote:

Sorry Kevin, but not true. If the hypothetical treadmill can keep up with the huge amount of thrust there is still something called gravity keeping the plane on the ground and in contact with the treadmill and in turn motionless. You will have to remove gravity to infer frictionlessness (is that a word).

Which is why the plane won't take off "normally." The engine forces overwhelms friction (despite a hell of a lot of spinning and rotation of both tire and treadmill), the wheels skid, bounce, skip, and screech as the plane invariably moves forward. The plane takes off, but it ain't smooth.

Ohio Rulz wrote:

.................... wrote:

Sorry Kevin, but not true. If the hypothetical treadmill can keep up with the huge amount of thrust there is still something called gravity keeping the plane on the ground and in contact with the treadmill and in turn motionless. You will have to remove gravity to infer frictionlessness (is that a word).

Which is why the plane won't take off "normally." The engine forces overwhelms friction (despite a hell of a lot of spinning and rotation of both tire and treadmill), the wheels skid, bounce, skip, and screech as the plane invariably moves forward. The plane takes off, but it ain't smooth.

You guys are either trolls, really stupid, or from ohio . . . oh, looks like one of you is from ohio, so I'll try to make it really simple.

The riddle states that the treadmill runs at the same speed as the PLANE, not the WHEELS. The engines push the plane down the treadmill runway (it's the length of a regular runway) - the treadmill spins the opposite direction - but this action hardly effects the body of the plane - it only makes the wheels spin faster. At takeoff the plane will be moving 150MPH - the treadmill will be moving 150MPH - and the wheels will be moving at 300MPH.

Got it?

Kevin Rosenberg wrote:

Hi, I am a physicist.

The conveyor belt is irrelevant to the question. If I were teaching an introductory physics course, we would say that the wheels in this problem are "frictionless." However, the small amount of friction is still irrelevant to the huge amount of thrust produced by a jet engine. The jet engine of a plane is "pulling" on the air, which is irrelevant to the ground. Engineers design the wheels of a plane to have as little friction with the axle as possible, so that its main purpose is just to keep the plane up in the air. That being the case, the plane will simply turn its jet engines on, the huge thrust produced would quickly overcome the relatively minuscule amount of friction of the wheels, and the plane will take off normally.

K.R.

Ph.D. Physics, MIT, 1999

You're an idiot, you should not be teaching physics. No air is moving over the wings, which makes it impossible to have lift. You need to have air flowing over the wings in order for it to take off... you just made all that up about being a physicist, cus you're actually retarded and don't have a clue about science.

It really disappoints me that there are still people out there who, even after many brilliant explanations to the contrary, still think the airplane would not move forward and (given enough runway space)take-off.

I hope those of you who are arguing that the plane will have no air moving under its wings are just kidding. Sadly, this may not be the case for some of you.

To help you visualize what happens, watch the following video of an experiment some guy did with his kids Air Hogs toy plane. Note that in the experiment the plane moves forward no matter how fast the treadmill is moving in the opposite direction. The only reason the plane does not take-off in this experiment is because of limited runway space. If I remember correctly, the initial riddle stated that it was a giant conveyor belt runway. So, if enough space is given the PLANE WILL F*CKING TAKE-OFF. End of discussion.

http://youtube.com/watch?v=-EopVDgSPAk

Neb wrote:

You guys are either trolls, really stupid, or from ohio . . . oh, looks like one of you is from ohio, so I'll try to make it really simple.

The riddle states that the treadmill runs at the same speed as the PLANE, not the WHEELS. The engines push the plane down the treadmill runway (it's the length of a regular runway) - the treadmill spins the opposite direction - but this action hardly effects the body of the plane - it only makes the wheels spin faster. At takeoff the plane will be moving 150MPH - the treadmill will be moving 150MPH - and the wheels will be moving at 300MPH.

Got it?

No genius. The riddle says treadmill speed = wheel rotation. As the planes wheels increase in speed, so does the treadmill.

Your supposedly simple solution violates the riddle. It ain't a math problem (150 minus negative 150 - you following me? = 300).

You crack me up trying to sound smart. HEY GUYS I FOUND THE ANSWER!!! THE WHEELS ARE DIFFERENT FROM THE PLANE! The answer isn't in deconstructing the semantics of the riddle. Trying figuring it out for real, moron.

Try it this way. Wheels = 300. Treadmill = 300. Now what?

The plane starts in a motionless state, wheels spinning equal to treadmill. Now thrust. How did your plane get up to 150MPH, without the wheels exceeding the treadmill speed? (Answer: only by skidding, which adheres to the riddle's principle of wheel & treadmill equal speed)

I may be from Ohio, but apparently we make 'em smarter here. Where you from? Jersey?

Ohio Rulz wrote:

Which is why the plane won't take off "normally." The engine forces overwhelms friction (despite a hell of a lot of spinning and rotation of both tire and treadmill), the wheels skid, bounce, skip, and screech as the plane invariably moves forward. The plane takes off, but it ain't smooth.

This riddle is either great, because it provokes such lengthy, heated discussion; or terrible because the ambiguity thwarts a definitive answer. Most riddles have a satisfyingly simple, but not immediately obvious solution. The issue here is largely semantic, because from the way the problem is stated, one can't know if the set-up allows the plane to move forward on the treadmill or not. If the answer above is correct, then this riddle is even dumber than I've thought all along.

Ohio Rulz, please be assured that I am not insulting you. My issue is with the bizarre inferences that we are forced to draw from the way the riddle is stated.

You are correct, oldXCguy. The version with the wheels spinning at the same speed as the treadmill is incoherent. The original letsrun version says the treadmill is moving at the same speed as the plane, which still isn't very coherent, but we can at least imagine the situation. I'm still not convinced that the wheel speed=treadmill speed thing is even possible with the engines on.

There is no doubt in my mind that this riddle was designed to provoke controversy, and it has succeeded marvelously.

I believe the point that people are missing is the type of conveyor belt being used. If you can set up a manual belt, similar to a manual treadmill, I don't see this plane taking off at all. It would be similar to a car on rollers. In this cirumstance the plane's engines would turn on and produce thrust and move the plane forward, but the wheels turning would then turn the manual conveyor belt. So you can't have a plane taking off without moving forward, but that's not the riddle.

The riddle is basically "a plane on a treadmill", a modern automatic treadmill or in this case a huge ass conveyor belt. The belt moves, the wheels moves, but once the engines turn on the power of the engines will produce enough forward thrust to move the plane forward. As long as the plane is moving forward and has enough space it will eventually take off.

Now, my question is once the plane leaves the belt what happens? The plane's engines are producing enough thrust to go say 100mph, but because of the belt the actual speed of the plane is only say 50mph. So when the plane leaves the belt you could see a slingshot effect of a plane going from 50 to 100mph in the blink of an eye!

Alan

You are wrong about the riddle - here is how it was stated on letsrun -

"An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?"

It states "the treadmill run in the opposite direction at the same speed the PLANE is moving."

It says nothing about the WHEELS.

There are some versions on the internet that say the treadmill runs at the same speed as the wheels - but I think in those cases the person just stated the riddle incorrectly.

And I'm from Michigan.

Ohio Rulz wrote:

No genius. The riddle says treadmill speed = wheel rotation.

No it doesn't. Here's the thread:

"An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?"

I've always assumed that the reference point is the ground. As you increase the speed from inside the plane, the treadmill moves at that speed. So the treadmill's "speed meter" equals the plane's speedometer.

And yes, the speed that the plane's body is moving horizontally can be a different rotational speed than the wheels.

Wow, my response is practically the same as Neb's. We're not the same person, I swear.

OldXCguy wrote:

This riddle is either great, because it provokes such lengthy, heated discussion; or terrible because the ambiguity thwarts a definitive answer. Most riddles have a satisfyingly simple, but not immediately obvious solution. The issue here is largely semantic, because from the way the problem is stated, one can't know if the set-up allows the plane to move forward on the treadmill or not. If the answer above is correct, then this riddle is even dumber than I've thought all along.

Ohio Rulz, please be assured that I am not insulting you. My issue is with the bizarre inferences that we are forced to draw from the way the riddle is stated.

No offense taken. See, this isn't just a riddle like the joke you tell around the middle school campfire - one that has a very simple answer that most people don't think about. (The disappearing icycle murder weapon, for example).

Since this one can be tested, I'm giving my best guess as to what would actually happen. What's yours?

The "set up" doesn't disallow the plane moving forward. If it were this way, then the answer is incredibly obvious - no flight. No discussion even needed. No movement, no lift. Dumb riddle.

Alan - the only "extra" force that the engines are fighting is the small amount of additional friction caused by the treadmill. This will not create a "slingshot" effect anymore than leaving the ground normally creates a "slingshot" effect.

In both cases there is some friction when the wheels are touching the ground that is not present when the wheels are in the air - but that force is rather small.

In other words - if the plane is flying at 150MPH and the treadmill is running at 150MPH - the engines don't need to produce 300MPH worth of thrust. They only need to have 150MPH worth of thrust plus a little bit for the small amount of friction caused by the wheels.

Alan-

I'm confident you are NOT a troll so I would encourage you to look back at the original thread, bearing in mind that a plane does not generate it's motion through its wheels.

hyman shoulder wrote:

So the treadmill's "speed meter" equals the plane's speedometer.

How does the plane have "speed" if it aint moving? It's not like a bike (speed measured by wheel rotation). If it isn't moving, then the speedometer = 0.

Your definition of speed is flawed.

Now that everyone has agreed that the plane takes off, what interesting quirks can we add to keep the discussion going? Would it be possible for the treadmill to go fast enough that the rolling friction becomes significant? I guess we would have to ignore the plane matching the treadmill speed for this question.