[LawnGnome]

I was surprised so many people got it right at first, but then in came the people with opposing views.

Once you realize the answer , you will wonder how you didn't realize it at first.

 

[Davesrose]

Quote:

Originally Posted by LawnGnome /img/forum/go_quote.gif Once you realize the answer , you will wonder how you didn't realize it at first.

Or how convoluted the question is to make the conditions hard to conceptualize, so that the answer can be "yes", "but"

 

[LawnGnome]

Quote:

Originally Posted by Davesrose /img/forum/go_quote.gif Or how convoluted the question is to make the conditions hard to conceptualize, so that the answer can be "yes", "but"

there is no "but", it is simply yes.

 

[Happy Camper]

Quote:

Originally Posted by LawnGnome /img/forum/go_quote.gif there is no "but", it is simply yes.

So give the explaination now. Please!

 

[Davesrose]

If the answer is only yes, then you're not also assuming the landing gear is not a super duper landing gear that has no coefficient friction on its axle? Because the thing that's getting me is that if we're dealing with absolute theory, while the force of the airplane is going forward, its wheels are going with the force of the conveyor belt. If this is totally hypothetical, the only forces are the forward direction of the engine and the backward direction of the belt.

The red herring is if the plane can go forward relative to the ground. There are not enough conditions to say yes or no. In an ideal system, it would be just like a runner on a treadmill, and wouldn't move.

I scanned some other sites on this subject...and well, here's the other variables that make this confusing...ultimately, I think it's just like this quote:

Quote:

If you wanted a situation like the the mental image the OP tries to invoke, you would, instead of a conveyor belt, want a giant fan to create a headwind at exactly the same speed as that which the plane would otherwise be flying. In which case the props/jets wouldn't have any forward push on the plane, and it would stay stationary (well, they would have a forward push on the plane, exactly enough to counter the backward drag of the wind), but the movement of the air past the wings would still give lift, and the plane would rise vertically and then just hover (assuming the fan was tall enough, and was able to react quick enough to tweak the wind speed).

Quote:

another thing to keep in mind is that the conveyor belt, presumably, has a finite coefficient of static friction. Thus, if the conveyor belt goes fast enough, the wheels will begin to slide as opposed to roll, becoming kinetic friction. Since forces of kinetic friction are less than those of static friction, the force of the conveyor belt on the plane would, at this point, be less than the force of the thrust on the plane, allowing for the plane to move once again. Of course, since we're already dealing with infinites, I suppose it's possible that the belt does have an infinite coefficient of static friction, which would make my last point. ...Maybe the answer's just "no"

Since scientists and aerospace people seem to still be debating this, this is a paradox. So instead of "yes" with conditions...I'm changing it to yes/no !!

 

[Febs]

Quote:

Originally Posted by Davesrose /img/forum/go_quote.gif In an ideal system, it would be just like a runner on a treadmill, and wouldn't move.

No. It is absolutely nothing whatsoever like a runner on a treadmill. The runner propels himself forward by using friction against the treadmill itself. The plane propels itself forward using an engine that is independent of the treadmill. There is at most a negligible amount of friction between the plane and the treadmill, because the plane has freespinning wheels that play no part in the propulsion of the plane.

 

[Davesrose]

Quote:

Originally Posted by Febs /img/forum/go_quote.gif No. It is absolutely nothing whatsoever like a runner on a treadmill. The runner propels himself forward by using friction against the treadmill itself. The plane propels itself forward using an engine that is independent of the treadmill. There is at most a negligible amount of friction between the plane and the treadmill, because the plane has freespinning wheels that play no part in the propulsion of the plane.

If a plane stays stationary on a treadmill, will it not just follow the treadmill? The friction of gravity is keeping the airplane on the belt as it's being pulled by the belt. The airplane is not magically hovering in mid-air because its free floating from the wheels.

The object of the runner on the treadmill is to stay stationary to the ground, not push forward above the speed of the treadmill. He's staying in place because of gravity. If as he took a big step, he suddenly became weightless, then he would be lunched off into infinite inertia. If he or the plane suddenly gave more forward acceleration then the treadmill could do back, then they would move forward and begin to have their own inertia(relative to the ground).

The forces for this analogy (in an ideal system) is that you have gravity pushing down on the plane. The free spinning wheels are following and staying grounded with the treadmill. For however much forward acceleration the plane is doing, the belt is down an opposite acceleration (with wheels attached). The axle is connecting the two and is making this system like the runner.

 

[Born2bwire]

Quote:

Originally Posted by Lazarus Short /img/forum/go_quote.gif What beast? Don't you be doing a Patrick82 on us! Laz

It's too late. It's already upon us.

All I will say is this, the OP worded this problem satisfactory with me and thus avoids many of the inanities that people pick up from their interpretation. One way that people state is that the conveyor belt matches the velocity of the wheels relative to the belt. This imposes an impossible condition since the only time that this can be satisfied is at standstill (which frustratingly some people claim that's proof of the negative... Ow my head). Another way that I've seen it is to claim that the belt moves in such a way as to keep the plane from moving forward, which makes it a pointless question to ask. Some people really need to bone up on their mechanics.

 

[Davesrose]

Quote:

Originally Posted by Born2bwire /img/forum/go_quote.gif All I will say is this, the OP worded this problem satisfactory with me and thus avoids many of the inanities that people pick up from their interpretation. ......Some people really need to bone up on their mechanics.

Including the aviators, engineers, and aerospace folks that continue to debate this argument eh?

Until I see a consensus of answers, I can't see how this is anything but a paradox.

 

[tator]

Somebody send this in to Mythbusters!

 

[Mrvile]

LMAO @ this thread continuing...

I think one of the main reasons why people aren't understanding the concept is because they aren't quite sure how a jet airplane works. The plane's wheels are free-spinning, just as everyone has stated. When you are sitting in an airplane and it starts to taxi around, it moves due to one of two reasons. The first reason is that there is a separate vehicle towing it. The other is that the engines pull air through them and create thrust. The thrust then pushes the entire plane around, causing it to move.

How does this all relate to the treadmill question? When the plane's engines start pushing it forward, the treadmill starts to move back. However, since the plane's engines aren't affected by the movement of the treadmill at all, the plane is still able to be propelled forward by the thrust emitted from the engines. It starts to move forward. It moves faster and faster. Finally, it moves forward fast enough so that air is moving over the wings and SHAZAM! the plane takes off.

Round 2, GG

Oh and tator - Mythbuster never tried this because it is widely understood that it's impossible to test.

 

[Davesrose]

Quote:

Originally Posted by Mrvile /img/forum/go_quote.gif LMAO @ this thread continuing... How does this all relate to the treadmill question? When the plane's engines start pushing it forward, the treadmill starts to move back. However, since the plane's engines aren't affected by the movement of the treadmill at all, the plane is still able to be propelled forward by the thrust emitted from the engines. It starts to move forward. It moves faster and faster. Finally, it moves forward fast enough so that air is moving over the wings and SHAZAM! the plane takes off.

You might be LYAO, but I'm going mad I tell you

Well after this, I'm going to sign off for the night.

This is an impossible answer and I can see 3 answers: NO, the plane doesn't take off assuming we're just talking about speed of airplane, conveyor belt, and gravity: YES, while the airplane doesn't move forward due to inertia, there's going to be enough lift on the wings with enough drag and lift from the propellers: YES, the airplane will move forward because there are more real life co-efficients that would give the plane an inertia advantage.

In your examples with airplanes taxing off with runways, an the airplane is not on a magic treadmill....obviously the wheels are going to move with the plane and its going to have forward inertia. It would even have forward inertia on this magic treadmill if it had more speed then the treadmill.

To say from the other viewpoint, we are saying that the plane is effected by the treadmill. Assuming the belt is some fancy thing that can keep up with the engine, as long as there is a forward acceleration from the engines, there is a backward acceleration from the treadmill. The wheels are going in the opposite direction of the airplane and are connected by axles. Gravity is pulling down on everything, so the plane is staying seated as the engines are wanting to go one way and the wheels are wanting to go the other. That is one model....now people will dream up other models, and this thread will go on for 50 pages.

But I'm done!!!!

[size=xx-small]ok, at least for tonight

[/size]

 

[tator]

Planes don't take off from thrust alone. The thrust pushes the plane forward, which causes air to go over/under the wings, giving it lift. If the conveyor matches the speed of the plane, so that it remains stationary, then the plane will never take off. If it did, I'm sure the Navy/Airforce would have figured out a way to use this on carriers a long time ago.

 

[PiccoloNamek]

Quote:

If the conveyor matches the speed of the plane, so that it remains stationary, then the plane will never take off.

Who said the plane remains stationary? What is keeping it from moving?

 

[Mrvile]

Quote:

Originally Posted by tator /img/forum/go_quote.gif Planes don't take off from thrust alone. The thrust pushes the plane forward, which causes air to go over/under the wings, giving it lift. If the conveyor matches the speed of the plane, so that it remains stationary, then the plane will never take off. If it did, I'm sure the Navy/Airforce would have figured out a way to use this on carriers a long time ago.

Heh, why would this help Navy/Airforce carriers? The plane still needs the same length of runway to take off, treadmill or not. Why? Because the treadmill doesn't affect the plane's movement.

Stop and think about the treadmill + rollerskates + rope situation I posed earlier. Just think about it. Then come back once you've figured it out.