[Mrvile]

Quote:

Originally Posted by Davesrose /img/forum/go_quote.gif 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]

Ok then Dave, here's something to tickle your brain in the morning

The whole situation really doesn't have anything to do with inertia. If the plane was turned off, then yes, it would move back with the treadmill. However, the plane isn't turned off, and it is actually moving forward with its engines. The treadmill and free-spinning wheels of the plane cancel each other out. The engine can then freely propel the airplane around as it pleases.

 

[tator]

OK I reread the question. I guess if the belt matches the absolute speed of the plane, then the question is pointless. So now that I understand the question, then yes, I think it would take off, and as others have stated, the wheels would be spinning way, way faster than they were meant to.

 

[LawnGnome]

I'll say a few things:

The principle of an airplane is the speed relative to the AIR, not to the ground.

An airplane engine pushes against air to make it go forward.

The wheels would only be spinning twice as fast as if the plane were on regular ground, at all speeds.(except zero obviously)

For those mentioning prop planes, They will also be able to take off. As would a rocket powered car be able to speed forward.

And yes, this has been posted on many physics and aeronautics forums, and in the end they all have concluded it will take off.

 

[Mrvile]

Quote:

Originally Posted by tator /img/forum/go_quote.gif OK I reread the question. I guess if the belt matches the absolute speed of the plane, then the question is pointless. So now that I understand the question, then yes, I think it would take off, and as others have stated, the wheels would be spinning way, way faster than they were meant to.

Which is why it can't be tested.

 

[fordgtlover]

To those who think it can't take off.

What would happen if you reverse the direction of the belt and don't use the plane's engine? Will the belt ever be able to go fast enough to allow the plane to take off?

 

[fordgtlover]

Quote:

Originally Posted by tator /img/forum/go_quote.gif OK I reread the question. I guess if the belt matches the absolute speed of the plane, then the question is pointless. So now that I understand the question, then yes, I think it would take off, and as others have stated, the wheels would be spinning way, way faster than they were meant to.

It doesn't matter about matching the absolute air or ground speed of the plane.

Example.
A jet needs to reach a speed of 150 mph in order to take off. Or more correctly it needs to have air flowing over the wings at 150 mph to generate enough lift for the plane to take off.

So, at the moment just before take off, the conveyor belt is matching the plane's wind speed and moving at 150 mph in the reverse direction to the plane. The plane is still moving forwards at 150 mph. The wheels are travelling at 300 mph, which is the speed required to balance out the forward travel of the plane and the reverse direction of the belt. If you doubled the speed of the conveyor belt to 300 mph, the plane is still moving forwards at 150 mph, but the wheels are now travelling at 450 mph.

The wheels are not bound by the speed of either the plane or the belt. They are able to compensate for the difference in speed between the two.

Apart from issues like burning up the bearings etc. The plane will still have enough forward speed to take off.

 

[Davesrose]

Quote:

Originally Posted by LawnGnome /img/forum/go_quote.gif I'll say a few things: The principle of an airplane is the speed relative to the AIR, not to the ground. An airplane engine pushes against air to make it go forward. And yes, this has been posted on many physics and aeronautics forums, and in the end they all have concluded it will take off.

Since you've removed the red herring, then yes, the airplane will take off. However, I don't see this debate has ended in the science forums. It seems part of the problem is that the original questions came from science books....many of which were stating that the plane would not move forward. Anyway, since this is utter theorizing, and it depends on how you're looking at it. Hence why this "simple" science question will always be around the internet:

http://forum.physorg.com/index.php?s...0&#entry231694

Maybe people assume that since you have a magic conveyor belt, it would also carry enough friction to keep the plane stationary as well.....as this is complete hypothetical land

 

[LawnGnome]

It doesn't really matter how much friction the belt has, as long as the wheels can rotate.(meaning the belt isn't sticky)

And that thread is 2 years old, the people who still argue it won't take off are the ones that just come into it partway.

There is even video links posted in that thread that show how it will work.

What most people fail to understand is planes need speed relative to the air, not the ground. Most people who say it wont take of, just cant comprehend that. They have a hard time separating themselves from the only point of view they know.

 

[Davesrose]

Quote:

Originally Posted by LawnGnome /img/forum/go_quote.gif It doesn't really matter how much friction the belt has, as long as the wheels can rotate.(meaning the belt isn't sticky)

Actually, the way I see it, is that the wheels naturally want to go in the direction of the belt. They will be getting a counter direction from the axle of the moving airplane. And there in lies the difficulty to visualize it. Now if the landing gear's bearings were infinitely strong and the wheels only went with the belt, then the forward direction of the plane is met by the backward direction of the belt.

The belt is always going to be a little "sticky"...otherwise the wheels wouldn't move (it has nothing to push off from)

If you think about a free spinning wheel, it has friction points on both the tread and the axle. Let us say that there's enough friction on the wheel tread to belt that they want to adhere to one another. They will provide resistance to any other force. With the plane analogy, lets say there's a really small forward air speed. We've got super wheels that are keeping up with the belt. Hypothetically, if the belt has infinity friction, and the bearings are hyper greased to never burn out, the free wheeling wheels would always be stuck on the belt and you just have the forward vs backward ground speeds

Instead of the roller skates on treadmill analogy, for me, it's easier to see a person wearing normal rubber shoes. The rules are still going to apply as the roller skate analogy. Only it's easier to feel it....if you hold a rope on a treadmill, you'll still stay still and be able to stand. Though you will feel the resistance of the treadmill against the soles of your shoes (and you'll have to balance yourself). In this example, you could have a sled on the airplane, and it wouldn't make a difference. It's the friction of the tread that reaches a critical point that the forward direction of the axle can overcome. For people who say that the plane will take just as much take off distance (on belt) compared to on the ground, I've got to beg to differ. The forward speed will be reduced slightly by the friction of the belt. No one has utilized this technology because it's utterly pointless

So I get that. It seems in that very long thread, people have said that some textbooks do phrase the question in a way that the plane does not move forward, and if it's possible to achieve lift by air over the wings alone. It stays long because people are assuming different conditions, and there is a lot of different ideas around. Free spinning wheels and whether the plane goes forward or not are the red-herrings. I know I've been thinking about this too much and have examined all the paradoxes

 

[PiccoloNamek]

http://www.youtube.com/watch?v=l__kEcvDir0

This says it all.

 

[Davesrose]

Quote:

Originally Posted by PiccoloNamek /img/forum/go_quote.gif This says it all.

Well I didn't hear anything from that link

Here's the type vehicle that would have the least amount of surface friction:

http://www.youtube.com/watch?v=6Q7ju...elated&search=

 

[Mrvile]

Quote:

Originally Posted by Davesrose /img/forum/go_quote.gif Instead of the roller skates on treadmill analogy, for me, it's easier to see a person wearing normal rubber shoes. The rules are still going to apply as the roller skate analogy. Only it's easier to feel it....if you hold a rope on a treadmill, you'll still stay still and be able to stand. Though you will feel the resistance of the treadmill against the soles of your shoes (and you'll have to balance yourself). In this example, you could have a sled on the airplane, and it wouldn't make a difference. It's the friction of the tread that reaches a critical point that the forward direction of the axle can overcome. For people who say that the plane will take just as much take off distance (on belt) compared to on the ground, I've got to beg to differ. The forward speed will be reduced slightly by the friction of the belt. No one has utilized this technology because it's utterly pointless

Alright then, then we'll phrase the situation this way...with today's airplane technology, taking into account all friction that would affect the bearings of the wheels of the plane, the plane wouldn't take off. Instead, the wheels would quickly break, causing the airplane to land on its underside and completely fall apart.

If we were able to invent a frictionless substance to apply to the axle and bearings of the wheels, and the tires were made of an indestructable rubber, then yes, the plane would take off. So in theory, the plane would take off. In real-life application, it would not even survive.

 

[Febs]

Quote:

Originally Posted by Davesrose /img/forum/go_quote.gif Instead of the roller skates on treadmill analogy, for me, it's easier to see a person wearing normal rubber shoes. The rules are still going to apply as the roller skate analogy.

No. They are not the same. Period. There is friction between the soles of your shoes and the treadmill that causes your shoe to move with the treadmill. There is no such friction with wheels. Friction may cause the wheel to turn, but it does not cause the skate to move along with the wheel like it does with the shoe.

Do you have a treadmill? If so, go turn it on, and place any wheeled object on it. Hold the object to the treadmill with your finger. Press hard. Does the object move with the treadmill, or does it stay still? Now, remove the object, and press your finger directly against the treadmill with the same amount of force. Do you see the difference?

Once you understand that the plane on the conveyor is just like the rollerskates on a treadmill, and nothing like the shoes on a treadmill, you will understand that there are no paradoxes in this problem.

 

[Mrvile]

Quote:

Originally Posted by Febs /img/forum/go_quote.gif No. They are not the same. Period. There is friction between the soles of your shoes and the treadmill that causes your shoe to move with the treadmill. There is no such friction with wheels. Friction may cause the wheel to turn, but it does not cause the skate to move along with the wheel like it does with the shoe. Do you have a treadmill? If so, go turn it on, and place any wheeled object on it. Hold the object to the treadmill with your finger. Press hard. Does the object move with the treadmill, or does it stay still? Now, remove the object, and press your finger directly against the treadmill with the same amount of force. Do you see the difference? Once you understand that the plane on the conveyor is just like the rollerskates on a treadmill, and nothing like the shoes on a treadmill, you will understand that there are no paradoxes in this problem.

Ok, Dave's analogy might be a little exaggerated but he does have a point. Take a pair of shoes and put them on a treadmill. Now hold onto the shoes by the shoelaces. They stay in place, but there is a little bit resistance due to the friction of the shoes. Now put the shoes on and step onto the treadmill. You can still stay in place if you hang on hard enough, but there is a lot more resistance due to the increased friction from your weight.

In a real-world situation, the plane would act in a similar way. Wheels by themselves would rotate on the treadmill relatively easily. However, once you factor in the weight of the entire airplane creating a lot of friction between the wheel's bearings and axles, the plane becomes much harder to merely keep in place. It would still be possible to make the plane take off, but it would require a lot more thrust from the engines, and the wheels themselves, well, they wouldn't stand a chance.

Since the airplane/treadmill situation is completely fantastical and is impossible to test, we are disregarding a lot of factors that actually would affect the situation, such as weight and friction. In a perfect simulation where there is no friction between the bearings and axles, the weight of the plane wouldn't matter and the plane would take off with ease. But in real-life, it doesn't work that way.

 

[Febs]

If you still don't believe me, here is another experiment that you can try. Go to the airport or a shopping mall where there is a people-moving conveyor. Bring one of those wheeled suitcases. Put the suitcase on the conveyor, and then walk alongside the conveyor at the same speed that the conveyor is moving, but in the opposite direction, pulling the suitcase alongside you. (To be clear, the suitcase will be on the conveyor; you will not be on the conveyor, but walking alongside it.)

Do you think that (1) the suitcase will stand still relative to an observer who is not standing on the conveyor, or (2) the suitcase will move along with you as you pull it, until you reach the end of the conveyor?