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The suitcase is on the conveyor, just as the plane in our example is on a conveyor. I am applying a force to the suitcase, just as the plane's engine is applying a force to the plane.

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You keep mixing up terms. I am not "supplying the energy" for the suitcase. Energy is not the relevant term here. I am applying a force to the suitcase that is in the opposite direction to the conveyor, just as the plane's engine applies a force to the plane in the direction opposite of the conveyor.

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There is nothing about this that moves it from the realm of reality to the realm of pure hypothetical. You could go to the airport right now and do exactly the experiment that I am describing. And I guarantee you that the movement of the conveyor belt would NOT negate the movement of the suitcase. The forces involved in moving the plane are exactly the same forces and the plane, just like the suitcase, would move.

Watch this. The speed of the conveyor belt has very little to absolutely no impact on the acceleration of the plane, because of the freewheeling wheels. Now you nay-sayers, please stop your argument, it's futile.

Febs, your suitcase analogy is perfectly accurate. Some people just can't acknowledge that, they can't get their minds around the facts of the case. If Febs is walking 10km/h in one direction, the conveyor belt would be going 10km/h in the opposite direction. This would not stop the suitcase from going the same speed Febs is going, the wheels will just be turning faster than if the conveyor belt stands still. Walking with the conveyor belt on or off only has effect on the turning speed of the wheels. The same holds for the plane. The friction of the conveyor belt might mean that Febs will be walking 9km/h with the same energy as walking 10km/h without the conveyor belt. He will still be going forward though.

Let's see if I understand this correctly:

Experts all over the internet would agree that a plane sitting on the runway while running the engines at full throttle for engine checks wouldn't lift off the ground. Yet, once you put the same plane onto a conveyor belt so that the only difference is that the wheels spin, and the plane will suddenly take-off? How do spinning wheels get that plane off the ground?

I figure I'm missing something, but this is hypothetical, and maybe I'm missing something critical, but since it appears that the question states that the wheels spin yet the conveyor counters any forward travel I don't see how this changes anything.

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Nowhere does it state that the belt prevents forward travel. The plane's engines push the plane along the conveyor until it takes off.

double post

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You have to supply energy to the suitcase, otherwise it will go with the conveyor...that is what I was saying. Whether you are going with or without the conveyor belt, you have to transfer momentum to the suitcase for it to move from its ground plane.


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OK, so after re-reading your airport experiment, I think I understand what you are saying to do now. It seemed like you were saying to send a suitcase from one side of the conveyor with yourself on the other side. But you're saying to go to the side of the conveyor that would be going in a reverse direction, and drag the suitcase with you. Yes, obviously, you would be able to drag it.

Now the question is, would you be able to feel a bit more drag over the opposing conveyor belt vs dragging on the ground? Wheels are reducing surface friction, but there still has to be a force acting on the axles (if the wheels are now going away from the object that's being dragged). Obviously you're going to be capable of dragging a wheeled object forward, but I would think there must be some drag from the wheels wanting to go the other way. With suitcases and model airplanes, they would be much easier to drag. Airplanes are bigger, so I would assume they would have more drag.

I would think this drag would be related to surface friction....if you constructed a RC plane with skis on it, it would be able to counter a treadmill as well: surface friction is reduced. I still think hover crafts would be the fastest

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There will be some, but it is really negligible compared to the force exerted by me pulling on the bag. But if you can see that I can pull a wheeled object forward when the wheeled object is resting on a conveyor belt moving in the opposite direction, you can see that the plane's engines could pull the plane forward under the same circumstances.

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Yes, but airplanes also have much more thrust. The mechanics are no different than the situation with the suitcase and the conveyor belt.

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Yep, but the point then of my previous ideas was that the surface friction of the wheels are being transfered to the axle. It's all about eliminating surface friction...and that's why my shoe analogy helped me understand it Because the airplane is having to exert more, it's going to take a little longer to take off then if it where on the ground. How much? Do we really want to argue that

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Wheels spinning forward with the plane reduces the surface friction and lets the forward air speed of the engines move the airplane forward right? If the plane didn't have any engines on, and the brakes weren't on the wheels, the plane would still stay seated on the belt as it moved. Friction is keeping the plane in place with the belt. If you applied the breaks and did an engine check on the moving belt, the plane would still just be moving with the belt. Now if you counter the direction of the belt, the wheels will spin with the belt and reduce surface friction. However, since they're opposing the natural forward tendencies of the plane, the plane is going to have some increased thrust to move the wheel along (it's not equal to the forward thrust of the airplane because the friction on the wheel can't keep up). Imagine an airplane on skis....it helps me understand it anyway Skis would reduce surface friction as well, so that when there's enough forward thrust, the airplane can go (granted on a regular surface, we're talking a serious forward thrust). If you put a hovercraft under the plane, it suddenly will be really easy to go forward because it is reducing surface friction to a fraction. While the free spinning wheel is going the opposite direction of the plane, it's still reducing surface friction. The axle would encounter more resistance then if the wheel was following the plane. How much resistance will the airplane get? Well that's been the debate recently it seems

People that keep saying their is infinite friction, that is WRONG.

The wheels will ONLY be spinning TWICE as fast as they normally would.

Because whatever the speed the plane is moving, it is doubled by the treadmill.

So if the plane is moving forward at 50mph, the wheels would be experiencing the friction of 100mph.

I'm sure the safety range of airplane wheels is greater than twice the speed needed to take off.

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Not in paradoxical hypothetical land

What helped me get this is to imagine that the plane was floating just above the conveyor belt - imagine the plane moving though the air rather than running along the ground. And this went against everything I thought at the start. Theres a video on youtube where a father and son conduct a little experiment at home that explains it fairly well

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

EDIT: this is the one posted a little back up the page.

Fran

the problem with the initial question is that the plane is supposed to stay stationary in respect to the surrounding enviroment, and this is not possible, unless it was attatched to the ground, say with a teather

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Ok then think about this:

The plane starts moving forward at 10mph. The treadmill moves back at 10mph to match. The plane's speed relative to the treadmill is now 20mph. The treadmill then moves back 20mph to match. The plane's relative speed is now 40mph. The treadmill the moves back 40mph to match. The plane's relative speed is now 80mph. Etc.

As you can see, as the plane continues to accelerate, its relative speed to the treadmill quickly reaches infinity. However, in a perfect world with no friction, this has no effect on the groundspeed of the plane and the plane can accelerate as it wants with ease, as if the treadmill wasn't even there. Then it takes off.

But in the real world, there is friction. With the plane travelling at an infinite speed relative to the treadmill, its wheels would be spinning at an infinite RPM, and the friction between the bearings and axle become infinite. So in real life, within seconds the plane would utterly die.

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Why is it supposed to stay stationary?