Author Topic: plane on a conveyor belt? (Read 19897 times)

eskimo2 · « **Reply #465 on:** January 27, 2007, 10:18:59 AM »

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Originally posted by WhiteHawk
Nice hijack effort. I am trying to decipher eskimos answer as yes the plane will fly if it were possible to tune the conveyor to the wheel speed, or no it would not, even if it were possible. Feel free to comment.

Depends on the question:

If the question were that the conveyor were matching the plane’s speed in the opposite direction, the conveyor would be traveling at the rate of 50 mph at take off and so would the plane. The plane would take off normally except its wheels would be rolling at twice their normal take off rpm. That consumes a tiny bit of energy that really can’t be felt by the pilot.

Conveyor matches the plane’s speed: plane will fly!

Our question, however, states that the conveyor must match the plane’s wheel’s speed.
In this case if the plane moves forward, its wheel has gone further, and therefore faster than the conveyor. The only thing that the conveyor can do about this is to speed up like mad!

Conveyor matches the plane’s wheel speed: plane will NOT fly!

john9001 · « **Reply #466 on:** January 27, 2007, 10:20:07 AM »

it's a paradox, the conveyor can only match the speed of the plane, so the plane has to move for the conveyor to move, if the plane does not move the conveyor can not move. so there is no answer.

it's like the "time travel paradox," if you go back in time and kill your grandfather you will not be born, if you are not born you could not go back in time to kill your grandfather.

WhiteHawk · « **Reply #467 on:** January 27, 2007, 10:27:31 AM »

Quote

Originally posted by eskimo2
Depends on the question:

Our question, however, states that the conveyor must match the plane’s wheel’s speed.
In this case if the plane moves forward, its wheel has gone further, and therefore faster than the conveyor. The only thing that the conveyor can do about this is to speed up like mad!

Conveyor matches the plane’s wheel speed: plane will NOT fly!

Ok, but only because it is not possible for the conveyor to match the wheel speed? The question asks us to assume that to be possible.

eskimo2 · « **Reply #468 on:** January 27, 2007, 10:33:31 AM »

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Originally posted by WhiteHawk
Ok, but only because it is not possible for the conveyor to match the wheel speed? The question asks us to assume that to be possible.

Sure its possible, the conveyor can even push the plane back if it wanted to.

WhiteHawk · « **Reply #469 on:** January 27, 2007, 10:36:58 AM »

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Originally posted by john9001
it's a paradox, the conveyor can only match the speed of the plane, so the plane has to move for the conveyor to move, if the plane does not move the conveyor can not move. so there is no answer.

it's like the "time travel paradox," if you go back in time and kill your grandfather you will not be born, if you are not born you could not go back in time to kill your grandfather.

But the question implies us to assume that it is possible for the conveyor to match the wheel speed. You are attacking the hypothesis which is assumed to be true, therefore, unattackable.

eskimo2 · « **Reply #470 on:** January 27, 2007, 10:37:55 AM »

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Originally posted by WhiteHawk
Ok, but only because it is not possible for the conveyor to match the wheel speed? The question asks us to assume that to be possible.

Tell me, isn't this question an impossible paradox too?:

A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.

The question is:

Will the plane take off or not?

(Note the word “wheel” has been removed)

In this situation the plane would take off normally except its wheels would be rolling at twice their normal take off rpm. But wait, how can the conveyor speed be exactly the same as the plane? If it waits for the plane to move even the slightest bit it is behind, which is not the same speed…? Or should we just assume that the conveyor stops trying as soon as the plane moves at all?

WhiteHawk · « **Reply #471 on:** January 27, 2007, 10:38:08 AM »

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Originally posted by eskimo2
Sure its possible, the conveyor can even push the plane back if it wanted to.

Lets put the question another way. A plane sitting on a conveyor with alien technology 0 friction wheels. Able to withstand unlimited rpms without failure. A conveyor belt is racing below at 100mph. If there is 0 friction then there would be 0 energy transmission and the plane would be stationary. Now lets fire the engines and give say, 2000lbs thrust. what happens to the plane?

eskimo2 · « **Reply #472 on:** January 27, 2007, 10:45:27 AM »

Quote

Originally posted by WhiteHawk
Lets put the question another way. A plane sitting on a conveyor with alien technology 0 friction wheels. Able to withstand unlimited rpms without failure. A conveyor belt is racing below at 100mph. If there is 0 friction then there would be 0 energy transmission and the plane would be stationary. Now lets fire the engines and give say, 2000lbs thrust. what happens to the plane?

It takes off.

eskimo2 · « **Reply #473 on:** January 27, 2007, 10:50:28 AM »

Note; this question does NOT depend on rolling resistance or bearing friction. In this area it only requires normal traction between the tires and conveyor surface. This is the same traction that would prevent a plane from moving if it applied full power with its brakes on.

Kuhn · « **Reply #474 on:** January 27, 2007, 10:58:23 AM »

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Originally posted by Kuhn
The wheels wont roll if an airplane does not move forward. Being that the wheel movement is is controlled by the prop moving through the air ,the plane will move forward nomatter what the wheels are doing. The conveyer can try all it wants to keep the wheels spinning fast enough to keep up with the rotation but in no way is it possible even in theory for it to keep up with the wheel speed. It will keep accelerating as the plane moves forward and the plane will lift off. So actually the original question is flawed and cannot be answered the way it was asked. It is not possible for the plane not to move forward when the prop is pulling it through the air. It is not possible for the plane to move forward if the wheels dont rotate when in contact with the ground.

This applies if there is no friction on the wheel bearings. But then I'm adding to the original question.

Kuhn · « **Reply #475 on:** January 27, 2007, 10:59:44 AM »

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Originally posted by JB88
i wish i was in the same bar ignoring all of you physics dweebs while making out with the hot blond in the corner.

This dweeb would do that AFTER the debate. Then I'd take her home and.....

lukster · « **Reply #476 on:** January 27, 2007, 11:03:52 AM »

I'm not attempting to influence the determination as to whether the plane flies or not. However, force is transmitted through friction. There can be no force transmitted from one mass to another without friction. You can define the coefficient of friction to be so small that perhaps only two atoms are interacting but you cannot eliminate it. To say that two objects acted on each other with no friction would be like saying they have no mass.

WhiteHawk · « **Reply #477 on:** January 27, 2007, 11:11:55 AM »

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Originally posted by eskimo2
Note; this question does NOT depend on rolling resistance or bearing friction. In this area it only requires normal traction between the tires and conveyor surface. This is the same traction that would prevent a plane from moving if it applied full power with its brakes on.

Ok good point, if the brakes were on, the plane wouldnt move in our problem right? Now lets fire the thrust at 2000lbs keeping the brakes on, WS=0; CS=0; increase the thrust to full power 18000lbs, brakes on, WS=0; CS=0; We take off the brakes in a fashoin as not to torque apart the plane, what happens to the plane?

eskimo2 · « **Reply #478 on:** January 27, 2007, 11:21:56 AM »

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Originally posted by WhiteHawk
Ok good point, if the brakes were on, the plane wouldnt move in our problem right? Now lets fire the thrust at 2000lbs keeping the brakes on, WS=0; CS=0; increase the thrust to full power 18000lbs, brakes on, WS=0; CS=0; We take off the brakes in a fashoin as not to torque apart the plane, what happens to the plane?

Depends on the mass of the plane. A light enough plane will skid and accelerate. If the skidding drag isn’t too much it might even take off. It has been stated; any plane capable of taking off with its brakes locked (negating tire blow out leading to prop hitting the ground, etc.) will also take on the conveyor where the conveyor matches the wheel speed (also negating tire blow out leading to prop hitting the ground, etc.).

WhiteHawk · « **Reply #479 on:** January 27, 2007, 11:23:01 AM »

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Originally posted by lukster
I'm not attempting to influence the determination as to whether the plane flies or not. However, force is transmitted through friction. There can be no force transmitted from one mass to another without friction. You can define the coefficient of friction to be so small that perhaps only two atoms are interacting but you cannot eliminate it. To say that two objects acted on each other with no friction would be like saying they have no mass.

And nobody is going to try to take a 200,000lb aircraft off on a moving conveyor. We are trying to solve problems based on a HYPOTHETICAL assumptions. What if. Try to work with us here.