think about this...

[Slow Play Ray]

wow, i left this trainwreck (planewreck?) for a little while and now i can barely catch up!

however, this is the most sensible thing i have read in this thread in a long time:


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just because theres no solution for the rate at which the wheels should be rotating, doesnt mean there is some force impeding the plane.

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nice post.

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We are assuming "no slip" between the wheels and the runway. If you are stating that a plane on skis on ice will take off, I fully agree.

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In that case I'm a bit confused too. It's been a long time since I took physics, but as I understand it...

As the plane moves forward relative to the runway, the wheels also move forward, because they're attached to the plane. This generates friction between the wheels and the runway. This friction causes the wheels to spin faster (the current rate of rotation is irrelevant).

If the wheels spin faster at the same rate as the conveyor belt, then causing the conveyor belt to accelerate will not change the acceleration of the wheels relative to the conveyor belt. Therefore the conveyor belt can't compensate for the faster rotation of the wheels as the plane is moving forward. Therefore as the plane is moving forward the speed of wheel rotation changes relative to the speed of the conveyor belt and the assumption in the original post becomes untrue.

Can someone please point out exactly where this line of reasoning goes awry? I don't doubt that Patrick knows 100 times more about this than I do, but I would really like to know where the flaw is. Am I making an assumption that isn't warranted? Is my understanding of physics off? Where is the issue?

[Oblivious]

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I assumed the conveyor would be just like sticking the plane on a dynamometer and the plane would never be able to move forward. I was wrong. What you're describing is more like a Harrier or frictionless runway.

Or maybe the conveyor ramps up to infinite speed and the universe collapses?

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BINGO!

[LAGmaniac]

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We are assuming "no slip" between the wheels and the runway. If you are stating that a plane on skis on ice will take off, I fully agree.

[/ QUOTE ]

In that case I'm a bit confused too. It's been a long time since I took physics, but as I understand it...

As the plane moves forward relative to the runway, the wheels also move forward, because they're attached to the plane. This generates friction between the wheels and the runway. This friction causes the wheels to spin faster (the current rate of rotation is irrelevant).

If the wheels spin faster at the same rate as the conveyor belt, then causing the conveyor belt to accelerate will not change the acceleration of the wheels relative to the conveyor belt. Therefore the conveyor belt can't compensate for the faster rotation of the wheels as the plane is moving forward. Therefore as the plane is moving forward the speed of wheel rotation changes relative to the speed of the conveyor belt and the assumption in the original post becomes untrue.

Can someone please point out exactly where this line of reasoning goes awry? I don't doubt that Patrick knows 100 times more about this than I do, but I would really like to know where the flaw is. Am I making an assumption that isn't warranted? Is my understanding of physics off? Where is the issue?

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You are exactly where I am at with this problem. The only way I can see it being reconciled is to assume that the conveyor belt will accelerate up to infinity along with the wheels. Then the plane moves forward with its wheels spinning at infinty + some speed sufficient for takeoff, which still equals infinty.

[NLSoldier]

I finally got hooded to admit defeat. The easist way for us to think about it was the rollerskate on treadmill example. If you are pulling yourself forward on the treadmill, you are going to go forward no matter what your [censored] wheels do. you are going to move forward! the hang up is that the OP suggests that the plane somehow is not able to accellerate forward but the fact is that it WILL accellerate forward. THe only thing stopping it is the wording of the problem which suggests its going to stay in place, but taht is not what is going to happen.

edit-actually, it doesnt even say anything about that in the OP. that must have been concluded by people later in the thread.

[bobman0330]

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It was assummed and I believe the OP specified a no-slip condition between the wheels and runway. I don't believe that condition can be reconciled with the condition that the wheel's speed = runway speed if the wheel has a tangential acceleration with respect to the runway.

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I just reread the OP, and I didn't see where it said the wheels don't slip. If the wheels can't slip (I'm not positive, but my intuition says this would be the case on perfect bearings too), then the conditions of the problem make no sense. If they can, then the plane takes off.

[SomethingClever]

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It is not theoretically possible to apply a force from the jet engine and have the wheels move at the same speed as the runway. The wheels will speed up, no MATTER how fast you make the runway, the wheels will move at that speed plus more, thanks to the force from the jet.

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Why is it that the wheels can't impart force to the plane, but the plane can apparently impart force to the wheels?

Hmm???????

[Oblivious]

[ QUOTE ]
wow, i left this trainwreck (planewreck?) for a little while and now i can barely catch up!

however, this is the most sensible thing i have read in this thread in a long time:


[ QUOTE ]
just because theres no solution for the rate at which the wheels should be rotating, doesnt mean there is some force impeding the plane.

[/ QUOTE ]

nice post.

[/ QUOTE ]

TYVM

[diebitter]

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There is a plane (Boeing 777, prop plane, whatever) on a moving, conveyor-type runway. The runway moves in the opposite direction of the plane at the exact same speed as the plane's wheels. There is no wind. Can the plane take off?

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Yes.

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Why is it that the wheels can't impart force to the plane, but the plane can apparently impart force to the wheels?

Hmm???????

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So... If the contacts between the plane and the wheels are all 100% frictionless, the wheels aren't attached to the plane at all. Is that the answer?