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Old 03-07-2006, 06:42 AM
zut zut is offline
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Join Date: Apr 2000
Location: Detroit, MI
Posts: 3,725
Originally Posted by David Simmons
I think it's a lousy thought experiment. Such experiments are those that you can do theoretically but not practically. I don't think you can even do this one theoretically.

Take the case where the only force opposing the engine thrust is the acceleration of the wheels. In order to stop the plane indefinitely you must constantly accelerate the wheels and that is not possible.
Why not? The realistic limiting factors that I can think of are: a) material limitations of the tire and bearings and belt and so forth, b) velocity limit of the motor or whatever power source moves the treadmill, c) compressibility effects (sonic shock waves) when the conveyor exceeds the speed of sound, and d) speed of light.

To turn this into a thought experiment, I'd be comfortable ignoring all of these (and Chronos pointed out that the last one--the speed of light--isn't a problem).

In any case, a thought experiment is useful in exploring the relative magnitudes and effects of various causes. To my mind, this airplane question is similar to "what happens when you approach the speed of light?" When you ask that question, you don't get hung up on how long it takes, or what kind of vehicle you travel in, or where the fuel comes from, or any of those kind of questions.