Posted on 06/15/2012 1:04:06 PM PDT by Mark Landsbaum
Actually, it would pass the center of the tunnel at maximum speed, then decelerate until it was almost to the far end (but not quite, because of frictional losses), and then if not stopped would begin to fall back toward the center.
Unaided, it would oscillate back and forth across the center of the tunnel, until all kinetic energy was dissipated due to friction, at which point it would (as you say) come to rest in the middle. But it might take a very long time.
With propulsion available, there would be no reason not to use a little of it during the trip so that it would make it all the way to the far end, where of course you would stop it.
For short tunnels, there would not be much pull from gravity, so you’d have to use mostly propulsion. But for long ones, you’d get a noticeable boost from gravity. How useful this amount would be, I have not bothered to calculate.
(Consider the angle of descent at the beginning of a loooong tunnel vs. a short one, assuming of course that they were straight-line through the earth.)
Of course this is all quite fanciful until they figure out how to make a thousands-of-miles-long vacuum chamber and transport people safely through it. This is 22nd century stuff, at best.
Oh. I thought that was the Bill Clinton Memorial Practice Gallery.
“well gravity trains are a bit different”
I’m not an engineer. I simply was referring to the concept of underground trains travelling many thousands of MPH. I gather that gravity trains rely exclusively on gravity for propulsion, limiting their speed so that LA-NYC would be 42 minutes instead of 30. The RAND train, I infer, relies on magneto-levitation propulsion, so it can go faster, but I presume that gravity must be doing a fair bit of the heavy lifting, so to speak. That is, if gravity alone can get you to high enough speeds to cross the country in 42 minutes, then the propulsion needed to speed it up even further to cross in only 30 minutes presumably is much smaller than trying to rely exclusively on such propulsion to achieve 10,000 mph.
Of course, gravity is an intrinsic feature of nature that has to be overcome/exploited by any transportation system over long distances, so perhaps the distinction I’m trying to draw is meaningless. But I guess my point is, any system that relies on trains running on the surface of the earth—even if enclosed in a tunnel to minimize air pressure/wind resistance—has an inherent disadvantage over a system that tunnels through the earth and hence can exploit gravity effects during the first half of the journey. It seems to me that a train running on the earth’s surface has to rely entirely on propulsion to get its speed, whereas a tunnel through the earth can use gravity to its advantage etc.
Again, I’m no engineer or hard scientist, so perhaps I have completely bollixed the truth of the matter in my description above.
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