Talk:Gravity train

message: The error has now been corrected (see previous version of talk page).--New Thought 10:16, 6 September 2005 (UTC)
thread_title: Error

message: How does the earth's rotation affect the line of travel? Particularly when the train would travel east/west as opposed to directly north/south.
207.245.73.241 13:53, 9 October 2006 (UTC)
thread_title: Earth's rotation

message: The comment "Evacuating the air to eliminate drag would require additional power." seems to over-state the difficulty of such a task. Once an extremely strong sealed pipe has been 'laid' through the earth, evacuating it and providing airlocks at each end seems a relatively trivial engineering feat. In fact it would probably be required to avoid the immense air pressure at the bottom of the hole (or even to stop it liquefying?).
I assumed, but didn't know the Gravity Train concept had been thought of before. As a kid I used to think about such things. I only considered the hole through the middle of the earth, the entire journey would be weightless. It seems to be on the limits of physical practicality, the main problem being the immense pressure (and currents), not the temperature which is (probably) much more reasonable. Perhaps someone could expand on just how immense this pressure is.
It's similar to my low earth orbit idea, where an evacuated pipe is laid on or under the sea, and vehicles are accelerated to mach 22 (or whatever LEO speed is) inside this pipe, where they will levitate naturally. It would take the same time to get to the other side, and assuming the energy is recovered, also lossless (and weightless of course). Again, not the safest form of travel. --Adx 01:31, 23 October 2006 (UTC)
replies: text: I don't think the trip would be weightless, unless the trip was directly through the centre of the world. The gravity vector would be on an angle which could be resolved into two forces only one of which would provide forward motion, the other would still act perpendicular to that motion. The ratio of those forces would make you weigh less, but not weightless. Vespine 02:47, 8 February 2007 (UTC)
thread_title: Vacuum

message: The non theoretical gravity train also needs an entry.  It was used a lot in the 1800's a horse would pull several cars to the top of a hill, it would be loaded with cargo and the train would coast down the hill with a brakeman or two (and the horse - see Dandy waggon).  Does anyone have thoughts on if both should be discussed in this article or if they should have separate articles and if separate how to handle the disambiguation?  Jeepday 03:59, 19 November 2006 (UTC)
replies: text: I think they should have separate articles. Probably call your article on the other practical one "Gravity train" because that seems to be the definite name of it. But I dunno what the best name for this theoretical article is. Took me a while to find that this article actually existed. As for disambiguation, all we'd need is links at the top of each article. There are templates, have a look at Wikipedia:Disambiguation. Vadmium 14:27, 17 June 2007 (UTC)
thread_title: The other Gravity Train

message: I just thought I would add an external links section
will add this link
http://www.damninteresting.com/?p=696
--Mrebus 19:46, 17 October 2006 (UTC)
thread_title: External links

message: Since gravity is a function of mass and distance from the center of gravity of that mass, an interesting thing would happen at the center of the earth: If you could hollow out the center, and position yourself in the hollow space, you would be pulled simultaneously in all directions, and float weightless.
What I'm wondering, is whether the acceleration assumed by the takes this into account. We think of acceleration due to the earth's gravity to be 32 feet/second/second, but this is not constant. As you proceed toward the center of the earth, the mass above you increases, and begins to offset the gravitational pull of the mass below you. As a result, the rate of acceleration will decrease over time.
Any thoughts on this?
hubby2debbie--Hubby2debbie 03:36, 5 December 2006 (UTC)
thread_title:  Important Fact Ignored? 

message: Several articles, such as Moon and Lunar_Laser_Ranging_Experiment both say the moon has a liquid core. -72.87.188.204 19:15, 11 February 2007 (UTC)
replies: text: Point taken.  Can anybody think of a substitute? Maikel (talk) 13:18, 23 May 2009 (UTC)
thread_title:  Solid moon 

message: Robert Hooke and Wes? Wes who? —The preceding unsigned comment was added by 66.27.69.167 (talk) 04:05, 9 March 2007 (UTC)
thread_title:  "and Wes"? 

message: Quote:  All straight-line gravity trains on a given planet take exactly the same amount of time to complete a journey (that is, no matter where on the surface the two endpoints of its trajectory are located). For Earth, this time would equal 42 minutes and 12 seconds if it were a perfect sphere.
Where does Friction come into this? For this example, assume friction is measured in units of Kevin (Kev), the amount of friction required to decelerate one clothed, 100kg human 1m/s. If System A has a friction of 25 kKev over the entire route, and System B has a friction of 30 kKev, doesn't system A run faster?  —Preceding unsigned comment added by 24.205.50.170 (talk) 10:41, 10 October 2008 (UTC)
replies: text: I'd guess that neither system would run very fast. Not sure how you're relating your Kevin unit to mass, speed, body shape, viscosity, etc. but maybe it's just intended to be a kind of qualitative idea? 25 or 30 kKev would slow a human being down by 25 or 30 thousand meters per second (over some period of time, hopefully not 1 second) Friendly Person (talk) 03:12, 10 April 2014 (UTC)
thread_title: Friction

message: "All straight-line gravity trains on a given planet take exactly the same amount of time to complete a journey (that is, no matter where on the surface the two endpoints of its trajectory are located). For Earth, this time would equal 42 minutes and 12 seconds if it were a perfect sphere." Is the first sentence true even if the planet is not a perfect sphere? --NE2 06:42, 27 October 2008 (UTC)
replies: text: No. Divide the height variation by the peak speed, and you'll get the time variation, or at least that's my first guess as to how to calculate it.  Jim.henderson (talk) 02:50, 1 November 2008 (UTC)
thread_title:  Perfect sphere? 

message: I've removed this argument:
<blockqote>
Another objection (argument) could be that the portions of the planet which are at the sides of the tunnel (see graphic annimation above) would also exert its gravitational pull on the sides of the train,which would be in opposite directions, thus slowing down the train.  In a graphic as shown above we are guided by our concept of UP and DOWN. But as far as laws of gravity are considered there is no up or down.  There is only gravitational pull of earth inside the tunnel from all the directions which has to be considered.  It is possible that the train may come to a halt much before it reaches the centre.  It may go on moving only as long as the mass and gravity of the earth in the direction in which it is moving is strong enough.  After that it may come to a halt.
</blockquote>
It seems to be based on a totally confused opinion about how one calculates the total force of gravity, as well as misunderstandings about how friction works; I thought about adding "citation needed", but decided to WP:BB and just remove it. --Alvestrand (talk) 06:09, 16 June 2013 (UTC)
thread_title:  Removing argument about "pull to the side" 

message: Please mention and cross reference Concrete Gravity Trains used for energy storage. See, for example: http://interestingengineering.com/concrete-gravity-trains-may-solve-energy-storage-problem/  Thanks!  --Lbeaumont (talk) 18:20, 20 August 2017 (UTC)
replies: text: Although it has a similar name, this article is about a specific type of train that uses gravity to travel through a tunnel deep below the surface of the Earth. ARES is already mentioned at Energy_storage#Mechanical_storage. Dlthewave (talk) 18:44, 20 August 2017 (UTC)
thread_title:  Please mention and cross reference Concrete Gravity Trains used for energy storage 

message: Based on ideal gas law and constant temperature at 15°C as well as an Earth with uniform density, my estimate of the air pressure at the center of the Earth is about 10164 (1 followed by 164 zeros) times that of the atmospheric pressure at the Earth's surface. Apparently, this number is too large to be realistic, but it implies that the pressure in the tunnel would be so large that the air would probably be in liquid or even solid or some supercritical state. (Would someone make a more realistic estimate?) This would make the environment for the digging impossibly difficult. --Roland (talk) 21:13, 19 March 2019 (UTC)
thread_title:  Atmospheric Pressure in the Tunnel 

message: I can envision a variant where something like subway cars are used, in short, non-evacuated tunnels, and the initial angle is just a bit more than needed to overcome rolling friction and air resistance.  The tunnel could then level off, near the bottom, allowing the trains to coast to a stop at a station there, where the passengers could switch trains.  Power would be needed to lift them back up to the surface from there, for the 2nd half of the trip.  The advantage would be only needing to provide power for half the trip.  Presumably more energy would be needed on the trip back up, than in a traditional subway, and the total would be comparably to a regular subway, but there is the initial cost savings of not having to lay down a third rail or other power source on half the tunnel, and the need for braking could be greatly reduced for both halves, perhaps saving a bit of energy there.  So, is there such a concept ?  And is this within the scope of this article ? SinisterLefty (talk) 15:59, 22 March 2019 (UTC)
thread_title:  Less ambitious, more practical versions ? 

message: I think this article needs to be rewritten, either to describe "a gravity train as a theoretical means of transportation", which is what the introduction says, or to describe the construction of such a means of transport. At the very least, the title of the first section should be changed to something like "Practical Engineering Difficulties" and it should be moved to the end of the article after the concept has been defined in detail. It is jarring to say the least to find that the first section "objects" to a perfectly valid, and interesting, "theoretical means of transport". Some diagrams would help as well.81.154.119.34 (talk) 11:38, 15 November 2019 (UTC)
thread_title:  Comment