If you start an Apollo LRV on the Moon and let it go on flat terrain (with motor off), would it be stopped by ground friction eventually, or doesn't that stop a car? Same question for a train on rails on the Moon. Cars and trains can't be stopped by air resistance on the Moon. Other than by hitting something or going up a hill, would something stop the LRV or would it drive permanently at the same speed until an obstacle might brake it?
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12$\begingroup$ There is no such thing as a frictionless environment $\endgroup$– Carl WitthoftJul 10, 2020 at 11:50
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1$\begingroup$ @OrganicMarble I've adjusted the question. $\endgroup$– LoveForChristJul 10, 2020 at 14:01
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1$\begingroup$ There is no such thing as an absolute vacuum. The vacuum of space is better far away the lunar surface than close to it. $\endgroup$– UweJul 10, 2020 at 16:08
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1$\begingroup$ So you did understand there is no absolute vacuum directly above the lunar surface? In intergalactic space the density of gas atoms is very, very small but the probality to find an atom there is not zero, it is only extreamly small. $\endgroup$– UweJul 10, 2020 at 16:18
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1$\begingroup$ Your statement "there is: absolute vacuum" was written as a comment to a question about a lunar surface environment. $\endgroup$– UweJul 10, 2020 at 16:34
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3 Answers
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It still has the resistance of terrain against wheels (well, weaker than comparable terrain on Earth due to lower gravity - but then the terrain is pretty awful for driving), the same friction of bearings and so on - a car driven through loose sand on Earth will stop really fast due to the sand resistance, and not due to air.
Now if instead of a lunar rover, you use a maglev train on a superconducting track on the Moon, it could move for a long, long time because the resistances it encounters are minuscule. But a rover in the lunar dust? Nope.
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$\begingroup$ And the train, what resistances might stop it? Also the terrain friction? $\endgroup$ Jul 10, 2020 at 9:26
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9$\begingroup$ @LoveForChrist: Eddy currents (magnetic resistance( inside the frame and incurred on terrain, elasticity of the track bending under the weight of the train, tidal forces, light pressure, solar wind, the minuscule amounts of gases of Moon atmosphere, electrodynamic force when passing through the tail of Earth's magnetosphere, and a couple more minuscule forces I forgot the names of. Mostly the same kind of stuff that deorbits satellites. OTOH while the train wouldn't need energy, the track still must be supercooled and that's pretty expensive. $\endgroup$– SF.Jul 10, 2020 at 9:47
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1$\begingroup$ Finding comparable fine dust layers on terrain on the earth might prove difficult. Lunar dust is awful for driving. $\endgroup$ Jul 10, 2020 at 11:04
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3$\begingroup$ @Polygnome Indeed. For those not in the know, lunar dust is particularly bad for mechanical parts for 3 reasons: 1) the particles are very small, 2) they are electrically charged so they static cling to everything, and 3) they are very jagged due to the lack of erosion on the moon. $\endgroup$ Jul 10, 2020 at 19:44
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2$\begingroup$ A fascinating fictional example of this shows up in the ballistic subways of Heinlein's stories. Leveraging the vacuum of space to cut down on friction, he had subways which were basically bullets that flew at orbital speeds just under the surface of the planet. Not real, of course, but a neat alternative to maglev! $\endgroup$ Jul 11, 2020 at 1:38
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Cars and also lunar rovers are slowed down by wheel bearing fricition and rolling resistance of the wheels on the ground.
The rolling resistance of the wheels is reduced by a perfectly flat terrain with no dust but it will never be zero.
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$\begingroup$ That would depend on how the wheels and the entire car is built, wouldn't it? $\endgroup$ Jul 10, 2020 at 9:32
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$\begingroup$ There are wheel bearings with more or less friction but there are no zero friction wheel bearings. $\endgroup$– UweJul 10, 2020 at 9:35
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3$\begingroup$ lunar regolith is fluffy, driving in it compresses it producing friction between all the particles, and that's going to be a substantial loss as well. $\endgroup$– uhohJul 10, 2020 at 13:31
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The other answers are good, but let me offer a more general answer. If the train or car did go on forever (effectively orbiting the moon, but on land), then you would have discovered a perpetual motion machine. See here for why that's impossible.
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$\begingroup$ Usually perpetual motion machines are meant to be machines from which energy can be extracted without the motion ceasing. The Earth orbits the Sun, essentially forever (ok, in some point of time the Sun will enlarge to a larger diameter than the Earth's orbital diameter). Why isn't that a perpetual motion machine? Because if you extract energy out of the system, the motion is slowed down. $\endgroup$– juhistJul 11, 2020 at 9:31
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$\begingroup$ Somewhere along the way, diagrams started to be drawn with power output in the middle of the machine, and that's what has piqued human interest ever since. However, in the strictest sense, perpetual motion machines don't have to put out power, and they're still impossible. For the example you gave: The Earth would eventually hit the Sun, given a long enough time span. There truly is no such thing as perpetual motion. $\endgroup$ Jul 14, 2020 at 21:53