That is equally incorrect.
What’s difficult to visualize is that the path will behave as you intuitively think a flat one will with there being no uphill or downhill. If the hula hoop were frictionless the marble could be pushed and roll around forever. The problem with a flat model though is there would be no centrifigual force from moving in a circle which would couter gravity holding the mable to the earth side of the opening.
I think maybe we’re on the same team. The total energy is conserved. As you say in the underlined portion, the energy in the box is still the same, it’s just that some of it now unavailable to do work. The energy in the box is all at the same level so there is no energy difference to allow movement of the energy through any kind of machine so as to produce a useful output.
[Sub]Argghhh…joke potential high…but GQ behavioral regulators in place… Must… not…post <explodes in ball of flame> [/Sub]
Well, let it out. We can’t have you exploding as we need you.
Just to get the hijack over with, so we can put this thread back on track, I think astro was going to say something about Mr. Simmons and Ms. Persson both being attracted to the same segment of the populace. Or at least, so I presume: No offense intended to Mr. Simmons if I was mistaken in his case.
And now, for that back-on-track I promised, there’s nothing which inherently guarantees that entropy will increase. It just can’t decrease, and as a practical matter, it’s exceedingly unlikely to stay the same. In a true vacuum, two orbiting bodies would in fact continue orbiting forever. Except for that minor detail about gravitational waves. Well, one might be able to deal with that by making the entire Universe topologically compact in an appropriate shape. But then, whatever they’re made of would have some nonzero (though perhaps fantastically small) vapor pressure, so they’d outgass and produce a (possibly very tenuous atmosphere). Well, we could make the objects black holes. But even they produce Hawking radiation. OK, but since our space is already compact anyway, they’ll eventually re-absorb whatever they emit. Except that they’d be preferentially absorbed by the larger one, leading to an imbalance and eventual coalescence into a single hole…
You begin to see the problem.
A device like this could tap into the energy that the earth uses to rotate, thus it isn’t nessesarily a PMM or a violation of the laws of thermodynamics.
There are many sources of more or less “free” energy. Solar power for instance. For all intents & purposes Solar power is a type of PMM. However, we can easily see the source of the outside energy. Energy can also be generated from Tides, from Ocean Thermal Gradients, Geothermal, and many more. Most of these are not cost effective at this point in time. But we are in no dange rof 'running out" of power- it’s just that the cheap sources have already been exploited and in some cases (fossil fuels) will run out some day.
Quite a few of what seems to be PMM’s are really tapping into a source of ‘free’ and not easy to spot energy. For example, on Mythbusters they had a device that tapped into radio waves to generate energy. The amount was tiny. Still, it wasn’t a violation of the laws of thermodynamics.
No, the sun will not shine forever. Saying that soler power is a PMM is different only in degree from a car with a really, really big gas tank.
Energy/momentum is conserved because of the translation symmetry of time/space.
When studying potential perpetual motion devices (PPMD), always follow the energy. Account for it all and you have your answer. Details (like forces) will just get you in a muddled mess, because PPMD are always over-complicated contraptions.
Another way to look at it is that the known laws of physics do not permit the existence of a perpetual motion machine. To build a perpetual motion machine, you would need to exploit some natural phenomenon that is unknown to or misunderstood by physics at this time.
Having said that, I remark that the “Second Law of Thermodynamics” has rightly been characterized as “a dismal swamp of obscurity” by a preeminent mechanician and historian of science. It doesn’t even have a universally accepted mathematical formulation.
Not quite - you need to add “that produces external work” or perhaps define a machine as something that produces external work or energy. There is nothing in the laws of physics that prevents a body moving forever in empty space. A hydrogen atom perhaps can be visualised as a tiny machine with electrons circulating around a nucleus , and as most have been around for 16 billion years or so I think we can count them as perpetual.
Or maybe science friction?
I remember that Arthur C. Clarke wrote a science fiction story where a large grid was submerged deep in the ocean, with solar collectors at the surface. The temperature differential was used to generate electricity. Until the grids were torn apart by the colossal squid…Sorry, I just love that thread about the squid and want to keep it alive…
The qestion about perpetual motion had already been answered. I was explaining why the “ball on the top of the hill” idea was invalid. Then clairobscur came along and did it much, much better and a lot shorter.
You want perpetual motion? Hook a motor up to a solar cell and orbit it around the sun.
As was already pointed out, this is close enough to perpetual for pretty much all practical purposes, but it’s not a perpetual motion machine. It will only run until the sun goes out, and then, about 8 minutes later, it will stop.
Perhaps it would be beneficial to define what a perpetual motion machine is. A perpetual motion “machine” (or process) is a closed loop system in which the same process can be repeated indefininitely without any loss in usable energy. They don’t violate the law conservation of energy insofar as they don’t create excess energy, but they do come into conflict with the generally accepted notion that entropy always increases in a closed system. (Most so-called PMMs that function to generate excess energy are actually over-unity energy devices which do violate energy conservation.)
Entropy, it should be noted, is essentially a measurement of the amount of energy one has in a system that is not available to do work. By the second law of thermodynamics, you need both a high temperature reservoir (to draw from) and a low temperture reservoir (to dump to) in order to do work. If the state of your closed system is isothermal (all one temperature) then you have no way to direct the energy to effect work, regardless of how much energy you have in the system.
As Chronos notes, there is no rigorous general proof that entropy increases, although for many specific analytical cases and virtually all observations of the real world we see entropy increasing as a function of work done. Even on the quantum level, where time does not appear to have a fundamental direction, the limitations to how well you can determine the state of a system (and the increase in those limitations with the more interactions or observations you make with the system) suggest that entropy is a fundamental property on all levels and with all physical processes.
There are certainly energy sources available in the Universe that would provide energy sufficient for any need for the foreseeable future. But they are not perpetual; in thirty or forty billion years, as space continues to expand and accelerate, and protons start decaying, particles will no longer be able to interact with one another, there will be no energy left for utilization. Long before that, of course, the Sun (and other stars) will have burned to dwarfish husks or collapsed to singularities, and the galaxy may indeed collapse into its central hypermass or be ripped asunder by collision or gravitational disruptions. Nobody lives forever, and if they did they’d have a pretty boring time of it.
Stranger
Note the term “for all intents & purposes”. Nothing is really forever- the Universe will end someday, eith from “heat death” or being swallowed by black holes or something else. Thus, one can say “nothing is a PMM as the Universe will someday end”. But that is a pointless answer.