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Perpetual Motion Answered

If perpetual motion is theoretically impossible, how is the earth still in orbit? Or how is anything in orbit? There falling at the rate at which the object there falling towards is rotating and they'll never stop rotating or falling so isn't that perpetual motion?

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Plasmana

9 years ago

About the magnets, how do they work? And their magnetic field, are they a continuous stream of energy oscillating through the magnets or it is "fortified" energy?

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Kiteman

10 years ago

All of the orbits we observe are changing, as are the rotational periods.

The Earth's day used to be only 16 hours long, but interactions with the Moon and Sun have slowed us down. Even friction between the oceans' currents and the coastlines is robbing the planet of rotational energy.

The Moon is receding from the Earth at approximately 4cm per year. Earth is receding from the Sun (I forget how quickly). The tiny gravitational twitches inflicted by the planets by each other render orbits chaotic, so that it becomes impossible to predict orbits precisely for more than the next few thousand years.

Looked at on a long enough time-scale, our steady, constant Solar System is, in fact, wildly unstable, with planets and Moons teetering on the cusp of being hurled into the Sun, flung randomly into interstellar space, or smashed haphazardly into each other.

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kelseymhKiteman

Reply 10 years ago

I have just one minor cavil with your discussion. The many-body interactions render the Solar System chaotic on time scales of millions of years, not thousands.

The non-perpetuality of orbits is not dependent upon the bodies' having complex internal structure, as your discussion about tidal friction assumes. A system of two perfect point masses (e.g., two black holes) in orbit has a dipole moment. Consequently, the system will radiate gravitational waves and slowly but surely lose energy, eventually spiralling inward and coalescing.

For a system like the Earth and Moon, the amount of gravitational radiation is immesurably small, and the consequent decay time from this process is probably at least as long as the lifetime of the Solar System, if not the Universe. For a binary pulsar system, that isn't true, and the lifetime is just a few hundred million years.

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NachoMahmakelseymh

Reply 10 years ago

. "Minor cavil" is redundant. Quibble, quibble, quibble. . But, seriously, thanks for the clarification.

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kelseymhNachoMahma

Reply 10 years ago

Cool! Have you really been around long enough to be a proper Usenet Grammar Nazi? Oh, look! I just launched a flamewar by bring Hitler into the discussion, just like in the Good Old Days :-)

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NachoMahmakelseymh

Reply 10 years ago

. ROFLMAO . Actually, Kiteman is The Grammar Nazi (and Master YEC Hunter). Goodhart handles the bad puns (with plenty of help from caitlinsdad). Adrian monk and jessyratfink show us the female perspective. I just stir up hate and discontent.

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GoodhartNachoMahma

Reply 10 years ago

I beg to differ, my puns are as good as puns can be, although I am not normally into begging ;-)

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LftndbtGoodhart

Reply 10 years ago

Oh, I always thought they were good...

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GoodhartLftndbt

Reply 10 years ago

as good as puns can be I guess ;-)

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KentsOkayGoodhart

Reply 10 years ago

if your old and can understand the ;P

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GoodhartKentsOkay

Reply 10 years ago

I am, and can understand :P as well as (::()::) for your booboo lol

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KentsOkayGoodhart

Reply 10 years ago

hehe, I was just teasing, but thanks for the band aid!

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GoodhartKentsOkay

Reply 10 years ago

I was having difficulties understanding the link between being old (having seen the START of the Web) to the ;P ? :-)

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GoodhartKentsOkay

Reply 10 years ago

Ah, that clears it up, you were referring to your NOT being old enough to be able to relate to some of my pun material......got cha :-)

Like, you could get your own Theodore Roosevelt action figure put him in skimpy woman's underwear, and you would have a Teddy in a Teddie ;-)

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GoodhartKentsOkay

Reply 10 years ago

Yeah, especially if you know about his reputation ;-)

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Lithium RainKentsOkay

Reply 10 years ago

Lol

BTW, I'm digging the new avatar. So awesome! I need more Doctor Who. I've only seen two episodes and I think I'm hooked.

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KentsOkayLithium Rain

Reply 10 years ago

Thanks, it's just temporary, you could call it my "costume". I got it from the BBC website, they have a plethora of Doctor Who avatars...

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starwing123kelseymh

Reply 10 years ago

Perpetual motion isn't possible. Orbits lose energy, but slowly compared to our time frame. Satellites have to be re-boosted once in a while to keep them from crashing into Earth. Also, perpetual motion isn't possible because one form of energy turns into another form, so you can't really make an 100% efficient machine. I still have a question though, what if you fire a bullet into space with nothing to block or deter it. Wouldn't that be perpetual motion?

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kelseymhstarwing123

Reply 10 years ago

Ah, so you've touched on the underlying issue here. "Perpetual motion" as commonly used by pseudophysics crackpots, really refers to devices which violate thermodynamics, such that you extract work out of them indefinitely. That is simply impossible (you can't win, you can't break even, and you can't quit playing).

"Perpetual motion" in the proper technical sense, is just what your last question suggests: Newton's first law of motion. If you were able to find a truly perfect vacuum (no residual matter at all, not even the one atom per cubic meter in the intergalactic medium), then yes, an object you pushed away would just keep going and going and going.

In real space, there's still residual gas and dust (a few 10-9 torr within the solar system, a few atoms per cubic centimeter in interstellar space far from molecular clouds, and so on). There will still be some amount of drag slowing down your bullet, albeit ever . . . so . . . s...l...o...w...l...y.

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Goodhartkelseymh

Reply 10 years ago

really refers to devices which violate thermodynamics, such that you extract work out of them indefinitely.

Oddly enough, some of Hawking's meandering propose a universe with properties that have no beginning nor end, a kind of an avant garde perpetual motion astronomical event, as it were :-)

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kelseymhGoodhart

Reply 10 years ago

Yes, indeed! Of course, if the Universe is infinite, then thermodynamics as such (even extended non-equilibrium thermodynamics) may not apply.

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Goodhartkelseymh

Reply 10 years ago

may not apply :-)
I will hold off forming an opinion, until I can say "they do or do not apply more definitely :-) Nothing against using an imagination, but it may just be as possible for unicorns to have existed too (after all, many many animals became extinct leaving no trace).
Ooo, sorry, I don't want to go off on another one of my tangents I tend to veer off the beaten path way too many times *sigh*

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Kitemankelseymh

Reply 10 years ago

The chaotic nature is on the millennia-scale. Whilst astronomers will be able to predict the radius of the Earth's orbit in that time, they cannot predict exactly where in that orbit the Earth will be.

I know orbiting objects lose energy no matter what, but having a fluid or semi-fluid structure makes the losses much more rapid, and much more significant than simple gravitational radiation.

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kelseymhKiteman

Reply 10 years ago

There are simple counterexamples to your first statement. Stonehenge, the Pyramids, various Mesoamerican observatories. In astronomy "predict" is nicely time-reversal-invariant activity. If a system is chaotic, then you can neither propagate the equations of motion into the future nor into the past. The position of the Earth along its orbit determines the relative positions of the Sun and stars in the sky, as seen by an observer, and it determines when during the day (night) the various stars/constellations rise and set. If Earth's orbit were unpredictable on a timescale of millenia, then we would not be able to relate worldwide megalithic sites to astronomical or seasonal observations at the time of their construction.

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kelseymhKiteman

Reply 10 years ago

The Wikipedia writeup is pretty good.

The 3:2 mean-motion resonance of Neptune and Pluto means that they become unpredictable on a timescale of 20-30 million years. The inner solar system is free of mean-motion resonances, and so the Lyapunov time for e.g. the Earth's orbit is about 100 millon years (see Laskar's work mentioned in the Wikipedia article).

For asteroids and comets, the situation is much worse, and millenia is a reasonable order of magnitude estimate.

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emuman4evrKiteman

Reply 10 years ago

I knew Kiteman was going to be all over this. Although I seem to like goodhart's simple answer but I appreciate the lecture.

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gimmelotsarobotsKiteman

Reply 10 years ago

Well If we're moving away I guss that makes global warming good.

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guyfrom7upKiteman

Reply 10 years ago

wouldn't that seriously mess up the oceans?

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NachoMahmaguyfrom7up

Reply 10 years ago

. Not as badly as it messes up the coastlines. ;)

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guyfrom7up

Reply 10 years ago

oh, and none of that physics crap, because we only except it as fact since no one has been able to oppose it. It use to defy the laws of physics to fly. Now whos laughing? I'll tell you who's not. People waiting at the airport. HA!

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Goodhartguyfrom7up

Reply 10 years ago

except it

Sometimes we even accept it ;-) just teasing a bit

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kelseymhguyfrom7up

Reply 10 years ago

It never "use [sic] to defy the laws of physics" to fly. It defied the engineering assumptions of the day. Engineers assumed they had to build a device that worked the way birds worked, rather than in a different (bouyancy or fixed airfoil) way. Later they assumed that anything they built would be heavier than the lift they knew they could get from airfoils.

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Goodhart

10 years ago

All orbits are decaying orbits.....they are not forever (perpetual)

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emuman4evrGoodhart

Reply 10 years ago

Well they sure take a long time to decay.

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Goodhartemuman4evr

Reply 10 years ago

Orbits can take awhile to decay, but they don't always do so (take our man made satellites for example).

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guyfrom7up

10 years ago

BTW I spent a long time on perpetual motion, my favorite is the out of balance wheel