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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
Plasmana

12 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
Kiteman

12 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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kelseymh
kelseymh

Reply 12 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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NachoMahma
NachoMahma

Reply 12 years ago

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

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kelseymh
kelseymh

Reply 12 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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NachoMahma
NachoMahma

Reply 12 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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Goodhart
Goodhart

Reply 12 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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Lftndbt
Lftndbt

Reply 12 years ago

Oh, I always thought they were good...

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Goodhart
Goodhart

Reply 12 years ago

as good as puns can be I guess ;-)

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KentsOkay
KentsOkay

Reply 12 years ago

if your old and can understand the ;P

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Goodhart
Goodhart

Reply 12 years ago

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

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KentsOkay
KentsOkay

Reply 12 years ago

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

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Goodhart
Goodhart

Reply 12 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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KentsOkay
KentsOkay

Reply 12 years ago

Winking and sticking tongue out

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KentsOkay
KentsOkay

Reply 12 years ago

but I meant "them" not "the"

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Goodhart
Goodhart

Reply 12 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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KentsOkay
KentsOkay

Reply 12 years ago

Harhar, I do get that one...

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Goodhart
Goodhart

Reply 12 years ago

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

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KentsOkay
KentsOkay

Reply 12 years ago

*dashes to investigate*

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Goodhart
Goodhart

Reply 12 years ago

yes, I am familiar with it ;-)

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KentsOkay
KentsOkay

Reply 12 years ago

oops, you are

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Lithium Rain
Lithium Rain

Reply 12 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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KentsOkay
KentsOkay

Reply 12 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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Lithium Rain
Lithium Rain

Reply 12 years ago

*Runs to BBC site*

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whatsisface
whatsisface

Reply 12 years ago

Question answered.

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starwing123
starwing123

Reply 12 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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kelseymh
kelseymh

Reply 12 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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Goodhart
Goodhart

Reply 12 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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kelseymh
kelseymh

Reply 12 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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Goodhart
Goodhart

Reply 12 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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Kiteman
Kiteman

Reply 12 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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kelseymh
kelseymh

Reply 12 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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kelseymh
kelseymh

Reply 12 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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emuman4evr
emuman4evr

Reply 12 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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gimmelotsarobots
gimmelotsarobots

Reply 12 years ago

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

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guyfrom7up
guyfrom7up

Reply 12 years ago

wouldn't that seriously mess up the oceans?

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NachoMahma
NachoMahma

Reply 12 years ago

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

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Goodhart
Goodhart

12 years ago

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

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emuman4evr
emuman4evr

Reply 12 years ago

Well they sure take a long time to decay.

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Goodhart
Goodhart

Reply 12 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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NachoMahma
NachoMahma

Reply 12 years ago

. And that's a good thing. ;)

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Plasmana
Plasmana

Reply 12 years ago

Yeah.

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whatsisface
whatsisface

Reply 12 years ago

Yep. Galaxies are big old places.

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guyfrom7up
guyfrom7up

12 years ago

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