# Inertia and Perpetual Motion. **URL:** \ **Category:** Factual Questions\ **Created:** [October 19, 2016, 12:51pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020 "2016-10-19T12:51:15Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Jim\_B](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jim_b/32/4338_2.png) [@Jim\_B](https://boards.straightdope.com/u/Jim_B)\ **Post date:** [October 19, 2016, 12:51pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/1 "2016-10-19T12:51:15Z")
_Objects at rest tend to stay at rest._ _Objects in motion tend to stay in motion._ Every high school physics class teaches you those two rules. That is why the earth will continue revolving around the Sun basically forever. And it certainly sounds like perpetual motion to me. Where am I wrong? :):)🙂 ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [October 19, 2016, 1:01pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/2 "2016-10-19T13:01:06Z")
By itself it is perpetual motion. Note that perpetual motion machines are usually described to generate excess energy, not just keep running forever. However the rules you quote only apply if there is no external influence. And your planet-sun pair isn’t running is isolation. It is embedded in space-time, and that spacetime has some additional rules. What will eventually run things down is that the motion is distorting space-time, and that distortion is radiating energy away as gravity waves. Eventually, long after the sun has gone cold and pretty much the rest of the universe along with it, the pair will have radiated all the energy away and it will stop. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [October 19, 2016, 1:08pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/3 "2016-10-19T13:08:03Z")
> [@Jim\_B](#): > > the earth will continue revolving around the Sun basically forever. For values of “forever” that include “until it’s about twice its current age”. That’s roughly when Earth is destroyed by an expanding sun. Which is not the end of all orbital motion - just this example. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [October 19, 2016, 1:12pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/4 "2016-10-19T13:12:20Z")
Well you learned it cincompletely. 1. In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, \*unless \*acted upon by a net force ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [October 19, 2016, 1:16pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/5 "2016-10-19T13:16:08Z")
> [@Jim\_B](#): > > That is why the earth will continue revolving around the Sun basically forever. And it certainly sounds like perpetual motion to me. Tides formed by the gravitational interactions pull rotational energy from the system and will eventually cause the motion to stop. And the trick isn’t perpetual motion - an object moving in space essentially gets you that. The trick is perpetual motion while getting work out of the system. In the earth-sun system, that work is in the form of tidal energy (eventually radiated away as heat) which is in fact slowing the system down. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 19, 2016, 1:29pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/6 "2016-10-19T13:29:55Z")
There are [different types of perpetual motion](https://en.wikipedia.org/wiki/Perpetual_motion#Classification). A **perpetual motion machine of the third kind** is one that keeps going because nothing stops it - no friction or resistance. This does _not_ violate the laws of thermodynamics, so it _is_ possible. A planet revolving around the star falls under this category. But usually, the term “perpetual motion” refers to the **perpetual motion machine of the first kind** , which does _work_ without any energy input. Which means you can extract energy from it without putting any into it. This _does_ violate the laws of thermodynamics. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [October 19, 2016, 1:48pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/7 "2016-10-19T13:48:55Z")
> [@Grey](#): > > Well you learned it cincompletely. > > 1. In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, \*unless \*acted upon by a net force Normally we say “external force”, but that’s it. The earth is being acted upon by the gravitational pull of the sun, and is constantly accelerating. It’s not an example of inertia, where there would be no acceleration. Still, the orbit will eventually decay. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [October 19, 2016, 2:05pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/8 "2016-10-19T14:05:04Z")
Ummmmm - **scr4** said the F-word … ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [October 19, 2016, 2:24pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/9 "2016-10-19T14:24:29Z")
As a kid, I had the bright idea of a perpetual motion machine: Solar powered airplanes! Just fly them fast enough that they stay under the sun, badabing badaboom, perpetual motion! It took me a few years to figure out what was wrong with that idea. And then earlier this year [somebody did it](http://www.usatoday.com/story/tech/2016/07/25/solar-powered-global-flight-set-complete-final-leg/87530736/) anyway. But it wasn’t perpetual motion then, either. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [October 19, 2016, 3:21pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/10 "2016-10-19T15:21:14Z")
> [@DrCube](#): > > As a kid, I had the bright idea of a perpetual motion machine: Solar powered airplanes! Just fly them fast enough that they stay under the sun, badabing badaboom, perpetual motion! > > It took me a few years to figure out what was wrong with that idea. And then earlier this year [somebody did it](http://www.usatoday.com/story/tech/2016/07/25/solar-powered-global-flight-set-complete-final-leg/87530736/) anyway. But it wasn’t perpetual motion then, either. And it was nowhere near flying fast enough. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [October 19, 2016, 3:32pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/11 "2016-10-19T15:32:11Z")
When I was a kid, I learned that a motor converts electrical energy to mechanical energy, and a generator converts mechanical energy to electrical energy. I had the brilliant idea to hook a motor to a generator, and presto, perpetual motion! I later learned what was wrong with this idea, and also that pretty much everyone who learns enough comes up with this thought at some point. –Mark ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [October 19, 2016, 3:59pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/12 "2016-10-19T15:59:54Z")
> [@John\_Mace](#): > > Normally we say “external force”, but that’s it. The earth is being acted upon by the gravitational pull of the sun, and is constantly accelerating. Correct. There is a HUGE amount of force acting on the Earth to keep it in orbit. It is not in any way perpetual motion. If the Sun somehow vanished, the Earth would adios for the stars at 67,000 mph. Dennis ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [October 19, 2016, 4:14pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/13 "2016-10-19T16:14:58Z")
> [@markn\_1](#): > > When I was a kid, I learned that a motor converts electrical energy to mechanical energy, and a generator converts mechanical energy to electrical energy. I had the brilliant idea to hook a motor to a generator, and presto, perpetual motion! When we were designing a solar source to test the Space Station solar array in the early 1990s someone mentioned the output of the arrays could be fed back into the lights to cut down on the power requirements. They were kidding, but the calcs are easy. Power supply: 20,000 watts. Visible light output: 1600 watts Solar array output: 150 watts. Dennis ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 19, 2016, 5:39pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/14 "2016-10-19T17:39:33Z")
Not even 1%. Wow. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 19, 2016, 6:26pm UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/15 "2016-10-19T18:26:20Z")
> [@](#): > > When we were designing a solar source to test the Space Station solar array in the early 1990s someone mentioned the output of the arrays could be fed back into the lights to cut down on the power requirements. They were kidding, but the calcs are easy. If you’re just doing it for testing purposes, sure, that could indeed be useful. Maybe it’s not enough to be practical, but it’s still something. It’s only when you try to install such a system on the ISS that the laws of thermodynamics bite you in the butt. > [@](#): > > Quoth **Francis Vaughan** : > > By itself it is perpetual motion. Note that perpetual motion machines are usually described to generate excess energy, not just keep running forever. However the rules you quote only apply if there is no external influence. And your planet-sun pair isn’t running is isolation. It is embedded in space-time, and that spacetime has some additional rules. What will eventually run things down is that the motion is distorting space-time, and that distortion is radiating energy away as gravity waves. Eventually, long after the sun has gone cold and pretty much the rest of the universe along with it, the pair will have radiated all the energy away and it will stop. Yeah, if nothing else, gravitational radiation will eventually kill an orbit… but there will almost always be something else first. Gravitational radiation is _slow_, especially for a weak-field situation like a typical planet orbiting a star. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [October 20, 2016, 1:19am UTC](https://boards.straightdope.com/t/inertia-and-perpetual-motion/769020/16 "2016-10-20T01:19:43Z")
> [@LSLGuy](#): > > Not even 1%. Wow. Yeah, the halogen projector lamps we were using put out: IR 90% UV 2% Visible 8% And the solar cells were 8-9% efficient. In case anyone is interested, here is the lamp bank I designed and the very first solar panel for the ISS in 1991. The panel was pretty much priceless at that point. It is being tested in NASA Lewis’ Tank 5. I’m the guy in the one photo along with my electrical engineer. The design specs called for an insolation of .33 suns with no more than 5% variation on any solar cell. I was able to get .50 at less variation then spec. > **[Photo Storage](https://s36.photobucket.com/user/mixdenny/media/1991_08000_D_Thompson_VF5_zpsqhbjthmd.jpg.html)** > > Store your photos and videos online with secure storage from Photobucket. Available on iOS, Android and desktop. Securely backup your memories and sign up today! > **[Photo Storage](https://s36.photobucket.com/user/mixdenny/media/1991_07998_D_Thompson_VF5_zpsevzrf9ga.jpg.html)** > > Store your photos and videos online with secure storage from Photobucket. Available on iOS, Android and desktop. Securely backup your memories and sign up today!