Inertia and Perpetual Motion.

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?


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.

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.

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

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.

There are different types of perpetual motion. 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.

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.

Ummmmm - scr4 said the F-word …

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 anyway. But it wasn’t perpetual motion then, either.

And it was nowhere near flying fast enough.

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.


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.


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.


Not even 1%. Wow.

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.

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.

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.