oh lord help me: Is orbit perpetual motion?

Ok ok ok call off the dogs. I realize that perpetual motion machines are impossible and most likely always will be but this thought just occured to me and I can’t figure out why it wouldn’t be considered perpetual motion.

Orbit is basically just falling around the Earth over and over again, powered by gravity, right? So after the initial investment of energy to get yourself up there you would stay your course indefinately without any other outside power source, right?

Can someone explain why this isn’t perpetual motion? :confused:

The tidal forces of any orbiting bodies suck energy out of the system.

Only orbit of point bodies could be perpetual motion.

Well not out of the system but turn it into heat. That is why the moon has been slowly pulling away from the Earth for the past several billion years. That is why many moons (and Mercury) have fallen into an orbit-rotation ratio where one side is always facing their more massive partner.

Ok this is where you lost me.
What is the tidal force of an orbiting body?

What is a point body?

Sorry, I just got up so my brevity is a little high.

The force of gravity is based on distance. So if something if further away it has a smaller force pulling on it then something closer. So for the moon and the Earth the point closest to each other expereinces a larger pull then the point furthest from the other resulting in a stretched bodies.

Now the deformation of the Earth and Moon are pretty small but it still takes energy to make. And that energy comes from the orbital energy of the system and becomes heat. And that is what keeps orbiting bodies like the Earth and Moon or The Sun and Mercury from having perpetual motion.

Now for point bodies where there is no difference in distance from one part of the body to another it can occur. A point body is something where all the mass exists in one point. So it’s one dimensional and there can never be any difference in the force of gravity for that object. Things like electrons and their opposite positrons are supposed to be points. That’s still true, right anybody?

It is my understanding that although the orbiting body appears to be in perpetual motion, you could not use that motion to do any useful work (otherwise you would be taking energy from the body so it orbits at a lower altitude). Therefore, no laws of thermodynamics are being violated.

Tidal force is basically the forces caused by gravity that cause objects to deform. Deformation in the body(planet/moon) uses energy(creates heat). This energy comes from the movement of the two bodies. What happens to the bosied then probably depends on the system. It sounds like the moon is slowing down and moving away from the earth.

A point body doesn’t exist. They are mathematical entities that simplify calculations. It means you get to assume all mass of a body is located at an infinitely dense point in the middle. Since you can’t have deformation of a single point, you can’t have any losses caused by tidal forces.

BTW, some one will be along shortly to contest my claim that point bodies don’t exist. When they do, ask them to show you a picture of one.

Damn!! No way I can compete with the speed of OSIRIS.

And for good reason. How about this?


From the article you sited:
“If they are indeed both point particles, the question does not apply. If they are not point particles, the answer is that I do not know…”

Sounds like he leaves the door open for arguement.

Gotta picture??:stuck_out_tongue:

wouldn’t a quantum singularity be a point particle?

Getting back to the OP, isn’t a “perpetual motion machine” generally considered to be one you can take more energy out of then you put in? What the OP is describing sounds different; an apparent (but not really) frictionless machine.

I usually go with the dictionary.

So yes, the OP is about an apparently frictionless machine. But it isn’t really frictionless unless:

a) the bodies are perfectly rigid, so as not to lose energy to tides, and
b) space is a perfect vacuum.

(a) and (b) do not hold in reality, so it isn’t perpetual motion.

Now, in real life, of course there are tidal forces to worry about, and there’s no such thing as a perfect vacuum and all complications do arise.

However, the Second Law of Thermodynamics doesn’t just apply to the real world. It also applies to thought experiments. And in such experiments, you can consider idealized systems where these complications can be ignored. So this is a valid question, and it addresses the spirit of what the OP is asking:

Why can’t an idealized two-body system undergoing orbital motion be considered perpetual motion?

And of course the answer is that it can, because it is perpetual motion. However, as bobk2 points out, you can’t get any net work out of it, and so it doesn’t violate the Second Law. There’s nothing wrong with perpetual motion per se, but that’s the limit. If you have a system stays the exact same speed, no problem. If you have a system that gets slightly faster and faster, violation.

Beatya, Archenar.

Well, yeah, if I had seen your post, I probably would have cited you in my first paragraph, but I wouldn’t have changed what I said. :smiley:

Just to complete the moon story, and to do that you need to introduce the concept of the conservation of angular momentum. The reason the moon is slowly receding from the earth is that the sum total of angular momentum for the earth-moon system must be constant. Since the earth’s rotation is slowly decreasing, there is angular momentum that must be gained elsewhere, and the only way for the system to achieve this is for the moon to move farther from the gravitational center of the earth-moon system.

MSU 1978, why is the earth’s rotation slowly decreasing?

The Earth’s rotation is slowly decreasing due to the friction of water in the ocean. As the Earth spins, the water wants to stay still, hence there is friction. Not much, but it’s there. See this page http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html

Yep the “perpetual motion machine” for which the basement inventors strive is really a PERPETUAL ACCELERATION machine. It only moves constantly if you slap a mechanical governor on it, or give it a frictional load.

Perpetual motion is trivial: look at quantum motion at the micro scale, or big spinning rocks which are far from any solar system. That’s not what they’re talking about.

If anyone stumbled across a real perpetual motion machine (otherwise known as an anomalous energy source,) the machine wouldn’t just move by itself. If they were lucky, it would only speed up maniacally and fly apart. If unlucky, the energy output wouldn’t be so limited, and the neighbors would be mystified by the large smoking crater where the crackpot’s garage once stood.


I think there was a Heinlein short story about something similar. A weird circuit which had been repeatedly discovered, but which always killed its builders until someone first discovered it theoretically and realized what would happen if they tried to build an uncontrolled version.