# Isn't gravitational orbit perpetual motion?

1. Jun 15, 2011

### Haroldingo

I was wondering... Suppose that the mass that was being orbited, in place of the earth, was stable and constant - then substitute the moon with another constant but suppose the orbit is the same as the mass and distances between them are the same - would/could this be an example of perpetual motion?

2. Jun 15, 2011

### Drakkith

Staff Emeritus
This doesn't make any sense. What does "substitute the moon with another constant" mean? Why do you even need to replace the earth and the moon in this scenario?

In any case, an orbiting body, ignoring any slight deceleration from impacts with atoms and subatomic particles in space, is "perpetual motion". There is nothing against that. The issue is with perpetual motion "machines". And even more so with machines designed to generat an outpute greater than their input while using no fuel. Stay away from either of those and you will be fine.

3. Jun 15, 2011

### Haroldingo

@ Drakkith , sorry, my wording was poor. I mean to say that over time the moon and the earth will deteriorate and therefore the orbit will change and eventually cease to exist. So by a constant I just mean something with the exact same properties apart from deterioration.

Ok I getcha. But would it not be possible to reproduce gravitational orbit on a smaller scale ?

If we ever manage to understand how gravity really works then it should be possible.. maybe even know with enough experimentation. The key to perpetual motion should be gravity, because if it's an accepted possibility as you said, then the fuel output of a replica should be next to 0? Considering that the earth doesn't run on fuel, nor the moon..

4. Jun 15, 2011

### rcgldr

"Perpetual motion" is somewhat of a misnomer applied to devices that can supposedly output more energy than input. It's also unusual for any real process to be completely free of any losses, since for example space isn't a perfect vacuum, so there is some drag on any object moving in space. A current flowing through a true superconductor would be perpetual motion, and such a device could be useful for storing energy, but not for producing energy.

5. Jun 15, 2011

### Drakkith

Staff Emeritus
The earth and the moon formed from the collapse of a nebula. This collapse of this mass of gas and dust and such gave it rotational speed that resulted in the current velocities of the planets. The formation of the moon was similar, except that it is theorized that a large object struck the earth, ejecting the material that formed the moon. This is where the initial energy came from for the planets and moons to stay in their orbit. It is not infinite and does not require a continual expediture of energy.

The key to perpetual motion is the elimination of all resistance to said motion. There isn't any real benefit other than storing energy in something like maybe a flywheel or something.

6. Jun 15, 2011

### Pengwuino

Gravitational orbits are probably not even perpetual motion machines in the sense that Drakkith is mentioning. Orbiting bodies probably give off gravitational radiation which would cause the orbits of the planets to eventually cease and cause a collision.

7. Jun 15, 2011

### treehouse

A stable atom is a perpetual motion machine - its electron(s) perpetually orbit its nucleus.

8. Jun 15, 2011

### Pengwuino

Although quantum mechanically, orbits don't even make sense to talk about.

9. Jun 15, 2011

### Drakkith

Staff Emeritus
An atom is not a machine, nor are electrons similar to little planets in orbit. The wave function of the electron is more like a standing wave I believe. (Or so my illustrated guide to quantum physics book tells me. It's got pictures!!)

10. Jun 15, 2011

### treehouse

Whatever: the electron(s) in a stable atom will perpetually keep moving the way they move or those waves we represent as electrons will perpetually do what they do in a stable atom.

11. Jun 15, 2011

### Pengwuino

That's not how atoms work.

12. Jun 15, 2011

### treehouse

How is that not how atoms work?

13. Jun 15, 2011

### pallidin

Why do some think that the moon orbiting the earth is somehow an example of "perpetual motion" ??
Eventually(a long, long time) the moon will impact our earth.

14. Jun 15, 2011

### Pengwuino

Protons and electrons are not these pointlike particles that orbit each other. If they were, the electron would radiate away and almost instantly fall into the proton. Bohr's theory/quantum mechanics addresses the fact that both particles are not point-like orbiting bodies like planets.

15. Jun 16, 2011

### Drakkith

Staff Emeritus
The issue here is where most people use the word machine.

Machine: A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, a molecular machine and a biological machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work. A simple machine is a device that transforms the direction or magnitude of a force.

A stable atom simply sitting there is NOT a machine. You can make it into a machine, but to get work out of it you have to input energy. Perpetual motion by its very definition cannot apply to a machine and its output.

16. Jun 16, 2011

### Ryan_m_b

Staff Emeritus
Perpetual motion as in moving in perpetuity is not the same as what is meant by the term "perpetual motion machine".

The idea behind a PMM is that a circular system is constructed (i.e. it has components that interact with each other, the last interaction restarts the first). In real life such a machine would inevitably stop working because the initial energy is lost to friction, resistance, waste heat etc. So for such a circular system to work there must be absolutely zero energy loss. This in itself is not possible.

But in addition to this proponents of PMMs advocate that not only can they build a machine that has zero energy loss (i.e it can be in motion perpetually) they can extract energy from the machine to do work without losing energy. This is even more impossible.

To use the analogy of an orbiting system. Imagine we have one magnetic object orbiting a mass in a stable orbit. The orbit may last perpetually. Now imagine that we try to extract energy from such a system, perhaps by building a torus around the magnet's orbit, as the object moves through it it generates electricity. But in the process of extracting this energy we are creating resistance and slowing down the magnet. This will change it's orbit, ultimately making it fall.

17. Jun 16, 2011

### sicarius

Actually the moon is moving away from the earth and will eventually break free (unless we are swallowed up by the sun becoming a red giant first).

18. Jun 16, 2011

### Haroldingo

Am I incorrect in the assumption that electrons don't actually orbit the nucleus?

Electro-statical energy keeps the electrons within a perfect distance of the nucleus, Schrödinger's wave function principle (Kinetic energy + Potential Energy = E), the electron is both a wave and a particle at the same time, and as it exists in more than one physical state simultaneously it isn't actually orbiting, and even in the aspect that it is orbital, gravity isn't the force that causes it to be so.

19. Jun 16, 2011

### maverick_starstrider

The word orbit does not apply exclusively to gravity, a positively charged ball could orbit a negatively charged ball. That being said saying electrons "orbit" can rub physicists the wrong way since the math (and thus the physical reality) of quantum mechanics is nothing like that (for example, the position eigenvalue of an electron has no time dependence which is completely unlike some dot orbiting a bigger dot, and yet the have an angular momentum). Though for historical reasons we do still say electrons are restricted to ORBITALS.

However, in response to your main question, you're parsing the words "perpetual" "motion" "machine" but you don't seem to actually understand what the phrase as a whole means. Perpetual motion exists, that's not even close to the issue, any system in some sort of stable equilibrium could probably be said to be in some form of perpetual motion. The issue is crackpots who claim they have invented a perpetual motion MACHINE. What this means is they claim they've found some cycle (usually involving magnetism, because magnetism is mysterious to a lot of layman) in which usable work can be taken out of the system without in any way diminishing the energy of the system. Perhaps we should call them "Machines undergoing perpetual, undiminishing, motion despite constantly giving away energy to their environment". But that's kind of a mouthful.

20. Jun 16, 2011

### Pengwuino

No, you are right.

21. Jun 16, 2011

### Haroldingo

Ok, I get it :) So Perpetual motion occurs in nature, but cannot (as we know so far), be replicated mechanically?

Yay! I got something right :')

22. Jun 16, 2011

### Drakkith

Staff Emeritus

I guess it depends on what you count as perpetual motion. Orbits of planets and stars are not perpetual, but an electron in it's orbital is unless disturbed, but its up in the air whether you could consider that perpetual motion.

23. Jun 16, 2011

### Staff: Mentor

Well, no, not really. "Perpetual motion" just isn't a term used by scientists so it's tough to say what exactly it means. If taken as literally as possible, it just means "moving forever", which sounds a lot like Newton's First Law:
http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

So there's nothing all that special about that definition.

But when used by budding crackpots, it typically refers to a machine that can operate forever and generate energy. That's what's impossible. Please read the wiki on that subject:
http://en.wikipedia.org/wiki/Perpetual_motion

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