# Perpetual Motion

#### Alkayus

I am sorry if this is a weird or stupid question, as I am currently in college and majoring in physics, but have not taken any physics classes yet. The weird thing I was thinking about was the impossibility of perpetual motion. Are not atoms a form of perpetual motion? What about solar systems? I am not asking this to be controversial or anything of the sort. Im just trying to learn a bit more. Thank you so much for any input.

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#### Doc Al

Mentor
When we speak about the impossibility of 'perpetual motion' we really mean perpetual motion machines: something that can do useful work without energy input or with perfect efficiency. It's not motion per se that is the problem.

#### Bloodthunder

When we speak about the impossibility of 'perpetual motion' we really mean perpetual motion machines: something that can do useful work without energy input or with perfect efficiency. It's not motion per se that is the problem.
I thought the machine is something that you get more output than input energy.

#### Alkayus

Ah, I see. Thank you very much for clearing up that tiny, but important detail.

#### Redbelly98

Staff Emeritus
Homework Helper
I thought the machine is something that you get more output than input energy.
That's pretty much the same thing. If the input energy is zero, then it fits your description (output is greater than input). And something that fits your description can use part of the output energy for the input energy, requiring no input energy from an outside source. So a machine fitting either description can fit the other as well.

#### BruceW

Homework Helper
entropy is important. It is a measure of disorder - meaning that high entropy would be heat energy, low entropy would be useful energy stored in a battery.
Total entropy always increases or stays the same in an isolated system. Therefore the useful energy always decreases.
Doc Al is right, when people talk about perpetual motion machines, they mean machines that make useful energy "for free".
You might think that a solar power cell makes useful energy for free, but you must include the sun in the system, so then you see total entropy actually increases.

#### 256bits

Gold Member
I find it kind of funny, discouraging, when someone comes up with a mad hatter scheme of free energy for the masses. If you think about it, all of the energy resources we use are free- oil, wind, solar, geothermal, nuclear, hydro - just waiting there for us to use. It is the harnesing of these resources that costs a lot of money.

#### JaredJames

I find it kind of funny, discouraging, when someone comes up with a mad hatter scheme of free energy for the masses. If you think about it, all of the energy resources we use are free- oil, wind, solar, geothermal, nuclear, hydro - just waiting there for us to use. It is the harnesing of these resources that costs a lot of money.
I think you misunderstand the concept of "free energy".

#### Ryan_m_b

Staff Emeritus
I thought the machine is something that you get more output than input energy.
Generating electricity etc. basically takes energy in one form and uses some of it to do work.

A perpetual motion machine would have energy put into it and be able to use it to do work without using any of it.

#### BruceW

Homework Helper
I find it kind of funny, discouraging, when someone comes up with a mad hatter scheme of free energy for the masses. If you think about it, all of the energy resources we use are free- oil, wind, solar, geothermal, nuclear, hydro - just waiting there for us to use. It is the harnesing of these resources that costs a lot of money.
Sorry if I caused confusion by saying 'making useful energy for free' I don't mean Gibbs free energy or any kind of technical term. I literally mean that the useful amount of energy in the universe must decrease or stay equal.
In our case, it is the sun that is causing a decrease in useful energy (from potential to light). The energy from the sun's rays eventually becomes all the energy resources you mentioned above. And then we decrease its usefulness further by turning it into even more disordered forms of heat.

#### Bloodthunder

That's pretty much the same thing. If the input energy is zero, then it fits your description (output is greater than input). And something that fits your description can use part of the output energy for the input energy, requiring no input energy from an outside source. So a machine fitting either description can fit the other as well.
Was actually just wondering about the perfect efficiency part. What does it mean by perfect efficiency?

Mentor
100%

#### Bloodthunder

Wouldn't that be for one where total input = total output? Like matter-antimatter annihilations? In a perpetual motion machine, I think the efficiency would be >100%, would it not?

#### Drakkith

Staff Emeritus
2018 Award
Wouldn't that be for one where total input = total output? Like matter-antimatter annihilations? In a perpetual motion machine, I think the efficiency would be >100%, would it not?
It would have to be greater than 100% to overcome things such as friction, yes. If it were possible of course.

#### Alkayus

Thank you all very much for the insight. I'm quite happy that not only was my question answered, but sparked a very interesting conversation.

#### Drakkith

Staff Emeritus
2018 Award
Thank you all very much for the insight. I'm quite happy that not only was my question answered, but sparked a very interesting conversation.
Hit up wikipedia and look up perpetual motion if you want to know more about it. Just be VARY wary or sites online that propose a working perpetual motion machine or something related. Many will use the example you gave, that of an electron around an atom, as evidence that it is possible.

#### curious1409

I was also interested in this:

Physics says its impossible to achieve perpetual motion but planets are constantly in motion and so are electrons? If your objective was to get a planet from one side of the sun to the other without wasting any energy than this is possible, therefore perpetual motion has been achieved. Or am I mistaken?

#### Ryan_m_b

Staff Emeritus
I was also interested in this:

Physics says its impossible to achieve perpetual motion but planets are constantly in motion and so are electrons? If your objective was to get a planet from one side of the sun to the other without wasting any energy than this is possible, therefore perpetual motion has been achieved. Or am I mistaken?
There is a difference between an object in freefall and free energy machines. If you were to try to tap the energy of an orbiting planet (perhaps by building a huge torus around the planet's orbit that generates a current as the planet moves through it) you would sap energy from the speed of the orbit. It would shortly fall into the sun.

Perpetual motion machines don't just refer to objects that move perpetually, the idea is you have a system where energy is recycled 100% and yet energy is taken out to do work.

#### my_wan

I once met a maintenance guy at a factory that thought he had come up with a perpetual energy machine, using thermocouples. Insulate a cold storage very very well. Then use a thermocouple to generate electricity.

I tries to explain to him that even if it was perfectly insulated, so that it would stay cold in a hot room forever, then as soon as you started drawing power from the thermocouple the cold storage would start heating up. He refused to believe that.

#### russ_watters

Mentor
I was also interested in this:

Physics says its impossible to achieve perpetual motion but planets are constantly in motion and so are electrons? If your objective was to get a planet from one side of the sun to the other without wasting any energy than this is possible, therefore perpetual motion has been achieved. Or am I mistaken?

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