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I know there is no such thing as perpetual motion? Why is that? Answered

Okay, no such thing as perpetual motion. Why is that? What about something that will run for a year before it has to be started up again. I guess what Im saying is people just slam perpetual motion and ignore it. But, what if? What if you only have to start it again once a year, a month, a week or even once a day. Don't you think we should look into this result?



Best Answer 6 years ago

Try reading through some of the analysis here. We don't "just slam" perpetual motion; that perception is a result of ignorance and poor education by your science teachers.

There are very broad, and mathematically provable theorems (Noether's theorems) which absolutely preclude violations of conservation of energy, momentum, and angular momentum. Some violation of all three of those principles is required in order for any claimed perpetual motion device to function.

And of course there is Heinlein's rule, or Niven's curse:



Everyone's answers are informing and integrating. I don't think I could even find a best answer, for all of them are the best. The saddest thing I think I have ever read was, All things to be discovered are discovered, being all things are discovered... We can only improve on these... Well, with that attitude, there is nothing left to find... but one day, we will see this as being short sighted and ignorant.

About the closest you may see is the water cycle where the sun (perpetual as far as we will know - although it will have a finite life in terms of billions of years) evaporates water from the sea and deposits on mountains to run back as a river from which we can extract energy to do work.

Hydroelectric plants work like this.

Yes, indeed. This sort of cycle is "perpetual," but only because there is a continuous input of energy to keep the cycle going. Without that energy input, the hydrological cycle wouldn't exist.

(I know you know this already, Rick. My comment is for the less educated posters.)

The reason why perpetual motion does not happen is something called friction. Eventually everything will come to a stop because of friction. Also in our universe everything has a tendency for chaos known as entropy , therefore in our universe such machines can only exist in our imagination.

There is a law in physics that says it won't work.  Still, it's probably been investigated more than any other function of physics.

Even if it did work what good would it be.  If a machine could generate enough energy to keep itself running then you take just a tiny bit of power from it then it would stop.

Personally, I think that someday, someone will discover a new energy source that we don't know about yet that will be over unity and prove the law wrong.

What you "think" is interesting, possibly even amusing, but irrelevant to reality.

Conservation of energy is not just an "empirical" law, deduced from and supported by experimental observation. It is a mathematical law, a direct consequence, via Noether's Theorem, of the fact that the laws of physics are invariant with respect to displacement in time.

That is, because the laws of physics (in particular, Newton's laws of motion) are the same yesterday, today, and tomorrow, the mathematical quantity called energy must be conserved.

I think that almost everyone seems to confuse perpetuum mobile, the perpetual motor, and perpetual motion.
The first one is impossible due to energy conservation law - you need a motor to give you energy, but if you take energy from it, it either has to be replaced, or the thing will run out of it.
However, if you don't want the energy and only want the thing to go on forever, theoretically, nothing stops you (remember the Newton's first law), while practically, you can get very close to it. A satellite can stay in almost the same orbit for millions of years, as the only force that acts on it are collisions with dust and other stuff flying around. In a ring of superconducting wire current hadn't changed for years (until someone got sick and tired of pouring liquid helium on it).

Without wishing to argue - Near earth Satellites actually have a relatively short orbit life of a few years and need fuel (energy) to keep boosting them back into orbit.

Even the moon will eventually slow in it's orbit and the earth will loose it.

Making the liquid helium puts energy into the system to keep the super conductor cool and working - The resistance although very small isn't zero until absolute zero is reached.

A spinning top far out in space would spin without resistance and go on for a long log time BUT the solar wind, gravity, (even far out in space), and micro dust and gas would eventually slow it's spin.

Ok, my examples are bad, but there's no law that says it's impossible, and you can get as close to it as you want...


6 years ago

I agree with kelseymh and rickharris but in direct response to your question, if you have to start something, "...once a year, a month, a week or even once a day," that is not perpetual motion.
Perpetual motion means that once an action is started, that action will never, ever again require an external source of energy to continue that action. What you are talking about is creating something that is highly energy efficient but certainly not perpetual.

You can't under the laws of thermodynamics get something for nothing i.e. take out from a system more than you put in.

In essence friction is the biggest enemy as is trying to make the system do "work" this uses energy your not replacing.

In a working system the loss of energy (work) is replaced by:

Burning petrol (gas)
Wind power
Water power.