I asked my physics teacher, and he said, “Because there’s always some loss of energy due to friction.” Ok, I’ll accept that now we can’t create perpetual motion right now, but isn’t it kind of presumptuous to say that its impossible since we can’t do it now? Isn’t that a bit like saying “It’s impossible to go to the moon.” in earlier centuries? Granted our current knowledge prevents perpetual motion, how can we say that it’s impossible to do it in the future?
And why isn’t space travel perpetual motion? Theres no force acting against the direction of something moving in a void. My teacher said, because probability says that it will eventually end up in the field of gravity, where it will be sucked in and stop moving (collide?). According to probability, if given infinite time and infinite trials, shouldn’t at least one object moving through space not be pulled into a gravity field? And also, isn’t it sort of unscientific to say something is “impossible”, especially due to outside interference? To continue the “to the moon” analogy, wouldn’t that be like saying, “Its impossible to go to the moon because there’s a chance that when we lauch the shuttle it will collide with an airplane.”?
Note: I post this not to insult or argue against scientific fact, but rather because I desire a more comprehensive explanation of why scientists have deemed PM impossible. However, if the mods feel this is more of a GD than a GQ, by all means, move it and accept my apologies.
A body in motion tends to stay in motion. This is a basic law of physics. For all pratical purposes, the earth is in perpetual motion around the sun (at least for the next couple billion years)
What your teachers are trying to learn ya is that perpetual motion machines do not exist. They don’t. The patent office won’t even consider them. All examples on earth run into friction of the parts against each other and air resistance. In short, in order to work, they would need to have additional energy added to keep them going against these friction sources that are drawing away energy.
Doesn’t this have to do with the Law of Conservation of Energy? If a machine were capable, under ideal operating conditions, to produce the same amount of energy that it used, all that would be needed was to give the little bugger a shove to start it off and it would run forever. Unfortunately, there are no ‘ideal conditions’ and a portion of the energy produced would be used to overcome some loss (i.e. friction, counter-EMF, etc.) so, eventually the machine would sigh to a halt.
There still is a part of me, however, that really wants to be the one to prove this false.
It depends on what you mean by “perpetual” and by “motion”. As has been explained to you, the reason is friction. So, if friction is zero, perpetual motion is possible. Note that this does not mean you can get any energy out of the system. You can’t. But an ideal planet in orbit could stay in orbit forever. In real terms there is still some loss of energy caused by tides etc, so even a planet will slow down over long periods of time.
Because space isn’t a perfect vacuum; there’s (IIRC) about one atom per cubic metre, which will cause friction (albeit very small, but the tiniest deviation from ‘perfect conditions’ will cause the perpetual motion machine to stop eventually)
The Second Law of Thermodynamics says “In a closed system, entropy tends towards a maximum.” Entropy is the measure of the energy that is unavailable to be used for work. I.e.: Waste heat (the heat created by friction that cannot be trapped and used to power a machine). Work is obtained from order, so entropy can also be used to measure the orderliness of a system. That is all entropy means, cheesy cliches, bad analogies, and outright lies aside.
So, since any perpetual energy machine would be a closed system (not getting any energy from the outside), it must run down, even if it is in space or another hypothesized low-friction environment. The Universe is also a closed system, hence the ‘heat death’ (all mass-energy evenly distributed throughout the Universe) postulated by certain models of the Big Bang.
Even though the Laws of Thermodynamics are called laws, they might be disproven at some time in the distant future. My shit could also begin singing Verdi’s arias. It’s not likely, but anything can happen.
I’ll second what DPWhite said. Perpetual motion machines are not possible.
In the example of a spacecraft moving at constant speed with no net external forces acting on it (which in truth, is not really possible–see below), the spacecraft is not doing any useful work, in the sense that we use the term in physics. If the spacecraft were doing any work, a net external force would be acting to slow it down.
Please tell me that a physics teacher didn’t say (or imply) that. No object in the universe is unaffected by gravity. The force of gravity drops off as the square of the distance between the center of the two masses in question. The force of gravity never drops to zero.
In any event, it is irrelevant. The spacecraft is not doing any work, so no problem. It’s not a perpetual motion machine.
There could be, but such a machine would violate the Laws of Thermodynamics, as we know them today. As for comparing this to saying, “It’s impossible to go to the moon,” that’s more of an issue of technology rather than a violation of physical principles. For instance, we technically could send a spaceship to a distant star today, but the journey would take so long that all the passengers would be dead by the time they got there. It’s not impossible in a physical sense; it’s impossible in a technological sense. A perpetual motion machine violates accepted physical laws.
Getting back to your original question, I suppose it’s more accurate to say that, given our current understanding of physics, it should be impossible to build a perpetual motion machine. The day someone does build one, we’ll have to throw the First and Second Laws and formulate new ones that accurately reflect these new observations. That’s all science is, after all, an attempt to explain our observations. To date, no one has ever observed a violation of the Laws of Thermodynamics. And that’s a huge body of supporting observations, to say the least.
And echoing robby, a spacecraft drifting in space is not a machine because it does no useful work on a system.
Do we know if we will be able to violate the laws of nature (and physics) in the indefinite future? Are there loopholes. As a lawyer and not a physicist, I have to be on the side of loopholes. As we now understand what we know, we can say with great confidence that such a machine does not exist and cannot exist. But we hardly know everything. But nothing we have seen gives us the slightest reason to believe this would be possible. Faster than light travel has a better possibility through theoritical wormholes that would use up star clusters of power than a perpetual motion machine.
I think that the technical explanations above are much better than I could come up with. If you are in high school physics, you are probably learning Newtonian mechanics. If I remember my H.S. Physics (Hi Joe Housek!) remember that there is no free lunch and angular momentum is a bitch.
I dunno; this sort of question/position seems to pop up very often and it generally assumes that human progress will have no limits, not even if imposed by the physical laws of the universe we live in:
Want to travel to distant stars? it’s impossible, but wait, one day we’ll find a way to break the lightspeed barrier.
Want to teleport matter(particularly matter in the form on people) from one location to another? it’s impossible, oh, but one day it might not be.
We’re so in love with the idea that we can overcome any problem if we throw enough resources at it, that we have swallowed things like Moore’s Law without really thinking about it and we accept that certain things will be possible just because we’ve seen them on Star Trek.
And what’s wrong with that, Mangetout? I think that speaking in terms of probabilites that something will happen, rather than absolutes of what can never be and what must be, makes sense given incomplete information. I don’t think it’s very likely we’ll ever be able to get around the speed of light in a vacuum barrier, and I’d dismiss claims that somebody had almost out of hand, but I can, with sufficient evidence and studies performed at other locations by different groups of people, be convinced. The same applies to everything else.
We have a very good knowledge of how our Universe works. We know how things happen in all but the most exotic circumstances. But it is incorrect to assign absolutes to events that may/may not happen in the future.
I think the only thing that’s wrong with it is that it isn’t an informed position; usually somebody will give (what I believe to be) a flawed analogy along the lines of “people in the stone age wouldn’t have believed that air travel would one day be possible”; the reason I believe this is flawed is that there is a difference between saying something is impossible because you’re ignorant of the factors involved, and saying that something is impossible from a knowledge of limiting factors.
Although it might appear that my last post was defeatist, I think it’s at least equally unconstructive to just assume that somebody in the future will work it out.
Mangetout, there is solid theory now behind wormholes and “transporters”, and with a single photon. It doesn’t mean it will ever be done, and I certainly don’t think in our lifetime. A perpetual motion machine, however, assumes you get something for nothing, that is a set up of mass and a little motion (energy) will generate energy without consuming further energy or mass. Just isn’t going to happen. No observations have ever suggested that this may happen.
Sorry, upon rereading what I said, that wasn’t a 100% accurate description of what he said. He didn’t say “the field of gravity.” The jist was that it would be so influenced by the gravity of a planet or star or black hole as to be pulled into it.
Also, for clarity, is it perpetual motion, or perpetual motion machines that are impossible, (although I still think its irresponsible to use that word in scientific context.)
Ah yes, one more thought, isn’t the motion of electrons perpetual in that, in order for them to stop “moving,” wouldn’t it have to reach absolute zero.
The issue of absoulute zero being impossible is another law (the 3rd law of thermodynamcs, right?)
Just wanted to point out that in any system consisting of two objects orbiting each other which are not point-masses there is friction. One place where this is most evident is Io where it’s orbit around Jupiter causes a whole heck of a lot of heat. So even if both the Earth and the sun were black bodies there would be a small leak of potential energy into kinetic energy (heat) causing the sun and earth to move gradually close together.
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*Originally posted by Qwertyasdfg *
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Also, for clarity, is it perpetual motion, or perpetual motion machines that are impossible, (although I still think its irresponsible to use that word in scientific context.) **[/QUOTE]
Both in real terms. Unless there is a perfectly frictionless environment perpetual motion cannot be maintained. As others have pointed out, low friction environments such as outer space can allow objects to stay in motion over billions of years, possibly even to the collapse of the universe, but it is by no means “perpetual”. There is no place in the universe or the laboratory, even conceptually, with true zero gravity and no matter to impede motion.
My reading of the 3rd law is not that absolute zero is impossible. The main clause of it is “the entropy of a perfect crystal at absolute zero is zero.”