Something I have wondered about at least since high school.
Supposedly there is no such thing as perpetual motion. But isn’t that basically what inertia is?
‘Things in motion tend to stay in motion. Things at rest tend to stay at rest.’ Surely I am not the only person who has had that drummed into their head.
I mean take the earth. Isn’t the earth a perpetual motion machine basically? In a billion years it is still going to be moving. And no one is adding any more energy to it. Think about it.
My HS physics teacher said you can’t get something for nothing. But with inertia you are really. Aren’t you?
I know friction has something to do with it. But I think there’s friction in space. And I am sure that doesn’t nullify my point in any event.
When someone talks about Perpetual Motion, they mean that energy is being extracted and used, not just a device moving forever.
But, even that is impossible, due to friction.
The Earth is slowing down…
Probably not the best way to kick off your question.
No. Momentum is basically what inertia is. Other than momentum/inertia based ‘perpetual motion machines’, I don’t believe the two are related they way you’re thinking they are.
Friction absolutely nullifies your point. As soon as you attempt to extract energy from something, it’s going to slow down. Whatever concept of perpetual motion you can dream up, try to find a way to get it to do something. Spin a generator, climb up an infinitely long hill, charge a battery, power a car that will only drive in one direction without stopping forever etc.
Can’t be done.
In fact, if you go find the old Simpsons clip where Lisa created a perpetual motion machine, it didn’t just spin, it was continuously going faster and lighting a light bulb (extracting energy). That would be the forever famous ‘in this house we obey the laws of thermodynamics’ scene.
ETA, it’s the first law of thermodynamics that make the body at rest/body in motion statement. However, the same law also explicity states that if energy leaves the system, the system’s energy changes (ie, no such thing as a perpetual motion machine).
Yes, that is exactly what confuses people Things tend to keep moving perpetually.
And the second part of it is, our everyday experience that it takes work/power/effort to move things.
The problem is, that it’s just wrong. There isn’t power required to keep things moving. There is only power required to accelerate things, and to balance the things that are slowing it down.
Since things that are moving naturally tend to continue moving, since there is no power required to keep things moving, there is no power than can be extracted from perpetual motion. And specifically, making things run better and longer doesn’t prove that a new source of power has been found.
The phrase “perpetual motion” is only magic because of the second, erroneous belief. Until I recognize the error of the second idea, inertia seems like magic.
If you had no friction, things would run forever. But there’s always friction. When there’s very, very low friction, like for things in orbit, things run for a very, very long time. But even then, friction isn’t zero.
I think our misfortune is that we often use the phrase “perpetual motion machine” in place of the phrase “free energy machine”.
In human experience things may tend to stay in motion for a while, like a flywheel with really nice bearings, but still run down eventually because of friction or other energy drains. The earth is a really really nice flywheel, but it still radiates gravity waves because it isn’t perfectly uniform, and it generates eddy currents due to magnetic fields from stars and the galaxy as a whole, and the tides… well, don’t get me started on those. The point is that it is still going to run down, not because perpetual motion is in principle impossible, but because there are still plenty of imperfections in this practical flywheel.
But a flywheel that never stops, or its various thermodynamic equivalents, was never going to be especially valuable anyway. The holy grail here is the free energy machine. It’s a flywheel that keeps going forever even after you hook a generator to it. Or the battery that never needs recharging even after it runs a flashlight, or the solar cell that works in absolute darkness, or the energetic radioisotope with infinite half-life.
So, perpetual motion isn’t magic, it’s just not realized in any physical system we are aware of. The reasons that specific examples of systems that are very long lasting motions aren’t quite perpetual are always boring phenomena like tides, not fundamental impossibilities of perpetual motion itself.
But I think our current understanding of existence says that free energy is actually magic, or more accurately impossible – and broadly speaking many of the efforts at perpetual motion machines were really intended to create free energy machines.
Soar into interstellar space, far from the madding crowd of solid objects and their gravitational fields. Set a globe spinning. How long till it stops?
No, there ain’t no free lunch, no way to extract energy from a source without depleting it. Everything that moves eventually runs down (thermodynamics); everything put together eventually falls apart (entropy). “Eventually” is the key word. Whatever stays intact and active long enough seems perpetual unless we measure carefully.
Yeah, that’s the thing, isn’t it? Nuclear energy is just freaky that way. A piece of plutonium you could fit in your pocket puts out a constant one horsepower all the time, and is less than half spent after an average human lifetime. It’s not a free energy machine, but it’s close enough for human purposes. If only it weren’t so… inconvenient…
I don’t understand why so many posters are twisting the OP’s question into something he didn’t ask. Chronos (as usual) has the answer:
The earth will not stay in orbit forever. Even a googolplex years is still not forever. If the earth would hit only one molecule each million years, that would still be enough friction to slow it down eventually, and then to stop. Even the earth will not remain in motion perpetually. 'Nuff said.
No, you’re really not. If by inertia, what you’re referring to is ‘a body in motion will remain at motion’ (which is an fine way to describe it), you have to ask why it’s in motion. It’s in motion because an outside system put energy into it. A ball rolling down a hill requires someone or something to move it to the top of the hill first (work). An ICE engine that’s been spinning for hours requires fuel (energy), a glowing lightbulb requires electricity etc.
Even the earth moving around the sun required the Big Bang to get it started.
Any perpetual motion machine you can think of, particularly inertia/momentum based ones that just seem to have something spinning or swinging needed an initial push from an outside force (ie, you) to get them started. From then on, all they’re doing is dissipating that energy as slowly as possible.
Even the pitch drop experiment, which has been in constant motion, by itself, for nearly a hundred years, required someone to lift the pitch into the funnel (work).
So, no, inertia isn’t something for nothing, it’s something for something and the two somethings are exactly equal. Conservation of energy is still a thing, even if you don’t believe in it.
He mentions the rather old-fashioned idea that the planets are being pushed along in their orbits, which implies they require continuous work (and energy) to keep them moving, hence they require some inexhaustible force to keep them going. Such as Angels beating their wings and pushing the planets around. The new theory from the time of Kepler was that the direction of the angels’ pushing was changed, from along the orbital path direction to toward the central body, the sun.
Jokingly, Dr. Feynman says we still have the angels pushing, it’s just the direction that has changed.
The motion of the planets around the sun, in their almost friction-free environment, may be the closest we can get to a perpetual motion machine. The only way I can think of that we get “something for nothing” (practically speaking) is with planetary gravity assists.
Space engineers have figured out how to get spacecraft to incredibly high speeds in order to visit the outer planets by flying close to one or more planets and “stealing” momentum from these planets in order to gain speed. It works. The spacecraft are sped up and thus can reach Saturn, Neptune and even Pluto within our lifetime. They have done this since, oh, the Pioneer missions and maybe even before that. The planets slow down a tiny bit (probably not measurable). So yeah, we kind of do get something for “nothing” in this limited case.
