Why would/wouldn’t an object with mass m moving through intergalactic space at velocity v in a straight line be undergoing perpetual motion?
Assume that the object was set along that trajectory by a close-encounter gravitational interaction with a star, after which it did not substantially interfere with any other stars and escaped the galaxy altogether. This is the hypothetical machine’s activation. There are negligible external forces (by premise before leaving the galaxy’s gravitational field, and by virtue of distance in intergalactic space), and negligible friction (vacuum of both interstellar and intergalactic space).
Is it valid to refer to the inertial motion, namely in a straight line at constant velocity, of this object as “perpetual?”
I thought the law was that a body in motion will stay in motion until something acts on it. In your scenario, it’s traveling through a hypothetical void (one not only of sight and sound but of mind. A journey into a wondrous land of imagination).
Intergalactic space is not a vacuum, and on time scales of billions of years, the external forces are not negligible, nor is the mass in intergalactic space.
The more interesting answer is that you cannot do work using something that is just flying through a void; the moment you couple it to anything in order to get energy from it to use to do work, it’s no longer flying through a void and will, therefore, be stopped by the mechanism you’re using once all of its kinetic energy has been extracted to do work and make heat due to the losses in the mechanism.
That said, yes, eventually anything flying through the void will lose its kinetic energy, if for no other reason than it’s constantly acquiring momentum contrary to its trajectory by impacting with photons.
I’ve always heard a perpetual-motion machine is an impossibility. But isn’t the universe itself one big perpetual-motion machine? Does that not count? Or is it being powered … from outside?
See Derleth’s answer. Perpetual motion does not mean simply movement. Continual movement is certainly possible, as in the case of a perfect superconductor. Taking useful work out of a system defeats the perpetualness: something always runs down.
The universe is not itself perpetual - it will probably suffer heat death, or some other ending. I’m not sure what it would mean to use the universe to produce work, either, although black holes have a definable entropy. And vacuum energy can be thought of as something outside the universe.
Anyway, the issue is the same as the OP’s. Perpetual motion is a widely misunderstood term. If you can’t use the energy to perform work then it can’t be perpetual motion. It’s a very specific usage, but like so many other scientific terms, uses a familiar word in an unusual sense.
ETA: friedo snuck in with the same link while I had this open.
This is a bit complicated, but conservation of energy is a local property, not a global one. What conservation of energy means is that if you draw a box somewhere, the change in the amount of energy inside that box is the same as the net amount of energy that flows through the walls of the box. But the entire Universe cannot be treated as a single really big box. The “total energy content of the Universe” probably isn’t even well-defined, and even if it is well-defined, it’s probably not a constant.
As a followup question Chronos - does it happens instantly ? Or do conservation of energy follows the speed of light. That is if I add 1J energy to a cube of 3x10^8 m through one wall - and measure the energy in the cube - is the energy in the cube instantly increased by 1J ?
It’s not in a closed environment. In theory, its motion is perpetual, but in practice it will eventually hit a star, a planet, a comet, or whatever, or just come into contact with a gravity well.
In a theoretically “perfect” environment, with zero external forces acting on it, an object in motion will stay in motion forever. Once again, this doesn’t mean that useful work can be extracted from this motion.
The total energy in the box instantly increases. It won’t be evenly distributed in the box, but nobody said anything about the distribution of the energy.
As I said above, the term “perpetual motion” in this context has a specific definition. It does not mean constant movement; the definition requires that work can be extracted. So what you are talking about is by definition not perpetual motion. It’s that false understanding of the definition that leads to so much of the confusion.