I know perpetual motion devices can’t work. Still, I can’t figure out the problem with this very simple one that I came up with.
The device is an extremely large hula hoop that you put around the earth. It is supported by nanotube towers that hold the tube up at the height of low earth orbit. Inside the tube is a ball that orbits the earth like a satellite except the ball cannot fall out of that orbit. The ball hits a flipper every orbit to provide energy outside the system.
I know this is not practical. I am just wondering why it wouldn’t be a successful perpetual motion device if we could build it.
Hitting the flippers causes it to lose energy, so it will tend to go into a lower orbit. Since the “hula hoop” is preventing that, it will just rub against the inside of the hula hoop. That friction will cause it to lose even more energy. Eventually it will come to a stop, and no more energy will be provided.
The ball would eventually lose all of its kinetic energy to the flipper. The only thing putting it in orbit does is eliminate friction when the ball is at the exact correct velocity. Once that velocity is changed its no different from a hula hoop on the ground.
I’m glad this question was asked because I have a similar design I’ve been meaning to ask about. It’s a bit more complex though. Imagine, again, that you have an enormous ring around the Earth, or even out in deep space for that matter. The ring is not hollow, but it is lined with permanent magnets, so that as you travel along the ring, you encounter alternating magnetic fields. Then you have a device with a coil in it that flies along this ring producing electricity like a generator. The energy can be transmitted via radio waves or similar. The device can use electromagnetism and/or gravity from a nearby planet to keep it on a circular course. I can see this design not working on scales we could actually build because the centripetal acceleration required would exceed the energy produced by the generator, but with a huge ring and even gravity helping it, hardly any acceleration would be required. What’s wrong with this design?
Thanks for the answers. I still can’t visualize why the ball would ever come to a rest however. The way I pictured it, the ball would be sitting at the top of a hill slanting down in both directions if it stopped. Wouldn’t the earth’s gravity always pull it down one slope or the other?
“Down” = closer to the earth. I thought your “hula hoop” was the same distance above the earth all around. (If it’s closer to the earth in some places, it’s higher in others, and you can’t have a downhill without an uphill.)
When you use a magnet to induce current in a coductor, a magnetic field is also created which pulls the magnet in the opposite direction. Thus, in order to maintain the magnet at a constant velocity, you must constantly be pushing it (which puts energy into it.) Without pushing it, it would slow down and stop.
I havern’t gone into the details but we must remember that if the ball is stopped relative to the ring which rotates with the earth then there is centrifugal force that might counterbalance gravity.
The reason a satellite orbiting (circular orbit) at a certain velocity is at a certain height is that at that height gravitation is just balanced by the centrifugal force. If the ball is going faster in the ring than orbital velocity it will tend to fly away from the earth and roll around the outer edge of the ring losing energy to friction until it slows down to orbital velocity. Then I think it will just stay at that distance since the velocity now matches the orbital height, assuming that there is a perfect vacuum. Actually it will slow down because of friction with things like the residual atmosphere, particles from the solar wind, etc. until it falls to the inner part of the ring and it will be forced around that surface by friction but no energy can be recovered under those circumstances.
OK - since we’re throwing stuff out for consideration.
Question 1: Lightning strikes earth all the time and packs quite wallop. Would it be possible to build a very tall tower and use the difference in electrical potential between earth and ground to generate an electrical field that would do work and be able to recharge itself?
Question 2: The earth is superheated magma a few miles beneath the surface. Why can’t we tap into this to generate useful energy? Is it a conservation of energy issue / limitation or simply an engineering problem?
We do but not from magma a few miles down. In the Coso Mountains north of here are three electric generating plants that use the heated water from volcanic activity near the surface for energy. And there are lots of other geothermal generating plants around.
Somehow the heat from the magma would have to be gotten to the surface from a few miles down. Pipes with water circulating through them might be used to heat the water and get energy that way. Maybe someday when other forms of energy become even more expensive it will be tried.
Theoretically, yes. Practically, lightning just packs too much of a wallop. You come up with a way of storing the energy from a lightning bolt (a few million volts with a few hundred thousand amps of current flowing, typically) and I bet a lot of people will want to talk to you.
This isn’t really a perpetual motion machine. You’re really just tapping into an energy source that just happens to already be present due to someone else doing the work (like wind power).
Someone already beat you to this one. Google “geothermal energy”. Again, like wind energy, this isn’t a perpetual motion machine.
The better question to ask is why would a perpetual motion machine work. Every propsed machine either losed energy or has it added from an outside source.
But since we’re throwing anything against the wall to see what sticks how about limiting the universe to two objects rotating in a stable orbit around a common center of mass? We also have to stipulate there are no floating particles, that “empty space” is truly empty, and that the objects don’t have magnetic fields and their atoms don’t decay. Would that qualify as perpetual motion? Since anyting that put the objects into orbit could effect the stability of those orbits we’ll say God did it. Right after microwaving a burrito so hot…
Maybe if neither of them rotated on its own axis. Because if one or both did so then tidal friction would result in a loss of energy.
The impossibility of perpetural motion depends upon energy being conserved. If energy is conserved and none is lost by the system since in your postulate there is nothing to transfer energy to, it should go on forever. I don’t believe there is any theoretical reason why energy is conserved. It just so happens that in all of our experience we have never come across a case where it isn’t. And assuming that energy is conserved allows us to solve practical problems so the assumption of conservation seems pretty safe.
Your initial condition of pefectly empty space is pretty restrictive and highly unlikely to exist in our universe.
I seem to remember something about orbiting objects emitting gravity waves, analogous to synchrotron radiation, the energy produced when the path of charged particles are bent. Given enough time, the two objects will spiral into each other and break up at the Roche limit.
That’s because you have in mind a hula hoop on earth, where the “down” is at the bottom of the hula hoop, so the ball will fall following the curve and stop at the bottom. But in your huge orbital hula hoop, “down” is everywhere the inside “wall” of the hula hoop, closer to the earth.
Picture yourself standing inside it. You won’t fall or slide because “down” is right under your feet. Actually it’s exacty the same than when you’re standing on the earth. Despite its surface being curved, you’re not going to slide around it. Except if the surface is very smooth and I push you, in which case you’re exactly in the same situation than the ball in your orbital hula hoop. You’ll slide around the earth until the friction stops you.
No, not really. You have to remember entropy is the key. Consider a perfectly insulated sealed box with a battery driving an electric motor - after the battery runs down to nothing, the box still contains all the same energy, but the entropy has increased to the point where the energy is no longer available for use, because there’s no differential of energy across the control volume. Quantum effects and random organization of molecules and energy excepted.