Imagine a spinning wheel in the void of space, with no air resistence or intereference. Wouldn’t the wheel keep spinning forever? If so, why haven’t we already used it to make energy or why do they say perpetual motion is impossible?
Even in space a spinning wheel would experience friction from stray atoms or dust. Granted, this would be negligible, but over time could slow the wheel down. This doesn’t even take into account solar wind disturbances.
The key thing is that it can’t do any work. If work is done, either on other particles through friction, or through heating by spinning in a field or whatever, then it must eventually stop.
>If so, why haven’t we already used it to make energy
You have answered your own question. It isn’t so. But if you HAD a perpetual motion machine you could not draw energy from it without adding more at the other end and if you did that it would be an engine, not a perpetual motion machine.
>or why do they say perpetual motion is impossible?
As far as we know, it is. Even my ex had to stop bitching long enough to eat.
Furthermore, even if you could make it work so that there was absolutely no drag on the system, any attempt to get energy OUT of the system would immediately produce such a drag (for instance, if you attached it to a mechanism with moving parts, there would be friction between the parts).
That’s why a “perpetual motion machine” - even if you could produce one - would be fairly uninteresting. In order to actually DO anything with it you need a machine that produces energy, not one that just doesn’t lose it.
Not to mention that this spinning wheel by itself is not a machine: it doesn’t do any work. It may be in perpetual motion (Guy Incognito’s point notwithstanding), but it only has the energy that was initially given it to make it spin.
Now, if we wanted to get work out of it, we could hook it to a dynamo to produce electricity, but the resulting friction would quickly drain it of its energy.
In short, a perpetual motion machine is thought to be impossible because you would get more energy out of it than you put into it.
Can’t we use the spinning wheel as a power generator? Attach magnets to the wheel and have coiled wire around it. As the wheel spins, it moves electrons. You wouldn’t even need to attach an axel to it.
And why can’t we put the wheel in a sealed box, so that stray atoms or whatever can’t interfere?
Dang! You guys beat me to it before I even finished my post!
Carry on.
Alietta - As the wheel spins, it would produce a magnetic field. That takes energy, and the wheel would slow down doing it.
Also, the sealed box won’t keep out particles - particles are created and disappearing constantly even in the hardest vacuum.
Even ignoring Aspidistra’s point, and pretending a perfect enclosure could be created. Electromagnetically coupling to a device to get energy will slow the device.
Don’t believe it? Get any hand (or pedal) run generator and put different loads on it. With no device hooked to the output it is pretty easy to turn, short the terminals out and it is nearly impossible to turn.
I think Aspidistra and scotth are saying the same thing. In other words, if you put magnets on a wheel and coils outside the vacuum enclosure, the wheel still interacts with the coil through the magnetic field. The wheel can generate current in the coils, but in this process the wheel is slowed down.
I think every youngster ponders this. My solution was simple. Take an electric motor, connect the shafts and wires, and start spinning. Pretend the losses don’t exist.
Welcome to here, Alietta. 
Peace,
mangeorge
Don’t forget folks that the solar system is pretty darned close to a perpetual motion machine. We have been spinning here for a few billion years with not that great of permutation.
The device described here exists: the Magnetic Flywheel.
For its weight and/or volume, better for storing electricity than any battery or capacitor.
Imagine a magnet impregnated carbon fiber rotor spinning in a high vaccuum canister with magnetic bearings…no parts touch.
It was supposed to be the new power source for electric cars, until one drove over a pothole and the rotor hit the inside of the casing…KABLOWIE!
My school physics teacher described this as “the sod’s law of the universe”.
Clarification: “Perpetual motion”, which seeming to refer to anything that moves forever, is used technically to mean a device that produces more energy than it takes in.
So things spinning in space are not candidates for perpetual motion since they produce no energy. (They do no work.)
Perpetual motion devices are excluded by the laws of thermodynamics (and the US Patent Office when it’s not havinng an off day).
Things just moving forever are in fact mentioned in Newton’s 3 laws of motion as a must (if nothing interferes with their motion).
>> Can’t we use the spinning wheel as a power generator? Attach magnets to the wheel and have coiled wire around it
That’s like saying “see that hamster going that way? Why don’t we just harness it to a train and have it pull the train along since it is going that way anyway?” The answer is that the train pulls the other way and stops the hamster.
The energy stored in a rotating wheel or any other moving mass is called kinetic energy. If you extract energy from the mass, then it slows down until the speed is zero.
Water moving fast can move a turbine but the turbine slows the water down. If your car is going fast it can coast uphill for a bit but it slows down as the energy is consumed. Then it will stop.
Old high school physics demonstration:
Have student turn a wheel that’s powering a generator. Then turn on a light bulb hooked up to that generator. Suddenly, student will be pushing much harder to keep the wheel going.
So, what everyone else said: electrical work is still work and will slow the wheel down.
What if say we build a ramp and put a magnet at the top. At the base of the ramp we have what looks like a roller with an arm sticking out the side. Lets say the magnet pulls the roller up the ramp and the arm moves past and ticks over a cog, (a dynamo) then the roller drops down a trap before the magnet and rolls down a slide to appear back at the bottom of the ramp to then start the process over again. If the magnet was just the right strength, surely it would be able to pull the roller up the ramp while allowing it to fall down the trap, especially if you build the base of the ramp from mumetal or something that will shield from the magnetic field to an extent.