Using Magnets to Create Perpetual Motion?

[eah2119]

Let's say I have permanent magnets set up in a sort of half sphere with the inside of the sphere pointing up and it laying flat on a table and then I have a ball with, let's say, half the radius of the half sphere. The ball has the same permanent charge as the half sphere. I place the ball over the half sphere. The ball and half sphere would repel each other and the ball would remain still and afloat. If I sent the ball on a spin, wouldn't it continue to spin assuming it is in a vacuum? I've always heard that perpetual motion is impossible on Earth, so what's wrong with this scenario? Would it even work? Would the ball's rotation slow down?

I originally posted this on Yahoo Answers, but I was suggested to post it on a science forum. Here's the original: http://answers.yahoo.com/question/i...tY.c6Mrty6IX;_ylv=3?qid=20110618150331AAp86o2

I did get quite a few good answers, but I expect there are real experts here on Physics Forums.

 

[arydberg]

I'm not sure what you suggest will work but there are various methods of levitating magnets. See here http://www.levitron.com/ If this was in a vacuum it would spin for a long time. Also it is possible to levitate a piece of graphite over cluster of 4 magnets. Brownian motion is also an example of constant motion.

The problem is that once you have something spinning what do you do with it? Any attempt to power anything, a light bulb or charging a battery etc will reduce it's energy and slow it down. The term perpetual motion usually means to keep going and produce useful energy. This is impossible.

 

[DragonPetter]

Even if you eliminated air friction, I think you would still generate eddy currents that would oppose the magnetic field and eventually slow down the ball. You also might have some kind of hysteresis effects creating heating as the objects spin.

Also, you would not be able to use this device to do any work even if all losses were removed, which is what makes a machine a perpetual motion machine in the first place.

 

[VihariP]

hello everyone!
This is completely alright eah! First of all let us see the definition of perpetual motion.
wikipedia defines it as "Perpetual motion describes hypothetical machines that operate or produce useful work indefinitely and, more generally, hypothetical machines that produce more work or energy than they consume, whether they might operate indefinitely or not."
You got it? It says it should produce useful energy or at least produce more energy than it consumes.So in your situation the ball is just rotating, it is producing no work( of course it swirls air around it because of air friction) because of which it may slow down after some time. It does n't contradict at all with the conservation of energy.
And by the way if you want produce some motion which you described you need not imagine something hypothetical. Take a flat plate wet it with a thin layer of water and try to rotate a flat glass(with less base area and of considerable geometry for balance).
it will rotate for 1 minute generally.

 

[Drakkith]

Here's the issue. Perpetual motion itself isn't an impossibility in science. (Well, not REALLY, but the difficulties in achieving an environment entirely free of friction and different forces that would cause an object to lose energy is pretty much an impossibility currently)

The problem lies within the usual use of the term. USUALLY it is mentioned in regards to a perpetual motion machine that can be used to generate power. This is 100% untrue.

Just be careful when discussing perpetual motion, as it is generally frowned upon because of the usual use of the term. Also, perpetual motion machines and related subjects are not allowed per PF rules, so don't stray too far.

 

[russ_watters]

Newton's first law: objects in motion tend to stay in motion...

 

[Antiphon]

I believe the overunity sawmill was perfected quite a while ago; on paper. Why doesn't someone set about building it already??

http://www.mcescher.com/Biography/lw439f14.jpg

 

[SteamKing]

I don't know from perpetual motion, but when you start making some money, I've got this bridge I'd like to sell you.

 

[arydberg]

hello everyone!
This is completely alright eah! First of all let us see the definition of perpetual motion.
wikipedia defines it as "Perpetual motion describes hypothetical machines that operate or produce useful work indefinitely and, more generally, hypothetical machines that produce more work or energy than they consume, whether they might operate indefinitely or not."
... .

IMO be careful using wikipedia as a source. There are no listed authors and as long as this is the case we have no way of knowing who is the author of what we are reading. As for their links they are what they wish us to follow. In controversial subjects (which this is not) it is generally accepted that wikipedia does have a strong bias.

 

[eah2119]

That's misleading. Knowing the definition of perpetual and the the definition of motion separately, and then combining them creates the definition: continues to move.

Somebody want to coin another term?

I wasn't thinking. Newton's first law proves it! Obviously anything will continue to move if there are no forces acting on it. Momentum is conserved. I guess I just didn't know the accepted definition of the term.

Another question: Would it actually levitate? Set up the way I've explained, would the ball remain in the air or would there be some kind of canceling forces?

Thanks for the responses!

 

[Ryan_m_b]

That's misleading. Knowing the definition of perpetual and the the definition of motion separately, and then combining them creates the definition: continues to move.

Somebody want to coin another term?

I wasn't thinking. Newton's first law proves it! Obviously anything will continue to move if there are no forces acting on it. Momentum is conserved. I guess I just didn't know the accepted definition of the term.

Another question: Would it actually levitate? Set up the way I've explained, would the ball remain in the air or would there be some kind of canceling forces?

Thanks for the responses!

Yes it is annoying, perpetual motion should technically mean something that moves in perpetuity. A literal perpetual motion machine would be one where the end result of a dynamic process activates the first again and every component is 100% efficient with zero energy loss (due to friction, waste heat etc).

A non-literal perpetual motion machine is what is used by proponents of free energy to describe a machine that not only 100% recycles the energy within it to start the process it just finished doing but also is able to have energy extracted to do work yet not loose any energy in other words if we imagine a spinning ball that looses absolutely no energy a free energy proponent would advocate drawing energy out of the balls spin without slowing it down. This is why such ideas are nonsense.

As for your idea of putting a ball in a bowl of magnets yes it should float. This is the principle behind http://en.wikipedia.org/wiki/Maglev_(transport)" .

 

[arydberg]

The problem with your ball in the center of another half ball is that a magnet monopole is impossible. Thus your "floating ball" would turn over and be attracted to the large ball. The link i posted earlier is an example of a levitating spinning magnet.