Perpetual Motion

[JayLechner]

Hi everyone i have a question. If i were to get a jar and put a magnet on the lid, bottom, and one suspended between the two, if i made a vacuum in the jar (get all of the air out) and had the suspended magnet spin would it spin for ever? (or at least a very long time!)

[Cap'n Refsmmat]

It would probably spin for a very long time. The trouble is that it's pretty much impossible to make a "perfect" vacuum -- one in which there are no particles at all -- so there'll be tiny bits of friction. Also, magnets gradually lose strength over long periods of time.

 

It would be a cool toy to have sitting on your desk, though.

[Mr Skeptic]

The spinning magnet would generate a changing magnetic field. This would induce electric currents in any conductors regardless of distance, though the closer ones would have a larger induced current. The induced current would have resistance, transferring some of the kinetic energy of the magnet into heat.

[SkepticLance]

In other words, no.

 

However, you have an example that comes close to what you are describing in planet Earth. It has been spinning for 4.5 billion years now, in vacuum. The spin is, however, slowing due to the pull of the moon, albeit to a very trivial extent.

[atheistoverlord]

I had one of these perpetual motion ideas that seemed revelationary at first, but friction destroys everything over time.

And by conventional physics theory would say NO.

 

Energy is neither created nor destroyed.

[JayLechner]

The spinning magnet would generate a changing magnetic field. This would induce electric currents in any conductors regardless of distance, though the closer ones would have a larger induced current. The induced current would have resistance, transferring some of the kinetic energy of the magnet into heat.

 

Ya thanks alot i didnt even think of the magnetic field.

[pioneer]

Relative reference can create perpetual motion. For example, we have a normal train leaving the station. The twin engines use 500 gallons of fuel to get the train to 30 MPH. From a stationary reference the fuel and the kinetic energy add properly. But if one is on the train, they will think their reference is stationary and will see the country side outside the window appear to be in motion at 30 MPH. With only 500 gallons of fuel and relative reference we can give mountains, lakes, bridges, etc. motion all the same time. If relative reference is valid, with no preferred reference, we can create endless energy this way. All we need to do is slow that mountain the background, from inside the train, and harness all kinetic energy to make electricity.

 

If we apply relative reference to space, are we on the train? Or is there a preferred reference so we don't create perpetual motion?

[ydoaPs]

Maybe that 500 gallons of fuel was used for the train to maintain its position by fighting friction.

[DJBruce]

Very fun I just had the exact same idea about a year ago.

[swansont]

Relative reference can create perpetual motion. For example, we have a normal train leaving the station. The twin engines use 500 gallons of fuel to get the train to 30 MPH. From a stationary reference the fuel and the kinetic energy add properly. But if one is on the train, they will think their reference is stationary and will see the country side outside the window appear to be in motion at 30 MPH. With only 500 gallons of fuel and relative reference we can give mountains, lakes, bridges, etc. motion all the same time. If relative reference is valid, with no preferred reference, we can create endless energy this way. All we need to do is slow that mountain the background, from inside the train, and harness all kinetic energy to make electricity.

 

If we apply relative reference to space, are we on the train? Or is there a preferred reference so we don't create perpetual motion?

 

By this reasoning we can create money simply by converting dollars to Zimbabwean currency

 

When you accelerate, you know it, and kinetic energy is not invariant under a Galilean or Lorentz transform.

[Sisyphus]

Generally more is meant by "perpetual motion" than "a constant nonzero velocity." Since obviously, as you say, you can pick any constant velocity you want by using a different reference frame. A basic definition of the usual sense is "a closed system that produces more energy than it consumes" or some such.

[swansont]

Generally more is meant by "perpetual motion" than "a constant nonzero velocity." Since obviously, as you say, you can pick any constant velocity you want by using a different reference frame. A basic definition of the usual sense is "a closed system that produces more energy than it consumes" or some such.

 

IIRC there is the machine of the first kind, which violates the first law of thermodynamics (over-unity machine) and the second kind, which violate the second law (entropy not behaving properly)

 

One should note that systems/processes that are reversible do not violate the second law; entropy remains constant. Macroscopic systems do not use reversible processes.