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109

In classical mechanics, absolute values of potential energy are meaningless. In your case of a skyscraper just sitting there, we could say it has a large positive amount of potential energy, no potential energy, or even negative potential energy. It doesn't matter at all. What is important is a change in potential energy. is it possible to convert the ...

74

[5/3 - Extended the answer, made some corrections, and responded to John Duffield's comment] This is actually the paradox that led Einstein to General Relativity. Consider a special case: An electron and positron are at the Earth's surface. Bring them together and they annihilate, creating gamma rays (which is very energetic light). The gamma rays travel up ...

70

Now, in my head, if you give the pendulum a little impulse, it will swing up in one direction and get attracted by the magnet just a tiny bit. You've neglected to account for the magnetic attraction as the pendulum bob goes back to its central position. On the outwards leg, you are correct that the magnet's attraction will pull on the bob and give it more ...

37

If the magnet is powerful enough to pull the ball up from the bottom of the ramp, the force on it will be quite strong at the top of the ramp. If so, why would the ball drop through the hole? The pull from the magnet will overwhelm gravity. Even if you constructed one where the ball could fall through the upper hole, I don't see any reason why it should ...

26

You do not need to invoke friction. The magnetic forces are in equilibrium by themselves so if you place the magnets in that configuration, they will not spontaneously begin to move. The reason is that there is a corresponding force on the magnets when they are vertical that matches the ones you've already drawn. Let me make a simple model. First of all, ...

26

An example of harnessing gravitational potential energy is a hydroelectric power plant which converts the potential energy of water falls, dams and the like into electrical energy. As far as harnessing the potential energy of a building sitting on the ground, I suppose if you caused the building to topple you could harness the energy of the falling ...

24

Your guess at the solution to this paradox is correct. "Pumping energy up" to the space station, regardless of the method you choose, would require an input of at least the amount of energy you would gain in kinetic energy on the way down. This is just a variation on the impossible perpetual motion machine concept. In practice, you would not only not gain ...

21

If gas A and gas B are of different density, then the situation sketched is not in equilibrium: the water level on the side of the light gas will be higher. There, the containers are moving down, and you have to push your containers through this net difference in level. You do need to put in energy here, which is probably the piece that you are trying to ...

21

Yes, you can convert the potential energy of the skyscraper into useful work. But, to extract useful work from the potential energy, must reduce the potential energy, that is: you must reduce the height of the skyscraper. You must tear the skyscraper down to get its energy. You should note that skyscrapers aren't free and that someone used a crane powered ...

18

There is a company in Switzerland that is developing an Energy Vault, which is a building sized stack of heavy blocks with cranes extending out from a central pillar. Each of these blocks can be lowered to the ground and the crane generates energy doing so. Re-stacking the block requires using energy to take it back up. There are, of course, losses when ...

16

If you could take from orbital energy, then it would decrease, until at some point in the future it would zero. Hence, it can't be perpetual.

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I have reconstructed your diagram computationally and numerically computed the potential energy as a function of the orientation of the ring: As you can see, when we neglect friction (something that you can't do in the real world) the machine is indeed capable of perpetual motion. However, some important points: The total change in energy of the system ...

16

At a given temperature, in your liquid water-air system, equal numbers of water molecules will enter the air from the liquid as return to the liquid from the air. The system will be in equilibrium and the air will be "saturated" with water vapor. There are two ways that condensation will form on your ceiling. If the air is supersaturated with water then ...

13

He's basically saying assume you have some complicated system of weights connect by pulleys, and each weight can be in only one of two states: up or down. But you can trade off which ones are up and down, for example you can make 3 light weights go up by having one heavy one go down, and there are many other moves like this you can do. Now his point is ...

13

If I were designing the experiment I would make the inside surfaces of the ramp out of of mu metal to shield the ball once it falls in the hole, otherwise a strong magnet will be pulling it back up the lower incline. I would use an iron ball with a smooth glass coating to reduce friction I would use a glass upper ramp , again to reduce friction A judicious ...

11

What a beautiful machine! It is obviously radiating away energy, because it is loud. I actually have a similar machine at my house. It is also a high-Q mechanical resonator with a very narrow frequency acceptance; I have compared it to standard frequency references and I believe its stability is $\Delta f/f \approx 1\times 10^{-4}$. Here's a photo of it: I'...

11

We already harvest energy from the Moon. It causes the tides and stress and strain and motion throughout the Earth. As a result, the Moon keeps getting farther away. (And it causes some heating in the Earth). The Moon at one time had a spin that was not locked to the Earth, and the tidal bulges in the Moon's shape caused by the Earth generated heat in the ...

11

Ignoring many of the limiting factors of this problem (like air and water resistance), this wouldn't do what you think it would because the screw driver would travel around Earth and come back to you from the opposite direction. When you catch it again, it would slow you down again by the same amount that throwing it sped you up. You'd come to a halt. So ...

10

The second law holds on average for systems of any size, large or small. If you have an isolated contraption containing just a few atoms, and you run it through some procedure (maybe as simple as waiting 5 seconds, or maybe more complicated), there is some probability that the atoms will wind up in a lower-entropy configuration at the end of the procedure ...

10

You are incorrect that there is a torque on the wheel. While the field from the permanent magnet may pull a bit on some sections of the wheel, the total pull averages to exactly zero. Yes, some of the bits of the wheel want to move clockwise to be closer to the magnet, but some other bits of the wheel want to move counterclockwise to be closer. We don't ...

9

At the point where the balls enter the water from the bottom they will experience "negative buoyancy" that exactly cancels out any work done by the balls floating up in the water - to enter the water column you have to do work against the pressure caused by the full column of water.

9

In the video I see that the author needs quite some force to load the gun. So here he puts the projectile in a high potential state, i.e. he provides the energy during the loading process. When released this energy accelerates the projectile. Hence, no violation of energy conservation. Edit Here a small representation with python and ten dipole magnets: ...

9

Is the reason we don't have any perpetual motion machines because of the first law of thermodynamics? That's one of the reasons, and it applies to a perpetual motion machine of the first kind, a machine that can produce work without any energy input in violation of the first law (conservation of energy). There is also a perpetual motion machine of the ...

8

I was surprised to see an effect that's actually real, and not a hidden motor or something like that. I believe this experiment can feasibly be repeated. The principles behind it make sense. The driving force comes from the density difference in the cup versus in the pipes. The cup has very few bubbles in it compared to the pipe. Why? Because: The ...

8

"... is there a way to show that you could build a perpetual motion machine from this?" Yes. Focus the radiant heat from a thermal reservoir onto a spot that is hypothesized to be raised to a higher temperature through its concentration into a smaller area. Now connect heat engine - a Carnot engine - between the hot spot as the engine's heat intake and the ...

7

But once it's moving in the vacuum of space, with no gravity or magnetic field nearby, could it spin nearly forever (aka billions of years) producing a magnetic field, No. A rotating magnet creates a changing magnetic field. Similar to an oscillating electric field, it will radiate electromagnetic energy. This energy will come from the rotational energy ...

7

We do. For instance in a hydrodynamic power plant we convert potential energy to kinetic and via a turbine to electric energy. Or what do you mean by harness gravity? It's not a radiation like light, where we use electromagnetical waves to produce energy. If you are asking why we don't do this with gravitational waves, the answer is simple: they are so weak ...

7

Instead of thinking about conservation of energy, you should look instead at the second law of thermodynamics (which perpetual motion machines almost always violate). The concept of entropy is entirely numerical (log of the number of microstates of a system), so violating the laws of thermodynamics/statistical mechanics is tantamount to violating the laws of ...

6

Nope, not if you use the standard definition for "perpetual motion machines of the first kind", which can indefinitely produce work. Entropy increases monotonically over the entire universe, and eventually all free energy will be gone. This is known as heat death. Will particles in the universe continue moving forever? Probably. But that's not what ...

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