Intuitively debunking a perpetual motion machine

Factual Questions
1

OK - fair disclosure, the device I’m describing is definitely a (putative) perpetual motion machine. It was designed by someone I care about, engineer by trade, who is concerningly obsessed with it.

It is a system designed to extract energy from buoyancy and recapture it via gravity. Basically, a water column where energy is extracted from a rising buoyant object via a chain attached to a spring or battery or something. Then the object is removed from the water, and dropped, where the energy of the falling object is extracted by a chain. The object is then placed back at the bottom of the column via a water-lock system to prevent loss of water from the system. Repeat ad infinitum.

I feel like if I invested a few weeks in re-learning college physics, I could give a detailed accounting of the energy loss, but that’s not really my strength. I am looking for an obvious intuitive hole I can point out.

Neglecting friction and viscosity, I feel like the energy extracted from a buoyant object of volume X, rising a distance Y, should be exactly equivalent to the energy extracted from a water of volume X falling that same distance Y. Since the mass of a buoyant object is by definition less than what it displaces, then the same object doesn’t get the same gravitational acceleration while falling, and cannot make up that energy. I know that’s a wordy way to say it, but am I basically on track?

Also, I fear I may be solving the wrong problem here, so if anyone has any thoughts on how to compassionately deal with a loved one who is obsessed with that sort of thing, I’d appreciate it. He seems totally fine other than this, but there’s a history of psychiatric issues that I can’t ignore. As distractions go, this one is pretty harmless, so maybe my best course is just to leave him to it.

So to recap, I’m looking for the most obvious and intuitive way to point out why this thing is a perpetual motion machine.

2

I didn’t quite follow the description. How is an object both heavy enough to sink and light enough to float?

If the design involves attaching weights, then you need energy to bring the weights back up, or just happen to be in a place with lots of heavy objects already suspended in some way (in which case that potential energy can be harvested in more efficient and direct ways).

Or I’m just not following.

3

OK so you have 2 tubes immersed in water. One filled with water and one empty.

The float travels from the bottom to the top of the flooded tube doing useful work and is returned to the bottom of the empty tube without loss. Now to reinsert the float into the water at depth will require the same amount of work you just extracted.

But, a clever inventor might use a U shaped tube and a valve system that replaces the float in the tube and then opens another valve in the bottom to flood the chamber. On close examination there is always a point where you initially have to over come the water pressure at depth.

4

The rise and fall of the oceans are powered by gravity. That’s adding energy to the system. That means it’s not a PM machine. Your friend should stop calling himself an engineer.

5

It’s been done.

Which is not to say that it works.

6

Don’t try too hard to make sense of it. Perpetual motion requires the observer to intuitively understand it, and it falls apart if you dig too deep.

[Album] Then a Miracle Occurs
7

A more common version of this machine is a wheel with a thick wooden rim. The wheel is mounted on the edge of a tank such that the center shaft runs along the top edge of the tank. The lower right quadrant of the wheel is immersed in water and the top of the wheel and it’s left side are not immersed. The rim will float in the water and then return in free air.

8

What work is it producing? A PM machine has to produce work/energy without further input to qualify.

9

Needs more sponges.

10

The sinking phase of the cycle isn’t in the water column, it’s in an air column. The object is laterally shifted once floated to the top, then (supposedly) we recapture the energy as it free-falls in air. Then we insert the object back at the bottom of the column via a water-lock and repeat the process.

I think when we insert the object back at the bottom, either we lose water that ultimately reduces that water column height when equalized, or somehow we are spending energy to force the object and raise the water column.

Intuitively I know it’s wrong but I’m just not fluent enough in physics to summon the obvious and concise explanation why.

11

How much energy does the lock system consume?

12

The catch in this proposed design is the point where the float is pushed back into the tube of water. In order to get that object back into the tube requires pushing the same volume of water out of the way. The only place for that water to go is up. Then that water flows back down to lift the float up. There won’t be enough energy available to keep pushing that object back into the tube, even before accounting for the energy lost into the water-lock system.

If your friend is actually an engineer, maybe you can get him to see sense by pointing this out? Probably not, but it might be worth a shot.

13

In both directions, the object pulls a chain that transmits the energy to a work output or storage output or whatever. The energy input is… just in environment, in the form of buoyancy and gravity, waiting to be extracted.

The more I write this out, the crazier it sounds, but I don’t know how to explain that you can’t just skim endless energy off gravity and buoyancy.

14

I would guess whatever is necessary to push down the entire object until it’s submerged. There will be friction and whatnot to actuate the doors, but I feel like that’s negligible compared to the broader energy balance problem.

15

The energy to raise a volume of water equal to the volume of the ball to the top of the column is not negligible. The ball will displace that much water and so doing will have to push the column up.

16

A PM machine has no continuous energy input, just the initial application to get it going. It also has to output energy (generating electricity, grinding grain, moving a vehicle) which will produce friction/drag and quickly bring the machine to a halt.

17

That’s the tough part

18

IANA physicist, but I recommend the Museum Of Unworkable Devices https://www.lockhaven.edu/~dsimanek/museum/unwork.htm

19

The float initially bobs to the surface - that’s the easy part

20

Okay, so you’re suggesting this machine can run indefinitely by extracting energy from gravity and buoyancy in the environment, without other inputs? Because I think that also will not work.

When I say “perpetual motion machine”, I mean a system with flaws that prevent it from working even in theory. I don’t think this thing works even in theory, but I can’t articulate why.