# Intuitively debunking a perpetual motion machine **URL:** \ **Category:** Factual Questions\ **Created:** [March 11, 2022, 4:01pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903 "2022-03-11T16:01:32Z")\ **Posts on this page:** 14\ **Page:** 5
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [March 15, 2022, 7:04pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/81 "2022-03-15T19:04:19Z")
> [@Senegoid](#): > > The idea has been mentioned already of putting this machine at the outflow of a dam to replenish the water or something like that. Some posts have commented, a little bit vaguely, on why this does or doesn’t work, or maybe it could provide some extra energy that can be extracted from the system. But I don’t think anybody has made the obvious point-blank observation, in so many words: > > If you do something like this, then you no longer have even a pretense of a Perpetual Motion machine. I think I am at the point of convincing him that the machine has a net negative energy output. (This is not an in-person discussion, it’s happening over text). It does nothing without an input of pressurized or overhead water. A person or otter pumping water is still an energy input. Having agreed on that, I hope to connect the dots that this is a hydroelectric system. It is in direct competition with hydro turbines and water wheels. AIUI those have an efficiency of at least 90%, so this needs to do better than that. Which – given that I calculated that it’s energy-negative _even neglecting friction and viscosity_, all the extra mechanisms are just going to make it even more negative. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [March 15, 2022, 7:26pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/82 "2022-03-15T19:26:07Z")
> [@HMS\_Irruncible](#): > > Which – given that I calculated that it’s energy-negative _even neglecting friction and viscosity_ , all the extra mechanisms are just going to make it even more negative. What factor(s) make it intrinsically net energy-negative rather than net-zero? > [@Chronos](#): > > The OP said that this was an engineer. If some tinkerer in his garage doesn’t understand the most basic fundamental principles of physics, well, no harm done. But if an engineer doesn’t understand the most basic fundamental principles of physics, then you get bridges falling down. I get the impression that even if OP succeeds in putting this guy wise to the impossibility of _even his_ perpetual motion machine, his bridges would still fall down. ---
**Author:** ![Kedikat](https://avatars.discourse-cdn.com/v4/letter/k/ed655f/32.png) [@Kedikat](https://boards.straightdope.com/u/Kedikat)\ **Post date:** [March 16, 2022, 12:28am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/83 "2022-03-16T00:28:51Z")
I have actually delved into this idea of an energy producing device. Quite a while back there are several posts of mine questioning aspects of buoyancy. Even with my crappy math skills, I came to the conclusion that the pressure present at the insertion point of the buoyant body will cancel out the uplift of any bodies in the water column. I still try and perfect the calculations from time to time to see just how close one can come to 0 in the system. It is at least more intuitively seductive than the rotating jointed arm contraptions proposed for free energy. As at least there are opposite forces acting positively on each side of the gadget. Unfortunately gravity still works on the water side as well. I enjoy imagining and then dissecting to destruction various things that are impossible, or at least so far impossible. I am inspired to return to this gadget and mentally find all the flaws. So what is the lowest friction O ring composition? ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [March 16, 2022, 12:43am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/84 "2022-03-16T00:43:51Z")
> [@Senegoid](#): > > What factor(s) make it intrinsically net energy-negative rather than net-zero? Well, the most obvious thing is the same problem that affects every perpetual motion machine: friction (or viscosity where fluid is involved). The machine stops if the lost energy isn’t replaced. The greater (and less obvious) problem with this specific machine is that it proposes to offset the cost of resetting the water column by recapturing the energy of the ball as it descends in air. That can’t work because in water, the buoyant force on the ball is a function of X liters of water, while in air it’s a function of X liters of ball material. If you imagine a floaty material like styrofoam, a liter of that is obviously much lighter than a liter of water. F=ma, therefore the forces aren’t anywhere close to balanced. That seems to be the biggest error in thinking here, the idea that buoyant force is somehow “free” just waiting to be harvested, rather than energy stored by pressurizing fluid to fill a tank, or to submerge a float under some natural reservoir. Also the notion that the water loss is negligible, and that moving water up and down, here and there, is something you get for free. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [March 16, 2022, 12:59am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/85 "2022-03-16T00:59:18Z")
> [@HMS\_Irruncible](#): > > That can’t work because in water, the buoyant force on the ball is a function of X liters of water, while in air it’s a function of X liters of ball material. If you imagine a floaty material like styrofoam, a liter of that is obviously much lighter than a liter of water. F=ma, therefore the forces aren’t anywhere close to balanced. This seems to get at the objection I was trying to think of. Think of the weights of the float and water instead of their volumes. Suppose the hollow float weight 1 oz, and has a total volume sufficient to displace 10 oz of water. So it sits at the surface, displacing 1 oz water. You need to push it down with an addition al 9 oz weight to submerge it. Altogether, this displaces 10 oz water, and that displaced water contains the potential energy you put there. Now let the float float, as floats are wont to do. As it pops up, 9 oz water can fall down, leaving the float floating, displacing the original 1 oz water. That just recovers the potential energy of the 9 oz you had to invest to push the float down. Now move the float off the water and drop in in the air. (Neglecting air resistance of course, because all cows a spherical.) You only have 1 oz worth of potential energy left in that float, and that’s all you’ll get by dropping it. You can never get that other 9 oz worth of energy that you wasted in pushing the float down in the water. So that’s where the massive net-energy-negative loss happens. Note the “intuitive” phrasing above, conflating “ounces” with some kind of units of energy. You wanted “intuitive”. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [March 16, 2022, 2:55am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/86 "2022-03-16T02:55:51Z")
> [@HMS\_Irruncible](#): > > That seems to be the biggest error in thinking here, the idea that buoyant force is somehow “free” just waiting to be harvested, rather than energy stored by pressurizing fluid to fill a tank, or to submerge a float under some natural reservoir. Ah, that’s an intuitive answer - at least for me. (Not an engineer) ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 16, 2022, 7:39am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/87 "2022-03-16T07:39:42Z")
> [@HMS\_Irruncible](#): > > That seems to be the biggest error in thinking here, the idea that buoyant force is somehow “free” just waiting to be harvested, rather than energy stored by pressurizing fluid to fill a tank, or to submerge a float under some natural reservoir. Also the notion that the water loss is negligible, and that moving water up and down, here and there, is something you get for free. Yeah, the whole idea is based on _what buoyancy seems to do_, rather than _how buoyancy works_. ‘Buoyancy makes light things go up’ is OK, but you can’t ignore the necessary ‘when, and because, a heavier thing goes down’ part. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [March 19, 2022, 2:50pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/88 "2022-03-19T14:50:48Z")
Epilogue: the individual I mentioned in the OP is now hospitalized for a psych issue. I’m glad he’s getting help and to know he’s not dumb, just temporarily unbalanced. Wish I’d listened to my gut last week. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 19, 2022, 5:59pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/89 "2022-03-19T17:59:16Z")
Oh boy. Hope he’s OK. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [March 19, 2022, 6:10pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/90 "2022-03-19T18:10:29Z")
> [@HMS\_Irruncible](#): > > the individual I mentioned in the OP is now hospitalized for a psych issue We often impute mental illness to those who have issues with scientific or political facts, but I don’t think very often do we think it’s literal treatable mental illnes, but rather the vernacular “you’re crazy” kind. This is kind of a first in my awareness. I hope this individual can make a full recovery, or at least be treatable to the extent of taking control of this illness rather than it having control of him. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [March 20, 2022, 2:54pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/91 "2022-03-20T14:54:30Z")
> [@gnoitall](#): > > This is kind of a first in my awareness. I hope this individual can make a full recovery, or at least be treatable to the extent of taking control of this illness rather than it having control of him. We’ll see. Not great at the moment. Apparently didn’t eat, drink, or sleep for 4 days, but getting proper treatment now. It was 100% treated and under control for decades, so we’re hoping things will return to that state. He had COVID recently and I’m afraid that some baggage may have shifted during takeoff and landing, neurologically speaking. An engineer pitching an apparent perpetual-motion-machine design… I had my suspicions, but it’s not easy to ask someone if they’ve lost a few marbles, especially me not being versed enough with the basic engineering. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [March 20, 2022, 3:40pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/92 "2022-03-20T15:40:52Z")
FWIW and I am not a psychiatrist… it sounds like a manic episode and that it was well managed for many years. With that track record there is every reason to expect that it can be gotten back under control relatively quickly. Best wishes to him and to you as his concerned friend. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 21, 2022, 12:30am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/93 "2022-03-21T00:30:31Z")
[Moderating] While we don’t usually close threads at the OP’s request, I think that the circumstances in this case warrant it. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 21, 2022, 12:30am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/94 "2022-03-21T00:30:38Z")
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