# 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:** 20\ **Page:** 4
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [March 14, 2022, 3:42am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/61 "2022-03-14T03:42:21Z")
> [@Monty](#): > > Isn’t buoyancy dependent upon gravity in the first place? It is (well, it’s dependent on an acceleration, which could come from gravity or some other source). However, gravity is not a source of energy by itself. Tidal energy comes from the rotation of the Earth relative to the Moon and Sun. Energy is extracted by slowing the rotation of Earth. Unless the buoyancy machine somehow extracts energy from somewhere else, it will not work. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [March 14, 2022, 4:11am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/62 "2022-03-14T04:11:11Z")
I think part of the challenge is to get someone to realise they’re promoting a perpetual motion machine rather than an efficient machine. ---
**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 14, 2022, 2:28pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/63 "2022-03-14T14:28:52Z")
> [@Senegoid](#): > > Let’s get a bit meta here: Why does it matter if someone believes he has a design for a perpetual motion machine? Is there any reason that every believer in perpetual motion needs to be shown the light? 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. ---
**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 14, 2022, 2:56pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/64 "2022-03-14T14:56:55Z")
Yeah. Maybe not even physics - belief in perpetual motion is pretty much a rejection of mathematical subtraction. ---
**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 14, 2022, 3:30pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/65 "2022-03-14T15:30:21Z")
> [@Chronos](#): > > “Slingshotting” the water is completely irrelevant, as is “scraping off” water. Those are places where inefficiencies will creep in, but you can make inefficiencies arbitrarily small: That’s just a matter of engineering. True, if I replace it with a hydrophobic surfaces or whatever then, yeah, the issue goes away… Sorry, I convinced myself that the discussion of “pushing new floats into the water” was basically talking about having the move water around, so the movement of the water was key. To confirm for myself, my understanding is that the basic process of flotation comes from Brownian motion of particles? It’s like if you have a box of dust and rubble, give it a shake, and you’ll see some objects work their way to the surface. Under that model, where we’ve got a bunch of small particles (water, but we might visualize it as a large number of small ball bearings) bouncing against each other and against our floats (which we’ll ignore that it’s also made of small particles and just envision as a metal tank of air), because the smaller particles are heavier they tend to shift under the float more than they tend to shift on top of it. That creates a pressure differential that moves the float upwards. My understanding of the given explanation, from this model, is that inserting an empty tank of air into a vibrating mass of ball bearings requires more energy than you can harness from the flotational (pressure differential) force of _any number_ of other floats that are already fully immersed (plus the force generated by an equal number of balls hanging on the other side of the contraption)? I feel like @HMS_Irruncible is giving some of the math to calculate this but he is only calculating for the weight of the balls on the downward side and doesn’t seem to calculate flotational force per ball (which could be thousands or millions of balls)? Am I correct that if we were to magically calculate that the flotational force of many many floats + their weight on the downward side was sufficient to pull new balls into the water, we would still be looking at a system that’s using the motion of the water particles to rise, and that motion comes from energy imparted into it by the sun? If you remove that external source then the water would freeze and - obviously - the system would stop. I.e., it is still not a perpetual machine. It’s just an elaborate solar panel? ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [March 14, 2022, 3:42pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/66 "2022-03-14T15:42:07Z")
You are on your way to deriving statistical mechanics. But the short answer is that if you do the maths, you will end up with exactly the usual gas laws, and that includes buoyancy. It doesn’t matter that we are talking about water, for the purposes of this device, we are not concerned with fluid flow, and we can treat the system without worrying about fluid flow. (No inertia or viscosity for instance.) That this can be done is perhaps one of the cornerstones of the idea of the “unreasonable effectiveness of mathematics”. Moving from ideal bouncing balls to gas laws by nothing more than the application of logic and mathematics is near miraculous. Getting to fluid properties is possible as well, but you need to add a whole raft of additional properties to the bouncing balls. Brownian motion is more the random motion of macroscopic particles that results from the motion of molecules in a gas or fluid, rather than the motion of the fluid components themselves. It is strong evidence that there such motion within the fluid, but not the motion itself. ---
**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 14, 2022, 7:25pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/67 "2022-03-14T19:25:28Z")
> [@Sage\_Rat](#): > > I feel like @HMS_Irruncible is giving some of the math to calculate this but he is only calculating for the weight of the balls on the downward side and doesn’t seem to calculate flotational force per ball (which could be thousands or millions of balls)? I actually worked up a spreadsheet of some of the more easily accountable details. The detail you mentioned is comparatively small. At the end of the day, the buoyant force of a 1L empty ball is identical to the gravitational force on 1kg of water. Ball goes up, water goes down. Ball cannot go up until that water is elevated again, and this is (by far) the biggest neglected factor in the concept. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [March 14, 2022, 7:40pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/68 "2022-03-14T19:40:09Z")
> [@pjd](#): > > Could otters be trained to do this bit ? Brilliant. I see this being the answer to many of life’s struggles. Let the otters do it! ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [March 14, 2022, 8:07pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/69 "2022-03-14T20:07:47Z")
> [@Lucas\_Jackson](#): > > Let the otters do it! Otterly brilliant! ---
**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 14, 2022, 8:17pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/70 "2022-03-14T20:17:06Z")
I suppose that, ultimately, Brownian motion is the origin of pressure in a fluid, but it’s not really relevant. All that’s relevant is that it somehow, in any way, has the pressure it needs to not collapse. You get the same buoyant effects for an object submerged in sand, even extremely cold sand-- It’s just that with sand, the friction is enough higher that you might not notice it. ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [March 14, 2022, 8:34pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/71 "2022-03-14T20:34:21Z")
> [@HMS\_Irruncible](#): > > Ball goes up, water goes down. Ball cannot go up until that water is elevated again, I think this is exactly right, especially since he is describing a closed system, with no water loss. All the water that goes into the water lock has to be pushed back into the water column, or is pushed upward when the ball is inserted into the water column. . With an outside source of elevated water, he could just drain the water lock to the open air, but at that point a hydroelectric plant will almost certainly be more efficient. ---
**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 14, 2022, 9:19pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/72 "2022-03-14T21:19:02Z")
> [@HMS\_Irruncible](#): > > Ball goes up, water goes down. Ball cannot go up until that water is elevated again, and this is (by far) the biggest neglected factor in the concept. Prediction: He’ll argue that the water can be recycled into the top of the column by evaporation/condensation (which of course it can, but that requires an energy input, and there are probably more efficient ways to exploit such an input) ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [March 14, 2022, 9:21pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/73 "2022-03-14T21:21:22Z")
> [@HMS\_Irruncible](#): > > 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. This is the showstopper. Assume your water-lock is full of water at the start, and open to the water-filled vertical tube. Your buoy is in the tube, at the bottom; you release it, it floats up, doing mechanical work for you on the way up via chain connection to a dynamo, and also adding gravitational potential energy to the buoy. Once the buoy surfaces, the water level in the tube decreases by an amount equivalent to the volume of the buoy. Now bring your buoy outside the tube and back down to ground level. Maybe you even harvest the buoy’s gravitational potential energy by getting it to do mechanical work on its way down, too. Back down at ground level, you now have to get your buoy back into the water filled tube. You close the inner door of your water-lock, and open the outer door. You were clever: you put the outer door on the top surface of your water-lock chamber, so water doesn’t spill out, and your water-lock remains filled to the brim with water. Except now you have to put that buoy into that water-lock. You’re gonna have to take some water out of the water-lock to make room for that buoy. Every time your buoy makes a round trip and gets back down here to the water-lock, you have to either: A) discard one buoy’s worth of water from the water-lock, which is ultimately replenished from what’s in the tube. The height of the water column in the tube will decrease over time, until it’s all gone and there’s no more water column with potential energy left to give to the buoy (so, not a perpetual motion machine); or B) pump one buoy’s worth of water from the water-lock back into the tube, raising the height of the water column back to its original level and providing an external input of energy to the system (so, not a perpetual motion machine); or C) lift one buoy’s worth of water out of the airlock, hoist it up to the top of the tube, and pour it back in, raising the height of the water column back to its original level and providing an external input of energy to the system (so, not a perpetual motion machine). or D) wait, no, there is no option D. Either the system runs out of energy over time as your buoy delivers mechanical work to your dynamo (and the water level keeps going down), or you keep replenishing the system’s energy by taking water out of the water-lock and either pushing it or lifting it back into the water column. ---
**Author:** ![abcdefghij](https://avatars.discourse-cdn.com/v4/letter/a/43a26b/32.png) [@abcdefghij](https://boards.straightdope.com/u/abcdefghij)\ **Post date:** [March 14, 2022, 10:13pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/74 "2022-03-14T22:13:38Z")
> [@pjd](#): > > Could otters be trained to do this bit ? I think this dagger-toothed little demon should be named “Maxwell” [https://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/A\_Fine\_set\_of\_Teeth\_(5619700893).jpg/732px-A\_Fine\_set\_of\_Teeth\_(5619700893).jpg](https://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/A_Fine_set_of_Teeth_%285619700893%29.jpg/732px-A_Fine_set_of_Teeth_%285619700893%29.jpg) ---
**Author:** ![Civil\_Guy](https://avatars.discourse-cdn.com/v4/letter/c/d78d45/32.png) [@Civil\_Guy](https://boards.straightdope.com/u/Civil_Guy)\ **Post date:** [March 14, 2022, 11:42pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/75 "2022-03-14T23:42:42Z")
Maxwell Demon, of course. How else? ---
**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, 2:08am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/76 "2022-03-15T02:08:30Z")
> [@Cheesesteak](#): > > With an outside source of elevated water, he could just drain the water lock to the open air, but at that point a hydroelectric plant will almost certainly be more efficient. 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. ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [March 15, 2022, 2:16am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/77 "2022-03-15T02:16:15Z")
> [@Senegoid](#): > > 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. - > [@running\_coach](#): > > 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. > [@running\_coach](#): > > What work is it producing? A PM machine has to produce work/energy without further input to qualify. > [@running\_coach](#): > > No, I’m not. A PM machine **does not** extract energy from the surroundings. Text to keep **discobot** happy. ---
**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, 2:19am UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/78 "2022-03-15T02:19:33Z")
> [@running\_coach](#): > > That means it’s not a PM machine. Okay, that one says explicitly and specifically that it’s not a PM machine. Your other quotes merely reiterate what a PM machine is. But to really emphasize the point, one needs to spell out **Perpetual Motion Machine** in full. ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [March 15, 2022, 6:35pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/79 "2022-03-15T18:35:37Z")
It’s not otters, it’s porcupines that are needed. [![](https://ic.pics.livejournal.com/black_samvara/3731370/27682/27682_original.jpg) ](https://ic.pics.livejournal.com/black_samvara/3731370/27682/27682_original.jpg) ---
**Author:** ![Civil\_Guy](https://avatars.discourse-cdn.com/v4/letter/c/d78d45/32.png) [@Civil\_Guy](https://boards.straightdope.com/u/Civil_Guy)\ **Post date:** [March 15, 2022, 6:49pm UTC](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903/80 "2022-03-15T18:49:16Z")
Genetically modifying porcupines so they _aren’t_ allergic to raisins. Well, let’s get on it! [Previous page](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903.md?page=3) [Next page](https://boards.straightdope.com/t/intuitively-debunking-a-perpetual-motion-machine/960903.md?page=5)