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Homework Help: Perpetual Motion (explain why it's impossible)

  1. Jan 14, 2012 #1
    Obviously it's impossible, but no one on a different forum understood why not in terms of forces. Also the solution doesn't need to be quantitative.

    1. The problem statement, all variables and given/known data

    The one thing I'd change from the troll science picture below would be to change the balls full of air on a string to a flexible string with no balls of uniform thickness. Also assume that friction is negligible.
    http://files.sharenator.com/1288271730828_Lots_More_Physics_Troll-s943x507-102899-580.png [Broken]

    2. Relevant equations

    Archimedes' principle
    force of buoyancy = (force of gravity)(density of fluid)(volume of object)

    3. The attempt at a solution

    I realize that if the system rotates that its energy doesn't change, but at the same time looking at the forces, I don't understand why it won't rotate counter-clockwise. Please help me to understand this.

    4. The solution explained

    Even when a fluid is at rest, its molecules are moving in all directions.
    But if it's at rest, just as many molecules are moving left as are moving right. Any solid in the fluid will feel a pressure on all its surfaces due to the impacts of fluid molecules.

    In the presence of gravity, the pressure in a fluid increases with depth. If the pressure were constant on all sides of an arbitrary volume of the fluid, then it would sink due to the force of gravity, and it wouldn't be at rest. Therefore for a fluid to be at rest, the increase of pressure with depth must perfectly balance the weight of the fluid.

    Any solid fully immersed in a fluid will be acted on by the force of buoyancy. This force is due to the increase of pressure with depth; the fluid molecules hit the bottom harder than they hit the top. If this force is greater than the force of gravity acting on the solid, it floats.

    However, if there is a fish tank full of water, with a cylinder that extends both above the surface and below the bottom of the fish tank (like in the problem in this OP), then the water doesn't produce a buoyancy force. This is because there is no bottom of the cylinder for the molecules of the fluid to impact; all the molecules do is hit the exposed sides of the cylinder (and compress it a bit). So Archimedes' principle does not generally apply.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 14, 2012 #2

    Filip Larsen

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    In short, if we ignore the loop for a second and just look at a vertical string of balls, the sum of the upward forces on the balls in the fluid will not exceed the force needed to pull a new ball through the seal.

    If you look at the pressure drop over the seal it is hδg, where h is the height of the fluid, g is the acceleration from gravity, and δ is the density difference between the fluid and the gas in the balls. If you now look at a small "rectangular" ball of height Δh, the pressure difference (head) from that ball will be Δhδg so even if you had balls from top to bottom with an infinite thin layer of fluid between them they could provide a total "ball height" of ƩΔh = h. In practice you need some fluid between the balls so the total head from the balls will be less than h, i.e. ƩΔh < h meaning that the total upward force AδgƩΔh will be less than the total downward force Aδgh on the ball that is just entering the seal.

    The only way you could make the balls in the fluid pull a new ball through the seal and leave enough surplus force to drive a generator would be if the new ball had less projected area than the ones above it, so for a continuous process the balls had to become smaller and smaller thus making a loop of such "shrinking" balls only providing less and less "surplus" force to drive the generator. When you sum up, you would only be able to extract the amount of energy that you initially had to deliver into it by submerging the balls on the loop.
  4. Jan 14, 2012 #3
    You missed one sentence I wrote: The one thing I'd change from the troll science picture below would be to change the balls full of air on a string to a flexible string with no balls of uniform thickness.

    So that downward force, Aδgh, wouldn't exist since there are no balls nor variations in the thickness of the string. I hope you can understand and explain this. Thanks.
  5. Jan 14, 2012 #4


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    Sorry. While I acknowledge that you wish to essentially debunk this PPM attempt, the problem with discussing PPMs at all is that it attracts crackpots who will weigh in on the topic. The fact that the thread started off rational and intelligent does not mean it will stay that way very long. We've had a looooong history dealing with this.

    Thread slated for lockage.

    Last edited: Jan 14, 2012
  6. Jan 14, 2012 #5

    Simon Bridge

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    The string immersed in the water has mass to pull down - what your modification had done is change from a discrete problem to a uniform one. The uniform problem can modelled as a series of discrete masses with a massless string between them... so you get the same thing replacing the deltas with d's.

    But there is a more elegant description - the vertical part of the continuous rope has no buoyancy because the water pressure cannot act above and below.
    see the floating rope for a more detailed description.
    Last edited: Jan 14, 2012
  7. Jan 14, 2012 #6

    Simon Bridge

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    OP is not advocating perpetual motion but want to know how this does not work.
    It's bordeline - we can think of it as a "what is the mistake in the reasoning?" puzzle.
    Thus not a discussion of pmm exactly, but of the force description of buoyancy.

    The discussion goes like this:
    Q "I can see the energy balances, but the forces don't: how can this be?"
    PF: "because you have left out a force somewhere?"
    Q: "but where - I can't see it?"

    We should send OP to Donald Simanek's web page.
    ... the trollscience pic and the continuous string modification are explained there.
    Explanation in terms of forces is the hard part and why pmm enthusiasts get tripped up. Also why physicists end up favoring an energy model instead of a force one.

    Also could try:
    Last edited: Jan 14, 2012
  8. Jan 14, 2012 #7
    Thanks, Simon. I'm going to read the link you suggested. And Dave, your attitude is disappointing. Understanding why this doesn't work is exceedingly important, and I'd bet dollars to donuts I'm not the only one who doesn't fully understand this yet would like to. I hope this thread does not get locked. If it doesn't, I'll edit my OP with the most elementary explanation that I'm capable of once I understand it.
  9. Jan 14, 2012 #8

    Simon Bridge

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    In PF, topics like PMM are problematic as they lead to protracted and acrimonious discussions. That, I suspect, is why they are banned. It also means you have to be careful about your wording.

    There are important questions, therefore, that do not get resolved here as a result - and many people must leave the forums thinking that scientists are a very intolerant bunch. But the alternative is a mess and there are other forums for dealing with these questions ... skeptical forums like the JREF are usually helpful.

    DaveC gave you links also to help you understand this.
    That is a perfectly reasonable response.

    You have to do a lot of reading to understand how PMM reasoning is flawed since it means overcoming issues with the way you have been taught physics.
    But I think it is worth the effort.

    I'd propose that threads being killed under the pmm rule should also include a link to the two pmm pages above.
    Last edited: Jan 14, 2012
  10. Jan 14, 2012 #9
    If I'm understanding you correctly, this is incorrect.
    Yes!! This is the kind of help I needed to understand the problem. Now I understand :) I truly appreciate your help.
    It certainly is worth the effort. And I propose that threads like this are not locked. If I'm permitted to write the explanation then many people will learn something very important about buoyancy like I just did.
  11. Jan 14, 2012 #10


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    You make a good point, but you ARE overlooking the point made by Dave and others. PMM threads eventually attract wingnuts and devolve into ridiculousness, and THAT is what is banned on this forum, not the valid kind of discussion you are looking for. I second Simon's suggestion that such threads CAN be continued, if the wording is careful.
  12. Jan 14, 2012 #11


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    To drive home the point, 99% of the time the original poster doesn't say:

    They will say "you're all stupid and wrong and this is big OIL preventing FREE ENERGY because GREEDY BUSINESS WANT MONEY!!!" or some other form of calling everyone who disagrees with them ignorant and alluding to giant conspiracy theories. In fact, I can't recall a single perpetual motion discussion, besides this, that did actually want to know why their machine couldn't work.

    A vast majority of people who start PMM threads don't want to know, they just want to tell. So considering the moderators are all volunteers, it's good policy to just ban the topic instead of having to monitor and close 100 bad threads so the 1 good thread could be allowed to run.
  13. Jan 14, 2012 #12

    Simon Bridge

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    Better to err on the side of banning - this one did not get banned right off, maybe it wasn't noticed in time or maybe it was well inside the right side of crackpot.

    Notice OP was not advocating a pmm and the example was a clear troll and the first post explicitly acknowledged conservation of energy effect and asked about a different representation. The whole tone was open ... it's not a fine distinction we are discussing here.
    The PMM folk who try to slip one by tend to be much more evasive.

    It's not as if there is a bot autolocking threads with the words "perpetual motion" in them so the moderator has to visit the thread and make a quick judgement. Adding links to pmm-skeptic sites before locking is still a good idea and no harder than copying and pasting from the rules.
  14. Jan 14, 2012 #13

    Simon Bridge

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    This is how a string falling off a table is handled ... in this case, the buoyancy terms will sum to zero.
    Sadly it is not as easy as that. We do lose out for this, however it cannot be helped.

    Sometimes I've felt frustrated in that I had an answer like the one above available that would help someone googling to the thread if not the OP ... but the thread got locked before I could type it in.

    But I understand the need to act quickly on these things and that the policy has been developed over a long time through hard experience.
  15. Jan 14, 2012 #14
    I just updated my OP with a solution that hopefully will satisfactorily explain the problem to everyone. Perhaps a mod can just edit the title to something buoyancy related.

    I just meant that there is no buoyancy.
  16. Jan 15, 2012 #15


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    No infraction. No warning. Just thread closed.

    Welcome to the PF.
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