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Air volume, compression and bouyancy

  1. Jan 15, 2007 #1
    Hi everybody! I'm new to this forum and i have a question i hope someone
    of u can answer...

    If i have a cylinder (made of plastic or some other light weight material) 1 meter long with an area of 1 dm2 that would be 10 liters right?

    Now, if i place the cylinder vertical and place a 8 kg piston on top of it (inside it on the upper end..), how much will the piston compress the air?

    Will it compress it to a volume less than 8 liter? If so, then the construction
    will sink if put in water right?

    If i let it sink to say 3-4 meter then turn the construction up side down, then
    the light weight cylinder will pull upward and the heavy piston will pull downward making the air decompress.
    Will it decompress enough to make the contruction move up to the surface again ?

    Hope u understand the scenario....
    Thanks for your reply :)
    Last edited: Jan 15, 2007
  2. jcsd
  3. Jan 15, 2007 #2


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    You are correct, with one caveat: water needs to be allowed to flow into the open end of the cylinder, above the piston, otherwise the cylinder is still full of air.

    Also, ther is a limit to how deep you can go, as the water pressure will start to compress the piston into the cylinder, making the whole thing sink all the way to the bottom.

    There are other complications too, such as the buoyancy of the weight itself.

    You're not hoping to find perpetual motion here, are you...?
    Last edited: Jan 15, 2007
  4. Jan 15, 2007 #3
    Yes, water should be allowed to flow into the open end. That wouldnt do any harm..

    How do i calculate this? I had a formula a couple of years ago but its lost and i just cant find any on the net...

    I dont like the word "perpetual motion". I would rather call it "extracting unused energy" or something like that... :)
  5. Jan 15, 2007 #4
    No. The air will exert a certain pressure on the piston, and the water will exert pressure in the opposite direction. The piston will not sink all the way to the bottom and push all the water out. It will find its equilibrium when the air pressure equals the water pressure.
  6. Jan 15, 2007 #5


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    KingNothing, that is not correct. Any scuba-diver will tell you that under-water buoyancy has no stable equilibrium. As the water pressure increases with depth, the volume of gas in a non-rigid object decreases and thus the buoyancy decreases with depth.

    To put a finer point on it, air pressure will always equal the water pressure (how can it not?). What determines whether something sinks or floats is the buoyancy and that isn't directly connected to pressure.
    Last edited: Jan 15, 2007
  7. Jan 15, 2007 #6


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    Bouyancy is the weight of water displaced, pressure is the height of a column of water times its weight density. If you want to find the depth at which you pass equilibrium (it is an unstable point, as I said), you'll need to solve simultaneous equations, equating the two. My LaTex isn't very good, so maybe I'll scan them in tonight if someone hasn't posted them first.
    Be very careful that you are taking into account all the forces and energies at work. Even things as simple as rotating the device upright or emptying the water from it require energy.
  8. Jan 15, 2007 #7
    To explain more in detail i will make a very simple drawing in ms paint and try post it here
  9. Jan 15, 2007 #8
    Here is a simple explanation of my idea...

    Attached Files:

  10. Jan 17, 2007 #9
    No comments? Is it a too foolish idea? Or is the picture difficult to understand?
  11. Jan 17, 2007 #10


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    Hi dille. I took a quick look at your picture. The left and right sides seem to be imballanced as you've noted. Most likely the problem is with the weight shifting as it is rotated. There are similar machines that have extended arms that allegedly produce a torque around the center of rotation. All these machines take some time to figure out why they don't work.

    There seems to be a steady stream of perpetual motion machines on this board, both here in the physics section and also in the engineering section https://www.physicsforums.com/showthread.php?p=1214757#post1214757"Needless to say, perpetual motion machines don't work and this one won't either. But the resolution is more of a riddle that will take more time to study than a person is apt to want to put in. These really are just riddles that ask, "why can't this perpetual motion machine work?" There is an answer, but it might take some time and there's so many of these things I suspect most people simply aren't interested in spending their time responding.


    On a side note, I think the board should have a new forum under the Skepticism and Debunking forum. Perhaps it could be named: Debunking Perpetual Motion Machines.

    The point is, such machines are just riddles at best. No one should take them seriously, and I don't think they should even be entertained in the physics or engineering section. It lends them too much credence and leaves the OP with the idea there may be some hope for perpetual motion machines.
    Last edited by a moderator: Apr 22, 2017
  12. Jan 17, 2007 #11
    Thank you for a great reply.
    But imo "perpetual motion" is a bad word to use. One could say that water power is perpetual motion, but it require the heat from the sun to work. If the sun was removed the water would stop to circulate.
    Same goes for my idea exept that gravity is required. Remove the gravity and it would stop.
    This guy have an interresting point:

    << crackpot link deleted by berkeman >>

    I read somewhere that the first rule of thermodynamic (conservation of energy) was based on the fact that no one had managed to build a working perpetual motion machine. And nowadays scientists say that perpetual motion cant be retrieved because of that rule....

    I don't really believe in "perpetual motion" but i believe that new ways to extract energy can be found.
    Maybe it is not as complicated as cold fusion or things like that....
    Last edited by a moderator: Jan 17, 2007
  13. Jan 17, 2007 #12


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    dille, I deleted the link you posted because the content was obviously crackpottery. We do not allow the promotion of crackpot ideas here in the PF. This thread had better get back to discussing real physics, because if it sways toward a debate about perpetual motion, it will be deleted.

    No warning points issued (yet).
  14. Jan 17, 2007 #13


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    Gravity isn't an energy source, though, so it doesn't power something any more than an oscillating spring does. That's why this is perpetual motion: it is extracting energy from where there is no energy to extract.
    No. The first law comes from very simple math. Whether it be a spring-mass system, a roller-coaster, or your device, it is a simple matter to add the kinetic energy to the potential energy and see that you always get zero.

    Try it. Take an object at height X, and apply Newton's laws of motion to calculate its speed when it hits the ground. Calculate the kinetic energy and potential energy and see if they are equal (and opposite).
    Certainly, but the point of the 1st law is that in some situations, there is no energy to be extracted.
    I did start to play with the numbers - it isn't hard, it just takes time and I wasn't particularly motivated. What I was finding is that the buoyancy of such a device is highly limited by the fact that water is so much denser than air: bouyancy difference starts off surprisingly small and quickly shrinks to uselessness.

    You really should learn to do these calculations yourself, though - this is something most people learn in junior high. For the amount of time you spend thinking and speculating about it, it is all utterly useless if you aren't learning anything. You're just spinning your wheels in place. Perhaps, then, you should post your calculations and we'll help you with them.

    For starters, you'll need to make some more assumptions about the things you are using. Ie, the cylinder's wall thickness can be ignored if it is neutrally buoyant, but the volume of the weight cannot. Assume constant water depth/pressure along the device (for now...). Try different aspect ratios and see how that affects it.

    Lead has a specific gravity of 11.4. Calculate the buoyancy of the cylinder upright and upside down using the following steps:

    -Calculate the pressure in the cylinder based on the area and the weight on it.
    -Ratio the pressures to the total volume to find the new volume.
    -Add the volume of the weight.
    -Now you have the new volume and mass (weight) of your device.

    Perform the same calculation with the cylinder upside-down.

    What you will find is that there is a buoyancy difference, but not that much.

    I'm not certain, but I suspect that where you lose your energy is in the rotation: The center of buoyancy and center of gravity are at two different places and the center of gravity moves as the device rotates. So as the cylinder rotates, its potential energy changes and you lose energy (you don't travel the same distance in both directions). That calculation is also very simple though: the center of mass is the geometric center of the weight and the center of buoyancy is the center of the volume (of the cylinder+weight).
    Last edited: Jan 17, 2007
  15. Jan 17, 2007 #14

    I have tried something simiilar to the concept you are attempting. it doesnt work, trust me
  16. Jan 17, 2007 #15
    yeah... when i was 12 i came here and ranted about how i thought gravity was a fifth dimension... can i have those warning points removed now. its been like two years :redface:

    energy needed to take one side of the contraption down would be equal to the energy needed to lift the other side back up, regardless of the densities, mass is constant until you approach light speed. SO your machine will be stuck at a standstill
  17. Jan 18, 2007 #16
    My calculations

    I will try to make some calculations from what i remember from the physics classes. Its been a long time since i studied so i really dont know if anything is right...

    To make it easy i use a weight (piston) with a density of 10.
    Piston volume = 1 cubic dm (1 liter)
    Piston weight = 10kg

    Cylinder area = 10cm2
    Cylinder height = 1m
    Cylinder volume = 1 liter

    Total volume uncompressed = 2 liter

    The pressure from the weight would be 1kg/cm2 and that would reduce the volume in the cylinder to 0.5 liter.
    Total volume compressed = 1.5 liter

    Since the piston density is 10x greater than water density and the piston is 1 meter long, water pressure would be the same as piston pressure at 10 meter deep.
    So we make the construction 9 meter.
    Every piston fall 0.5 meter in the cylinder so we can have 18 pistons on each side of the construction.

    The extra bouyancy on one side would be
    (uncompressed volume - compressed volume) x18 = 9kg

    So far so good. I know i have 9kg power on one side. But the rest i dont know how to calculate...
    I assume this:
    The power would be 9kg for a distance of 0.5 meter and the weight that should be lifted at the top is 10kg for 0.5 meter (meaning it wont work..)
    But at the same time there is a counter-weight at the bottom.. Those act like crank shafts. One 0.5 meter shorter than the other. Don't know how to calculate that...

    I believe that the volume difference will decrease depending on the depth?
    Then the "over power" would be less.

    Will the water pressure ADD to the piston pressure hence compress the air more ?
    What happens if the cylinder is turned at 10 meter when water and piston pressure is equal ? will the piston be like stuck in there?
    How to calculate the power needed to lift the top weight?
    Last edited: Jan 18, 2007
  18. Jan 18, 2007 #17
    Why must it be perpetual motion just because gravity is used ?
    Look at ordinary water power plants. What do they use?
    Gravity that makes the water fall.
    Heat from the sun that moves the water back up.

    Look at my idea.
    Remove the heat from the sun and the water wont be bouyant any more. And
    there would be no air pressure.
    So its NOT perpetual motion since it will stop work once the sun dissappear..
  19. Jan 18, 2007 #18
    Great answer!
    It made my brain function normally again lol
    Of course the masses are the same....no matter what medium they fall through...
    So....it can't work. Period. End of discussion.

    I was in to those gravity engines many years ago too and i came to a point
    where i realised they can never work.
    But now, for some strange reason i start discussing it again...

    Might as well close this thread..
  20. Jan 18, 2007 #19


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    In your example, it is gravity itself that is doing the work in both directions - no extra input from anything else.
    No. That isn't how pressure and buoyancy work.
  21. Jan 18, 2007 #20
    If the heat from the sun is removed then all water would freeze.
    Do you mean that it still is bouyant then ?

    And the atmosphere would dissappear too since the magnetic field that hold the atmosphere would dissappear, so no air to compress.....

    But it doesn't matter. Gravity engines can't work.
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