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Question related to the pressure of water

  1. Jan 17, 2012 #1
    Hi! I've been thinking about something, but I can't imagine if it would be physically possible. Hope you guys can help me out.

    Imagine there is a 8 meter-high rectangular container filled with water, and at the bottom of it (but on the side), there is an opening made of some sort of rubber material (or any other type of material that is flexible) that keeps the water from leaving the container, but could still let something in (much like it is on some water bottles, like powerades, when you need to either suck or squeeze the bottle to drink).

    My first question is: could this even be possible? Or would the pressure from the water not leave this opening closed?
    Second, if a heavy ball rolled relatively fast into this opening and got into the container, could it get in without letting too much water out?

    I've joined a drawing to help you understand what I'm saying (sorry about the quality of the drawing, I'm not too good at that haha)


    http://img28.imageshack.us/img28/5501/picture1dip.png [Broken]
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 17, 2012 #2

    Simon Bridge

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    This is the first stage of a line of inquiry that has lead many people astray.
    What you suggest is possible in principle - though in practise you have the stiffness of the valve at the bottom and water leaking out and so on.

    The next thing that occurs to everyone is to make the heavy ball less dense than the water (or use a dense fluid like mercury) so it floats up and pops out the end ... which can also happen.

    The next part is to wonder if this can be turned into a loop.
    Maybe the ball rises in the fluid, pops out the top, falls down to the bottom, where the energy it got from falling pushes the ball into the bottle and off it goes again?

    No it cannot - even with perfect seals and non-viscous fluids and zero friction and all the rest.

    What you want to do is go look at:
    ... where you will find many devices based on similar reasoning, including the one you are starting to think about.

    You are quite specific about dimensions ... where did you get this from?
    Last edited: Jan 17, 2012
  4. Jan 17, 2012 #3
    Thanks for your response, I will check out this website!

    As for the dimensions, I just came up with them to give more information and be more specific, in case the height might have something to do with the pressure level or something.

    What if we raise the valve to about half of the container for instance, and the ball rolls on a sort of platform into the valve? Would the water pressure be lower and the water leakage diminished?
  5. Jan 17, 2012 #4

    Simon Bridge

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    So I was right about the direction you were going with this?

    The height of the water does determine the pressure, you are correct.

    Raising the valve would decrease the pressure and the ball would enter more easily - but that still won't help make a circuit because you also have half the height to fall and so only half the momentum available to do the pushing.

    It is not a question of resistance getting in - turning this into a loop, which you can also tap for energy, would violate the law of conservation of energy. Even with a perfect valve, zero water leakage, zero viscosity, and no friction ... the best you can do is break even.

    Please be aware that perpetual motion is a banned topic on PF.

    This thread will be locked soon - go read the Museum of Unworkable devices site, all the physics stuff especially, I sent you to the explanation of the principles behind your idea but there's much more that will stop you going down blind alleys.

    You will find helpful people on the JREF boards who will discuss these things with you.
  6. Jan 17, 2012 #5


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    From the PF Rules link at the top of the page:

    Last edited by a moderator: May 5, 2017
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