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A boat in water, pressure, motion, et cetera

  1. Nov 16, 2003 #1
    A question (and no, it's not for uni work).

    A boat sits in the water. Attached to the front of the boat is a bucket (or some other vessel), submerged. The bucket is sort 0of a black hole for water. Anything going into the bucket vanishes, to another world, the Pentagon, Michael Jackson's basement, whatever. It just goes.

    Now, the water will pour into the submerged bucket due to gravity and pressure and all. That's happening at the front of the boat.

    Will the boat move forward?
  2. jcsd
  3. Nov 16, 2003 #2
    No, because water is entering the bucket from every direction.
  4. Nov 16, 2003 #3


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    You pose a non physical question and want a physical answer?

    With your magic bucket in the water it is anybodys guess as to what will happen. Since you made up the bucket you can make up the effect it has.
  5. Nov 16, 2003 #4
    The Shrodinger's Cat 'thought experiment' was hardly a physical question, yet it helped illustrate a good point.
    Integral, don't you think that questions such as the one posed help us to improve our grasp of Physics?
  6. Nov 16, 2003 #5


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    Schroedinger's cat, Ie a cat in a box, was completely physical.

    This proposes vanishing water, we have no mechanism for it, it simply vanishes. Ok, make it a black hole, then its not a bucket in the water, its a black hole. Perhaps something could be done with that. We need a real world mechanism to reach real world conclusions.

    The given statement is something out of sci fi, so the answer can be given from the same realm, that is anything is possible.

    No, I do not think seeking physical solutions to non physical situations aids in understanding the real world. As soon as you massively violate a basic physical principle we have exited the real world for a fictional one. I prefer to deal with real world problems.

    If you are writing a Sci Fi story where such a thing exists then you can also create the effects it will have.
  7. Nov 16, 2003 #6


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    Yes it was. That's exactly the point. A thought experiment consisting of nothing non-physical, can be devised, which has an ambiguous outcome. It's exactly why that experiment is such a valuable tool for understanding physics. OTOH, a bucket which simply causes water to disappear, is non-physical, or at best, an incomplete description. If it's a black hole, we can figure out what will happen. If water just disappears, we cannot. Does a vacuum exist below the not-yet-disappeared water? Does the disappearing force affect nearby water without disappearing it? Does gravity work normally near the bucket? Can the bucket really differentiate between water molecules and boat molecules? How? Unless we know these things, there is no way to calculate the rate of disappearance of water, and no way to say what will happen.

    Edit: Sorry, I was typing the same time as Integral, and said mostly the same thing.
  8. Nov 16, 2003 #7


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    If the "bucket" is afcing forward then yes, the boat should move forward. Water pressure in front of the bucket will be less than that behind it, creating hydrodynamic lift in whatever direction the open end of the bucket is facing.
  9. Nov 16, 2003 #8
    Okay, change "magic bucket" for "mobile drain" which is stuck to the front of the boat. It doesn't matter. Water is leaving from in front of the boat. It seems to me that would mean slightly less pressure at the front end, so pressure around the boat from everywhere else would force forward motion. Is this wrong?

    Another question. Have you ever filled the kitchen sink, or a bowl, with water, then poured some washing detergent on top, and made a small vaguely boat-shaped (pointed bow, flat stern) thing and sat it on the top?
  10. Nov 16, 2003 #9


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    Are you suggesting that there is some gravitation that only interacts with water? What it looks like your trying to do is contrive a sink for water, and then qualify the effects on the surrounding objects in the water. This is very close to a physical model, and it doesn't violate any physical laws that I know. There is a continuity equation that says:

    ∇*J = -∂ρ/∂t.

    (The asterisk is intended to indicate the dot product).

    In your thought experiment, ∂ρ/∂t is some value, a constant for simplicity, say Jsink. This gives you an equation for the flow of water. Now, just determine the coupling of the water to the boat (non-trivial), and you can determine the effect on the boat. Of course, the boat will probably introduce whacky boundary conditions when you solve the diff. eq. so I don't think this is an easy problem to solve in detail. I would imagine, as a first guess, that the boat wouldn't go anywhere.
  11. Nov 17, 2003 #10
    I agree with what Integral and Krab state about the non-physicality of the question, but don't we spend much of our time in Physics considering frictionless surfaces, point masses, infinite distances, field free areas etc......

    This helps us to understand the Physics, but they are hardly 'genuine' circumstances are they?
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