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I Fun question: Can this fish swim?

  1. Jul 22, 2016 #1
    Imagine a fish in a tank that is completely filled with water and closed on all sides. Can this fish swim?
    Since water is extermely difficult to compress, when the fish moves its fin to propel itself forward, the water can't move anywhere, so the fish should not be able to move right?

    Then I thought about the dissolved air in the water, and air can be compressed. So can the fish move if the water is saturated with air?
     
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  3. Jul 22, 2016 #2

    DaveC426913

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    Well, fish at the bottom of the Mariana Trench can swim, even with 11 kilometers of water on top of them.

    The fact that water is (almost) incompressible does not mean it is super-viscous.

    A fish will contract (or angle) its fin to make a smaller surface area in the direction of motion (like a slice pf paper on-edge). Then, when it moves its fin forward, the water molecules can flow sideways a tiny bit, out of the way of the fin. It then expands (or tilts) its fin to make a large surface aea, resutling in less water being able to escape around its perimeter. When it moves its fin backward, the now trapped water drives the fish forward.
     
    Last edited: Jul 23, 2016
  4. Jul 23, 2016 #3
    I think it depends also on the size of the fish, if the fish is small, and its fin is small, then as the fin and/or fish moves, the amount of water displaced is small, so the compression (if needed, not sure about that, check what DaveC says) will be also small, maybe a compression of something like 1% ratio, probably the water is compressible for such a small ratio.
     
  5. Jul 23, 2016 #4

    DaveC426913

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  6. Jul 23, 2016 #5
    If there is no where for the water to be displaced since its a completely closed tank, then it shouldn't be able to move. I didnt think about the fish km's below though
     
  7. Jul 23, 2016 #6
    Well, the water doesn't seem compressible at big depths , BUT, even in big depths the water isn't constricted and "forced to be compressed" when the fish moves like it will happen when the water is fully enclosed in a tank from all sides. When the tank is firmly closed the water (as the fish moves) will be forced to be compressed as it has nowhere else to move.
     
  8. Jul 23, 2016 #7

    jbriggs444

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    Water can move without changing its volume.
     
  9. Jul 23, 2016 #8

    DrGreg

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    When an object moves through water, it's true that water has to move away from the region in front of the object. It's also true that water has to move towards the region behind the object. The two effects cancel each other out as far as the total volume of the water is concerned.
     
  10. Jul 23, 2016 #9

    sophiecentaur

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    Trying to take real physical occurrences to extremes will usually let you down because it can often lead to apparent paradoxes that don't actually exist.
    Water is not 'displaced' in the wider sense. It is caused to circulate; if there were no friction, the water would be moving in closed loops (vortices) but, in real water, that circulatory motion dies down so the pressure on the fish's nose is less than the pressure of reaction on its tail.
     
  11. Jul 24, 2016 #10
    Yes. The fish being able to move has nothing to do with water compressibility. Even if water were totally incompressible, the fish would still be able to swim.
     
  12. Jul 24, 2016 #11

    CWatters

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    The water and the fish can just swap places. No need for the water to be compressed.
     
  13. Jul 24, 2016 #12

    DaveC426913

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    The issue comes down to understanding how water can move at all if it is (ideally) incompressible.

    If you had a bucket of marbles that were fully packed, you would not be able to move something through it. They cannot slide past each other - or swap places - without having at least a bit of wiggle room.
     
  14. Jul 24, 2016 #13
    I thought of that, but then again I thought that the swapping cannot be done (I suppose in the way that DrGreg says in Post #8) unless there is some free space for the water to move there first, or unless the water is compressible (what DaveC says more or less in post #12).
     
  15. Jul 25, 2016 #14

    A.T.

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    It's a continuous process.
     
  16. Jul 25, 2016 #15
    Are you saying that I can't swim under water in a fully enclosed tank? (Aside from running out of breath)
     
  17. Jul 25, 2016 #16

    jbriggs444

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    It is an elementary and well established experimental fact that water is a liquid (*). Please examine the definition of liquid to determine what that means.

    (*) Under a wide range of conditions in which it does not solidify as "ice" or evaporate into "water vapor". Maintaining a constant volume for a quantity of existing liquid water is not a condition which is observed to cause either transition.
     
  18. Jul 25, 2016 #17
    Let's forget about how the fish is being propelled for the moment. Let's just model the fish as a sphere that is moving forward in the tank at a constant velocity. Let's imagine that we are outside the tank and we are moving with the same velocity as the fish, so that the tank appears to us to be moving backwards. What does the streamline pattern in the tank look like to us (assuming the fluid is inviscid)?
     
  19. Jul 25, 2016 #18
    Got no clue...

    Ok well, I think I get it, water is liquid, the streamlines it will make will transfer water volume from the front of the fish to the back of the fish, in a continuous way without the need for extra free space or the need for compression...
     
  20. Jul 25, 2016 #19
    The streamlines will be parallel far ahead of the fish and far behind the fish. But in the vicinity of the fish, the streamlines have to pass through the gap between the fish and the sidewalls. So they have to converge (get closer together) and the velocity will have to speed up a little (so that all the fluid passes through the gap). At the hind portion of the fish, the streamlines will have to diverge again, and the velocity will have to slow down to again match the velocities of the front and back faces of the tank.
     
  21. Jul 26, 2016 #20

    CWatters

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    There is space. Molecules of water are nothing like solid glass marbles. They are spaced apart and at room temperature their average speed is >1000 mph.

    This (http://galileo.phys.virginia.edu/classes/304/h2o.pdf) has info on the properties of water such as the mean free path of a molecule (the average distance it can travel between collisions with another).
     
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