I saw this gif earlier and it bothered me how the plastic water bags with fish in it are not submerged in water like they would if we don't do anything special with the water inside and outside the bag:

several fishes, each in a separate plastic bag filled with water in somewhat spherical shape floating above the sea level, only slightly submerged in it

Then I read the comments on the corresponding Imgur page (I'm not affiliated with the post), asked my friends and figured that this could actually happen in some unusual conditions. I'm not very knowledgeable in physics, but from what I know, the sea water would have to be extremely salty so as to push out the plastic bags. Also the bags themselves must be very tight, so that the plastic material which they are made of contracts to form a spherical shape. Fish may not feel very well, as it might mean increased pressure inside the bag.

The spherical shape of the bags could probably be achieved with just high external pressure, but I'm not sure what could make the bags take a form of a drop. Perhaps if that little bit of air we see was actually so much lighter than the air outside the bags, that it would stretch up the whole bag and even raise its contents above water like that.

Are my guesses correct? What other effects could cause the illustrated state of things?


It looks like a bit of artistic licence was taken (gosh :-) ). You are correct that, the bags would not float that high up -- at least if I'm correctly viewing them as having only a tiny air pocket at the top.

The physics is simple: take the total mass of the bag+fish+bagwater, and calculate the equivalent volume of seawater for the same mass. That's how much volume the bag must displace, so if the bag 'assembly' has low enough density, the remainder of the bag's volume will be above the seawater surface. It's identical to the way icebergs float.

As a pal of mine says in situations like this, "because movie science magic" .


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