Start by filling a plastic bottle right up to the very top.
Then cut out a piece of tin foil of about 5cms x 3cms. Fold it carefully to make a little wallet and place it on the surface of the water right at neck level. Put the lid on the bottle and all you've got to do now is to squeeze the bottle hard to see your little submarine go down.
When you release the pressure the submarine goes up and by adapting pressure, you can bring it to whatever level you want!
NB: for any technical problems, see on the bottom of the page.
There you go!
Pa < P
Ca coule !
When you introduce your submarine into the bottle - lets call it ludion - its weight P ( which remains invariabvle ) tends to draw it towards the bottom whilst Archimedes force Pa tends to draws it to the top and which depends, amongst other factors, on the volume of the submarine.
When you exert pressure on the bottle, the water which cannot be compressesd, transmits entirely the force
because you have trapped a tiny air bubble whilst making it.
By being compressed Pa decreases and when it gets smaller than the weight then its the latter that wins and it sinks. When you let go the volume of ludion increases Pa as well and when it becomes greater than P, it goes up!
If you manage to immobilise the ludion half way up, then you have found the exact point where Pa=P
Pa > P
Ca remonte !