Here is a paradox that came to my mind during my fluid mechanics course last term. I don't know the solution to it: We have this experiment in which we hold two sheets of paper parallel to each other and blow between them. They are brought closer to each other: http://sepulki.net/loro/benoulli1.bmp [Broken] The air outside is stationary, and the air between the sheets moves, so from the Bernoulli theorem it follows that the pressure is higher outside and lower inside - this implies that the forces on the sheets point inwards and bring them closer to each other. Now let's consider this experiment in the reference frame of the air moving between the sheets of paper: http://sepulki.net/loro/benoulli2.bmp [Broken] Now the air in between is stationary, and the air outside is moving, so it would mean that the pressure is higher inside, and lower outside and so now the forces on the sheets point outwards and draw them apart. What is wrong with this reasoning in the moving frame? To be honest I'm not exactly sure if this situation can really be treated as an irrotational flow (and if the Bernoulli theorem is applicable).