What is the velocity of the water at the top of a glass of water? Is it really 0?
For example, there is a lot of problems which asks with what velocity does the water would leave that glass if I make a hole on the bottom. For these, we consider the velocity at the top to be zero. Why is that? Is it approximately zero or really zero? I am thinking now, if it is an incompressible fluid, it must be zero, because if the water is confined to that volume, cannot have speed. Nevertheless, it is clear that the water molecules are randomly moving. Can you clarify my doubts? Thanks in advance.