>>14420903>isn't it logicalYou don't know what "logical" means. You mean to ask, "Isn't it intuitive?" The first thought that jumps in your head and makes sense to you is not the logical one by the definition of logic itself, that's literally intuition and explicitly not logic.
On that note, the majority of intuitive observations about the majority of physical phenomena are wrong. The actual explanations largely defy natural intuition.
See
>>14420953.
>>14420983>>14421003There's a few different ways to understand it, but each of them is most easily expressed in terms of energy. The first part to understand is that, in the flow of an uncompressible liquid, the sum of kinetic, potential, and internal energy is always constant.
The water moving at A_1 has a certain amount of energy. This energy is conserved in the system no matter what. As the water moves to point A_2, the cross-sectional area of the flow decreases. This means a smaller amount of water is moving, with an identical amount of energy it had before in the larger diameter pipe. Since there is less mass of water to move but an identical amount of energy, the water in the thinner pipe is literally easier to move. More of the potential energy of the system is thus converted to kinetic energy in the flow rate. This is why velocity increases in the thinner pipe - it is quite literally and simply because there is less water to move with an identical amount of energy.
Pressure in the system also comes from the energy in the system. The pressure of the water in the pipe is caused by the force of the water against the walls of the pipe. Since the water in the thinner pipe is moving faster - that is, more of the energy of the system is going to flow velocity - there is less energy nearer to point A_2 for the water "to use" to push against the pipe.
That's basically it. There's less water to move, so the same amount of energy moves it faster, meaning less energy to exert pressure against pipe walls.
