High school student here trying to learn fluids. This is not a homework question, just a concept that I am trying to grasp but can't. The picture below is in my book: The book tells me that the acceleration of the fluid after experiencing the decrease in cross sectional area is due to a force that arises from the difference in pressures of the slower fluid and faster fluid. But the difference in pressures comes from the difference in velocities of the fluids. This leads me to conclude that the fluid accelerates because of a force that comes from the difference in pressure, which in turn comes from the difference in velocities. But the difference in velocities comes from the difference in pressure? This seems very cyclical to me. I asked an online tutor and he simply told me that it's a "chicken and egg" thing, but that didn't seem right to me. Can someone help me understand this? Does the pressure difference come from the change in velocity or does the change in velocity from the pressure difference?