WHEN a liquid or a gas is flowing through a horizontal pipe and encounters a constriction in the pipe, there is a higher velocity of the fluid and a lower pressure in the constriction than in the larger section of the pipe. At first thought, this is contrary to what one would expect, for the crowding of the fluid into a smaller section would apparently raise the pressure. Closer analysis, however, shows that places in the pipe where the velocity of the fluid is greater must be places of lower pressure and at places where the velocity of the fluid is less, the pressure must be greater.
Consider a definite mass of water as m in Fig. 1. When this piece of water moves from position A to position B, its velocity is increased
since the velocity of the water in the smaller section of the pipe must be larger than in the larger section if the same amount of water per second which flows through the larger section is to go through the smaller section. Since the velocity of this mass of water is increased (mass m is accelerated) there must be an unbalanced force acting on it. This unbalanced force is furnished by a higher pressure at position A than at position B. That is, the pressure behind the moving mass m is greater than in front of it and, consequently, the velocity is increased. As the piece of water leaves the neck in the pipe, the pressure in front of it is greater than the pressure behind it and it slows down to the lower velocity in the larger section of the pipe.
The generalization of the above described phenomenon is, that places in a fluid where the velocity is relatively greater are places of lower pressure and places where the velocity of the fluid is relatively