Water flows steadily through a horizontal pipe under a constant pressure difference. At a certain section the pipe constricts (narrows). At this narrow section, the pressure of the water:

stays the same
may increase or decrease
increases
decreases

Correct answer: decreases

Solution

Continuity equation A1*v1 = A2*v2 means a smaller cross-section gives a larger velocity. By Bernoulli's principle for a horizontal pipe, P + (1/2)*rho*v² = constant, so increased speed corresponds to decreased pressure. Hence pressure decreases at the narrow section.

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