Decreased pressure occurs in areas of high flow speeds. This is known as the:

• A. Continuity rule
• B. Reynolds number
• C. Poiseuille's law
• D. Bernoulli effect

Answer: (D)

The idea is Bernoulli’s principle: along a streamline in steady, incompressible flow, the sum of pressure energy, kinetic energy, and potential energy stays constant. When flow speed increases, the kinetic-energy term goes up, so the static pressure must drop to keep the total energy the same. That’s why areas with high flow speeds exhibit lower pressure — the faster the fluid moves, the lower the pressure it exerts. The equation P + 1/2 ρ v^2 + ρ g z = constant (with height held roughly steady) formalizes this relationship. The other options describe related ideas—continuity governs how speed changes with cross-sectional area, Reynolds number concerns flow regime, and Poiseuille’s law deals with viscous pressure losses in tubes—but they don’t capture the direct link between increased speed and decreased pressure that Bernoulli’s principle explains.