According to Bernoulli's principle, when air velocity increases, what happens to the pressure?

• A. Decreases
• B. Increases
• C. Stays the same
• D. Fluctuates

Answer: (A)

In steady, incompressible flow along a streamline, the total pressure (static pressure plus dynamic pressure) stays essentially constant. Dynamic pressure is ½ρv². So when velocity increases, the dynamic part grows, and the static pressure must fall to keep the total the same. That’s why faster-moving air has lower static pressure. You see this in action on a wing, where air speeding over the top surface creates a drop in pressure and generates lift, and in a Venturi tube, where constricting the flow raises velocity and lowers pressure. So the correct idea is that pressure decreases as velocity increases. This applies best to smooth, steady flow along a streamline; in other situations, behavior can differ.