static pressure
Summary
Static pressure is the pressure a fluid exerts on its surroundings through random molecular collisions, the P term in Bernoulli's equation. For an ideal fluid in steady flow at constant height, static pressure is lower where the fluid moves faster, such as in a narrow section of pipe.
Detail
Meaning: Static pressure is the pressure a fluid exerts on its surroundings through random molecular collisions, acting perpendicular to a surface. A gauge moving with the fluid, or a side opening in a pipe wall, reads it. In a fluid at rest it equals P0 + ρgh. In a moving ideal fluid it is the P term in Bernoulli's equation, separate from dynamic pressure, ½ρv². At constant height, faster flow means more kinetic energy per volume and so lower static pressure.
Equation: P + ½ρv² + ρgh = constant along a streamline. P is static pressure (Pa), ρ is density (kg/m³), v is flow speed (m/s), g is 9.8 m/s², and h is height (m).
On the MCAT: Expect a narrowing pipe, a Venturi meter, or vertical side tubes whose liquid columns show pressure. Use continuity, A₁v₁ = A₂v₂ (A is cross-sectional area in m²), to find where speed is higher, then conclude static pressure is lower there. Lift on wings and narrowed vessels use the same logic.
Common confusion: Faster flow does not raise wall pressure: dynamic pressure rises while static pressure falls. Bernoulli's equation assumes steady, nonviscous, incompressible flow; in a real viscous fluid, static pressure still drops along a horizontal tube of constant width (Poiseuille).
Sources
- Fundamentals of Physics
- Physics: Principles with Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.