Hydrostatic pressure is the pressure exerted by a fluid at rest due to the weight of the fluid above a given point, calculated as P = P₀ + ρgh, where ρ is the fluid's density, g is gravitational acceleration, and h is the depth below the surface.
Hydrostatic pressure is what you feel in your ears at the bottom of a pool. Every layer of fluid above a point pushes down on it, and the deeper you go, the more fluid weight is stacked on top of you. For a fluid at rest, the absolute pressure at depth h is P = P₀ + ρgh, where P₀ is the pressure at the surface (often atmospheric pressure), ρ is the fluid's density, and g is gravitational acceleration.
Two things about this equation trip people up in a good way once they click. First, pressure depends only on depth, density, and g. The shape of the container and the total amount of fluid don't matter, so a skinny tube and a giant tank produce the same pressure at the same depth. Second, pressure at a given depth pushes equally in all directions, not just downward. That's why hydrostatic pressure can push up on the bottom of a submerged object, which is exactly where buoyancy comes from.
Hydrostatic pressure lives in Topic 1.3 (Fluids: Pressure and Forces) in Unit 1 of AP Physics 2, and it's the foundation the rest of the fluids unit is built on. Archimedes' principle, gauge pressure readings, manometers, and even Bernoulli's equation all trace back to P = P₀ + ρgh. If you can reason about how pressure changes with depth, you can derive the buoyant force, read a manometer, and recognize that the ρgh term in Bernoulli's equation is just hydrostatic pressure showing up in an energy-density equation. It's also a favorite for conceptual questions because the container-shape and direction-of-push ideas are so commonly misunderstood.
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Gauge Pressure (Unit 1)
Gauge pressure is hydrostatic pressure with the atmosphere subtracted out. P_gauge = ρgh measures only the fluid's contribution, while absolute pressure adds P₀ on top. Most pressure gauges and manometer problems report gauge pressure, so always check which one a question wants.
Buoyancy and Archimedes' Principle (Unit 1)
The buoyant force isn't a separate law of nature. It exists because hydrostatic pressure is greater on the bottom of a submerged object than on the top, and that pressure difference produces a net upward force equal to ρ_fluid·g·V_displaced. If you can write P = P₀ + ρgh at two depths, you can derive Archimedes' principle yourself, which is exactly the kind of derivation FRQs love.
Bernoulli's Equation (Unit 1)
Set the fluid speeds to zero in Bernoulli's equation and you get P₁ + ρgy₁ = P₂ + ρgy₂, which rearranges into the hydrostatic pressure equation. Hydrostatic pressure is the static special case of Bernoulli, so the ρgy term you see there is an old friend, not a new idea.
Manometer (Unit 1)
A manometer is hydrostatic pressure turned into a measuring tool. The height difference between two fluid columns tells you a pressure difference directly through Δp = ρgΔh. Reading one is just applying the depth equation on both sides of a U-shaped tube.
Hydrostatic pressure shows up in multiple-choice questions that test whether you really understand P = P₀ + ρgh, like comparing pressures at the same depth in differently shaped containers, ranking pressures in fluids of different densities, or finding the pressure at the bottom of layered fluids. In free-response questions, it typically appears as one step in a larger fluids problem. You might derive the buoyant force from a pressure difference, justify a manometer reading, or set up Bernoulli's equation where one point is at rest. Be ready to do three things with it: calculate absolute or gauge pressure at a depth, explain in words why pressure depends on depth and not container shape, and use a pressure difference to find a net force (pressure times area).
Hydrostatic (absolute) pressure at depth includes the surface pressure, so P_abs = P₀ + ρgh. Gauge pressure leaves the atmosphere out, so P_gauge = ρgh. They differ by exactly one atmosphere when the surface is open to air. Quick check for your answer: gauge pressure at the surface of an open container is zero, but absolute pressure there is about 1.0 × 10⁵ Pa. If a problem says a gauge reads a value, you're being handed ρgh, not the total.
Hydrostatic pressure at depth h in a fluid at rest is P = P₀ + ρgh, where P₀ is the pressure at the fluid's surface.
Pressure in a static fluid depends only on depth, fluid density, and g, not on the container's shape or the total volume of fluid.
At any point in a fluid, pressure pushes equally in all directions, which is why fluid can push upward on the bottom of a submerged object.
The buoyant force comes directly from hydrostatic pressure being larger at the bottom of an object than at the top.
Gauge pressure is hydrostatic pressure minus atmospheric pressure, so P_gauge = ρgh for a fluid open to the air.
Pressure is measured in pascals (Pa), and atmospheric pressure is about 1.0 × 10⁵ Pa, a number worth knowing on exam day.
It's the pressure exerted by a fluid at rest due to the weight of the fluid above a point, given by P = P₀ + ρgh. It's the core idea of Topic 1.3 (Fluids: Pressure and Forces) and the starting point for buoyancy and Bernoulli problems.
No. Pressure at a given depth depends only on the depth, the fluid's density, and g. A narrow tube and a huge tank filled with the same fluid have identical pressure at the same depth, which is a classic MCQ trap.
Hydrostatic (absolute) pressure includes the surface pressure, P = P₀ + ρgh, while gauge pressure subtracts the atmosphere, leaving just ρgh. For water open to air, they differ by about 1.0 × 10⁵ Pa, one atmosphere.
No. At any point in a fluid, pressure acts equally in all directions, including upward. That upward push on the bottom of a submerged object is exactly what creates the buoyant force in Archimedes' principle.
Hydrostatic pressure is the special case of Bernoulli's equation where the fluid isn't moving. Set both speeds to zero and Bernoulli reduces to P₁ + ρgy₁ = P₂ + ρgy₂, which is the depth-pressure relationship.