Physics 2 · Unit 1: Fluids · 14 min read · Updated 2026-05-11
Pressure — AP Physics 2
AP Physics 2 · Unit 1: Fluids · 14 min read
1. Definition and Properties of Pressure★☆☆☆☆⏱ 3 min
Pressure is defined as the magnitude of the normal force exerted per unit area by a fluid on any surface it contacts. Unlike force, pressure is a scalar quantity with no direction, though the resulting force is always perpendicular to the contact surface.
Per the AP Physics 2 CED, Unit 1 Fluids accounts for 10-12% of total exam score, and pressure concepts appear in both multiple-choice and free-response sections. Pressure is foundational for problems involving buoyancy, fluid flow, or ideal gases, with 2-4 direct MCQ questions and at least one FRQ part expected.
2. Hydrostatic Pressure in Static Fluids★★☆☆☆⏱ 4 min
For a static, incompressible fluid in a uniform gravitational field, pressure increases with depth because fluid at a given depth must support the weight of all fluid above it. Pressure only depends on depth, fluid density, and surface pressure, not the shape of the container.
Exam tip: Always check if the question asks for gauge or absolute pressure. "Pressure from the water" indicates gauge pressure, while "total pressure on the diver" indicates absolute pressure.
3. Pascal's Principle and Hydraulic Systems★★★☆☆⏱ 3 min
Pascal's principle states that a change in pressure applied to an enclosed, incompressible static fluid is transmitted undiminished to every portion of the fluid and all walls of the container. This is the working principle behind hydraulic lifts and brakes, which act as force multipliers.
\frac{F_2}{F_1} = \frac{A_2}{A_1}
Where $F_1$ is input force on small piston area $A_1$, $F_2$ is output force on large piston area $A_2$. Energy is still conserved because the large piston moves a smaller distance, so work input equals work output.
Exam tip: Units cancel in the area ratio, so you do not need to convert length units to SI as long as both radii use the same unit.
4. Pressure and the Origin of Buoyant Force★★★☆☆⏱ 4 min
Buoyant force, the net upward force on any object submerged in a fluid, arises directly from hydrostatic pressure differences between the top and bottom of the object. Because pressure increases with depth, upward pressure on the bottom of the object is always greater than downward pressure on the top, creating a net upward force. Horizontal pressure forces on the sides of any object cancel out, leaving only the vertical net force.
Exam tip: If a question asks you to explain buoyancy using pressure concepts, do not just cite Archimedes' principle — you must explicitly reference the pressure difference between top and bottom to earn full credit.
5. AP-Style Problem Solving Practice★★★★☆⏱ 5 min
Common Pitfalls
Why: Most problems ask for pressure from the fluid alone, so students form a habit of only calculating $\rho g h$.
Why: Students confuse the size of the object with the depth of the point being measured.
Why: Students memorize "force scales with piston size" and forget area scales with the square of linear dimensions.
Why: Students confuse total weight of fluid with pressure (force per unit area).
Why: Students forget atmospheric pressure acts on both the top and bottom of the object.