Unit 8: Fluids

Fluids moved from Physics 2 into Physics 1

Fluids is part of AP Physics 1 in the current course. Before the fall 2024 redesign, this material was tested on AP Physics 2, so older Physics 1 prep books and question banks skip it entirely. Check anything you study from: if it has no fluids chapter, it predates the current course.

The unit is weighted at 10 to 15% of the multiple-choice section, the same band as Kinematics and Linear Momentum, so treat it as a full-size unit rather than an afterthought. It is also the last unit in the course, which means many classes reach it in a rush during April. If yours does, the AP Physics 1 fluids guide covers all four topics in one place.

The May 2025 exam was the first under the redesigned 8-unit course, and 67.3% of students scored 3 or higher, up from 47.3% in 2024. New material, but clearly learnable.

Topic 8.1: Internal Structure and Density

A fluid is a substance that flows and takes the shape of its container: liquids and gases both count. Topic 8.1 builds the one quantity everything else in the unit depends on, density:

Density is a property of the material, not the object. Cutting a block of aluminum in half changes its mass and its volume but not its density. Multiple-choice questions lean on this idea, asking you to compare densities of objects with different shapes or to reason about how density changes when mass or volume changes.

Two reference points worth memorizing: fresh water has a density of $1000 \text{ kg/m}^3$, and an object floats when its average density is less than the fluid's density. That second fact turns many buoyancy questions into one-line comparisons with no arithmetic at all.

Topic 8.2: Pressure

Pressure is force per area: $P = F_{\perp}/A$, where the force is perpendicular to the surface. Units are pascals ($1 \text{ Pa} = 1 \text{ N/m}^2$). The key equation for a fluid at rest is absolute pressure at depth:

where $P_0$ is the pressure at the surface (often atmospheric) and $h$ is the depth below the surface. Three habits keep this topic clean. Measure $h$ downward from the surface, not upward from the bottom. Remember that pressure depends only on depth and fluid density, never on the shape or width of the container. And keep absolute versus gauge pressure straight: gauge pressure is the $\rho g h$ part alone, the amount above atmospheric.

Pressure connects directly to force: multiply a pressure difference by an area and you get a net force, which is exactly where the buoyant force in Topic 8.3 comes from.

Topic 8.3: Fluids and Newton's Laws

Topic 8.3 puts fluids into the force framework from Unit 2. The buoyant force is

with two details the exam loves to test: $\rho$ is the density of the fluid, and $V$ is the volume of fluid displaced, which for a partially submerged object is only the underwater portion. Plugging in the object's density or its full volume is the classic Unit 8 mistake.

Otherwise, treat buoyancy like any other force. Draw the object, add the buoyant force pointing up and the weight pointing down, include tension or a normal force if something else touches the object, then apply Newton's second law. The free-body diagram guide is a quick refresher if you need one. A floating object in equilibrium gives $\rho_{fluid} V_{sub} g = mg$, the equation behind every "what fraction is submerged" question.

Topic 8.4: Fluids and Conservation Laws

The final topic applies two conservation laws to moving fluid. Conservation of mass gives the continuity equation, $A_1 v_1 = A_2 v_2$: where a pipe narrows, the fluid speeds up. Conservation of energy, written per unit volume, gives Bernoulli's equation:

Each term maps onto energy ideas from Unit 3: $\rho g y$ plays the role of gravitational potential energy and $\frac{1}{2}\rho v^2$ plays the role of kinetic energy. If the logic in the conservation of energy guide makes sense to you, Bernoulli is the same bookkeeping with pressure added.

The standard workflow: label two points in the flow, use continuity to relate the speeds, then substitute into Bernoulli to solve for the unknown pressure, speed, or height. Draining-tank problems are a favorite setup, since both points sit at atmospheric pressure and the equation collapses nicely.

How Unit 8 shows up on the exam

The exam structure is the same for every unit: Section I has 42 multiple-choice questions in 85 minutes (50% of your score), and Section II has 4 free-response questions in 95 minutes (the other 50%). The 10-15% weighting applies to the multiple-choice section, so expect roughly 4 to 6 of the 42 questions to involve fluids. Fluids can also appear in any of the four free-response formats: Mathematical Routines, Translation Between Representations, Experimental Design and Analysis, and Qualitative/Quantitative Translation.

A four-function, scientific, or graphing calculator is allowed on both sections, and every fluids equation you need is printed on the equation sheet: density, pressure, pressure at depth, buoyant force, continuity, and Bernoulli. Skim the fluids block of the AP Physics 1 formula sheet before test day so you know exactly what is given and what, like the floating condition, you have to derive on the spot.

How to study Unit 8

Work the topics in order, because they build. Density and pressure are definitions plus one equation each; get fast at them. Buoyancy is a Newton's laws problem with one new force, so lean on the same free-body habits you built earlier in the course. Continuity and Bernoulli reward setup discipline: pick and label points 1 and 2 before touching any numbers.

For a full walkthrough with more worked examples, use the AP Physics 1 fluids guide, and keep the formula sheet open while you practice so the symbols become automatic. Since Unit 8 sits at the end of the course, fold it into your final review alongside the earlier units at the AP Physics 1 hub rather than cramming it separately. Exam questions mix fluids freely with force and energy reasoning, and the unit makes far more sense when you see it as those same laws applied to a new kind of system.