Fluids
What this unit covers
The topics below follow the published Physics 1 course framework for Unit 8. This unit is worth 10–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Lessons in this unit
- Density & Pressure13 min · 3 objectivesCompute density as ρ = m/V · Define pressure as force per unit area, P = F/A · Find how pressure increases with depth, P = ρgh
- Buoyancy13 min · 3 objectivesState Archimedes’ principle · Compute the buoyant force, F_b = ρ_fluid·V_displaced·g · Predict floating versus sinking from a density comparison
- The Continuity Equation12 min · 3 objectivesState the continuity equation for incompressible flow · Relate cross-sectional area to flow speed, A₁v₁ = A₂v₂ · Predict how flow speed changes in pipes of varying width
- Bernoulli’s Equation14 min · 3 objectivesState Bernoulli’s principle relating speed and pressure · Apply Bernoulli’s equation along a streamline · Explain everyday lift effects with the speed–pressure trade-off
Formulas in Unit 8
Every term in Unit 8
All 15 terms we publish for Fluids, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Density
- ρ = m/V. An object floats in a fluid of greater density and sinks in one of lesser density.
- Pressure
- P = F/A, force per unit area, exerted equally in all directions at a point in a static fluid.
- Pressure with depth
- P = P₀ + ρgh. It depends on depth, not on the shape or total volume of the container.
- Pascal's principle
- Pressure applied to an enclosed fluid transmits undiminished throughout. A hydraulic lift multiplies force by the area ratio while conserving work.
- Buoyant force
- Equal to the weight of fluid displaced, F = ρ_fluid·V_displaced·g. Archimedes' principle.
- Floating condition
- A floating object displaces fluid weighing exactly its own weight, so the fraction submerged equals the ratio of the densities.
- Continuity equation
- A₁v₁ = A₂v₂ for an incompressible fluid. Narrowing a pipe speeds the flow, since volume flow rate is conserved.
- Bernoulli's equation
- P + ½ρv² + ρgh is constant along a streamline for ideal flow, so faster-moving fluid is at lower pressure.
- Applying Bernoulli and continuity together
- Continuity gives the speed change from geometry; Bernoulli converts that speed change into a pressure difference.
- Why pressure does not depend on container shape
- Pressure depends only on depth, fluid density and surface pressure, so a narrow tube and a wide tank at the same depth read the same.
- Gauge vs absolute pressure
- Gauge pressure is measured relative to atmospheric; absolute pressure includes it. A flat tire reads zero gauge but one atmosphere absolute.
- Apparent weight in a fluid
- True weight minus buoyant force. This is how density is measured by weighing an object in air and again submerged.
- Why ships float
- A steel hull encloses a large volume of air, so the average density of the ship is below that of water even though steel is far denser.
- Flow rate
- Volume per unit time, Av. Conserved along a pipe for an incompressible fluid regardless of how the cross-section changes.
- Limits of Bernoulli's equation
- Assumes steady, incompressible, non-viscous flow along a streamline. Real flows with turbulence or viscosity depart from it.
What examiners penalize here
- Distinguish gauge pressure (ρgh, the pressure from the fluid alone) from absolute pressure (P₀ + ρgh, which adds the atmosphere on top). Read the question carefully to see which is asked.
- For a floating object, the buoyant force exactly equals its weight (it is in equilibrium). The fraction submerged equals the ratio of the object’s density to the fluid’s density — a handy shortcut for iceberg-style problems.
- Continuity assumes an incompressible fluid and steady flow. On the AP exam it is almost always paired with Bernoulli’s equation: first use continuity to get the speeds, then feed them into Bernoulli for the pressures.
- Solve fluid-flow free-response in two steps: continuity (A₁v₁ = A₂v₂) gives the speeds, then Bernoulli (P + ½ρv² + ρgh = constant) gives the pressures. For horizontal pipes, drop the ρgh terms to simplify.
Practice Physics 1
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Physics 1: Algebra-Based exam is Unit 8?
Unit 8, Fluids, is worth 10–15% of the Physics 1 multiple-choice section according to the published course framework. Across all 8 units that makes it a substantial share — heavier than an even split would give it.
What topics are covered in Physics 1 Unit 8?
Fluids covers Density & pressure, Buoyancy, Continuity and Bernoulli’s equation. We publish 15 terms with definitions for this unit, all of them on this page.
How should I study Physics 1 Unit 8?
Read the 4 lessons below first — about 50 minutes — then drill the 15 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 8 units of AP Physics 1: Algebra-Based
Unit names, topics and exam weights follow the published College Board course framework for AP Physics 1: Algebra-Based. AP® is a trademark registered by the College Board, which does not endorse this site.