Everything you need to master Unit 8 — density and pressure, hydrostatic pressure with depth, Archimedes' principle and buoyancy, the continuity equation, and Bernoulli's equation for moving fluids.
10–15% of the AP exam
7 study resources
College Board aligned
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Seven free resources for Unit 8 — pick the one that fits how you learn.
Unit 8 is the culminating unit of AP Physics 1 — it applies forces (Unit 2), energy (Unit 3), and conservation laws to the special case of fluids (liquids and gases). The big ideas: solids push back, but fluids exert PRESSURE in all directions, and they flow.
The College Board breaks Unit 8 into 4 topics: (8.1) Internal Structure and Density, (8.2) Pressure, (8.3) Fluids and Newton's Laws, and (8.4) Fluids and Conservation Laws. The most important formulas are P = ρgh (hydrostatic pressure), F_buoyant = ρ_fluid·V_displaced·g (Archimedes), A₁v₁ = A₂v₂ (continuity), and Bernoulli's equation.
Unit 8 is 10–15% of the AP exam — one of the heavier units. Buoyancy and Bernoulli are common FRQ topics.
Key terms preview
A taste of what you'll find in The Essentials and Flashcards.
Density (ρ)
Mass per unit volume: ρ = m/V. Water has ρ ≈ 1000 kg/m³. Anything denser than water sinks; anything less dense floats.
Pressure (P)
Force per unit area: P = F/A. Pushes outward in all directions in a fluid. Units: pascal (Pa) = N/m².
Hydrostatic Pressure
P = P₀ + ρgh. Pressure in a fluid grows linearly with depth. The deeper you go, the more pressure.
Archimedes' Principle
The buoyant force on an object equals the WEIGHT of the fluid it displaces: F_b = ρ_fluid·V_displaced·g.
Continuity Equation
For an incompressible fluid: A₁v₁ = A₂v₂. Narrower pipe = faster flow. Conservation of mass for fluids.
Bernoulli's Equation
P + ½ρv² + ρgh = constant along a streamline. Conservation of energy for moving fluids. Faster flow = lower pressure.
Hydrostatic pressure: P = P₀ + ρgh. The deeper into a fluid you go, the more pressure from the column of fluid above. Pressure acts in all directions at any point, not just downward. This is why dams are thicker at the bottom and why your ears hurt deep underwater.
2. Buoyancy equals the weight of fluid displaced
Archimedes' principle: F_buoyant = ρ_fluid·V_displaced·g. An object floats if it's less dense than the fluid (it can displace a weight of fluid equal to its own weight while still poking out). It sinks if it's denser. A ship floats because its overall (hollow) average density is less than water's.
3. Moving fluids obey conservation of mass AND energy
Continuity: A₁v₁ = A₂v₂. Bernoulli's: P + ½ρv² + ρgh = constant. Where the pipe narrows, the fluid speeds up — and where it speeds up, the pressure drops. That's why a shower curtain pulls inward when the water's running, and why airplane wings generate lift.