Inside This Unit: The Full Breakdown
Fluids covers fluids at rest and in motion: density and pressure, buoyancy via Archimedes’ principle, and flowing fluids through the continuity equation and Bernoulli’s principle.
Why it matters
Fluids open AP Physics 2 and reward conceptual reasoning. The relationships here (pressure with depth, conservation of flow) appear in both multiple choice and free response.
Key concepts
- Pressure in a static fluid increases with depth: P = P₀ + ρgh.
- The buoyant force equals the weight of displaced fluid (Archimedes’ principle).
- For incompressible flow, A₁v₁ = A₂v₂ (continuity): narrower pipes mean faster flow.
- Bernoulli’s principle relates pressure, speed, and height along a streamline.
Pressure and Buoyancy
Pressure is force per area and, in a fluid at rest, increases with depth as P = P₀ + ρgh — independent of container shape. Pascal’s principle says a pressure change transmits undiminished through an enclosed fluid (hydraulic lifts). Buoyancy is the upward force equal to the weight of displaced fluid; an object floats when it can displace its own weight.
Fluids in Motion
For an incompressible fluid, the continuity equation A₁v₁ = A₂v₂ means flow speeds up where a pipe narrows. Bernoulli’s principle, a statement of energy conservation for ideal flow, shows that where speed is higher, pressure is lower along a streamline — explaining lift and venturi effects.
AP exam tip
When a pipe narrows, state continuity (speed up) and Bernoulli (pressure down) together — exam answers that connect both principles earn full credit.
Connections to other units
- Unit 2: Pressure and the ideal gas law both describe how fluids respond to conditions.
- Unit 3: Pressure-as-force-per-area parallels field-as-force-per-charge reasoning.
