Unit 1: Fluids

AP Physics 2: 61 practice questions with detailed explanations.

Unit Study Guide

Executive Summary

Fluids (liquids and gases) transmit pressure, buoy things up, and flow — with density, pressure, and continuity as the organizing ideas.

Density and pressure

Density $ho = m/V$ ; pressure $P = F/A$ measured in pascals. Pressure in a static fluid grows with depth: $P = P_0 + ho g h$ — same depth, same pressure, regardless of container shape.

Buoyancy

Archimedes' principle: the buoyant force equals the weight of displaced fluid. An object floats when its density is less than the fluid's; submerged fraction equals the density ratio $ho_{obj}/ ho_{fluid}$ .

Fluid flow

The continuity equation $A_1 v_1 = A_2 v_2$ says narrow pipes flow faster. Bernoulli's equation adds energy: faster flow means lower pressure ( $P + rac{1}{2} ho v^2 + ho g y =$ constant) — the physics of airplane wings, venturis, and sprayers.

Exam traps

Pressure depends on depth, not the shape or total weight of the container. The buoyant force depends on displaced VOLUME, not the object's weight or depth. Bernoulli applies to ideal (inviscid, incompressible) flow — real fluids have viscosity. Gauge pressure omits atmospheric pressure; absolute includes it.

Top 5 Concepts to Master

1Compute pressure at depth.
2Apply Archimedes’ principle to floating and submerged objects.
3Use continuity to relate speeds and areas.
4Apply Bernoulli’s equation.
5Connect buoyancy to density ratios.

Key Terms & Definitions

Practice with Flashcards

Density

ρ = m/V.

Pressure

P = F/A; grows with depth: P = P₀ + ρgh.

Pascal’s principle

Pressure changes transmit undiminished through a fluid.

Buoyant force

Fb = ρ_fluid · Vdisplaced · g.

Continuity equation

A₁v₁ = A₂v₂.

Bernoulli’s equation

P + ½ρv² + ρgy = constant along a streamline.

Viscosity

Internal friction resisting fluid flow.

Gauge pressure

Pressure above atmospheric.

Common Misconceptions: Exam Traps

Heavier objects sink.

Correct: Density comparison decides float vs sink, not weight alone.

Pressure depends on container shape.

Correct: At a given depth, pressure is the same in any connected fluid.

Faster flow means higher pressure.

Correct: Bernoulli: faster flow carries lower pressure.

Buoyancy increases with depth.

Correct: It depends on displaced volume, which is constant once submerged.

Question Bank Breakdown

By difficulty

easy 28medium 29hard 4

By topic

Buoyancy and Archimedes' Principle 19Density and Pressure 11Fluid Dynamics and Bernoulli's Equation 11Fluid Statics 6Applications of Fluid Mechanics 5