AP Physics 1 · Unit 8 · Fluids

Buoyancy

Measure displaced fluid and connect buoyant force with floating, sinking, a loaded brass tube and a helium balloon.

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Learn by doing

Three ideas you’ll put into motion

Use pressure differences and submerged volume to explain Archimedes’ principle.

Change volume, density, mass and depth, then use force balance for floating and tethered objects.

Calculate balloon tension and release acceleration, investigate a boat-and-ball puzzle, and design a target floating fraction.

The big idea

The pressure difference across an object produces a net upward force. F_B = ρ_fluidV_displacedg equals the weight of the displaced fluid.

A freely floating object at rest has F_B = mg. A tethered balloon also has a downward rope force; a sinking object still experiences upward buoyancy.

A quick comparison

Keep a specimen fully submerged in the same liquid while increasing only its mass.

Its buoyant force stays the same, while its weight increases.

Teaching this topic?Classroom fit, AP connections, and support

Classroom fit: Use for a concept introduction, guided independent practice, or a projected class discussion.
Before students begin: Density, hydrostatic pressure, Newton’s second law, equilibrium, and basic area and volume formulas. The Density, Pressure and Hydrostatics lessons prepare these ideas.
Built-in support: Prepared scenes, guided prompts, and feedback on lesson questions.
AP Physics 1 connections: Unit 8, Topic 8.3: fluids and Newton’s laws. Connect buoyancy with displaced-fluid weight and apply force models, mathematical reasoning and evidence from measurements.
Check for understanding: Ask students to distinguish total object volume from displaced volume, compare buoyancy with weight, and explain why the pool level falls when a ball moves from a floating boat into the water and sinks.

The first scene is set up. Start the lesson and explore.