8.3 Buoyancy and Fluids

Where buoyant force comes from

Fluid pressure increases with depth, so pressure on the bottom of a submerged object is greater than on its top. This pressure difference creates an upward net force called the buoyant force. It acts on any object partially or fully submerged in a fluid, regardless of the object's material.

Archimedes' Principle

The buoyant force equals the weight of the fluid displaced by the object: F_B = ρ_fluid × V_displaced × g. This depends only on the fluid's density and the submerged volume — not on the object's own weight or density. A denser fluid or greater displaced volume means a larger buoyant force.

Floating vs. sinking

An object floats when the buoyant force can equal its weight before it's fully submerged (average object density < fluid density). It sinks if even fully submerged, buoyant force is less than weight (object density > fluid density). A floating object displaces a volume of fluid whose weight equals its own weight.

Key terms

Buoyant force

The net upward force on an object in a fluid, caused by increasing pressure with depth.

Archimedes' Principle

Buoyant force equals the weight of fluid displaced: F_B = ρ_fluid × V_displaced × g.

Condition for floating

An object floats if its average density is less than the fluid's density.

Condition for sinking

An object sinks if its average density is greater than the fluid's density, even when fully submerged.

What does buoyant force depend on?

Only the density of the fluid and the volume of fluid displaced — not the object's own weight or material.

Try an AP-style question

A 0.01 m³ block is fully submerged in water (ρ = 1,000 kg/m³). What is the buoyant force? (g ≈ 10 m/s²)

1. 1,000 N
2. 1 N
3. 100 N
4. 10 N