Forces · Topic 8 of 17

Density, Pressure & Buoyancy

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Physics · fluids

Density is mass per volume, pressure is force per area, and a fluid pushes up with a force equal to the weight of fluid displaced.

01 · DensityDensity = mass ÷ volume

02 · PressurePressure = force ÷ area

03 · ArchimedesBuoyant force = weight of displaced fluid

Density is a property of the material, not of the amount

Buoyancy: more pressure below than above → net upward push

Less dense than the fluid → floats; more dense → sinks must know

Archimedes: buoyant force = weight of the fluid displaced must know

Rearranged: m = ρ × V and V = m ÷ ρ

Same force, smaller area → more pressure

Buoyant force > weight rises; < weight sinks; = weight hovers

Density ρ = m / V (g/cm³ = g/mL, or kg/m³) must know

Water: 1 g/cm³ = 1 g/mL = 1000 kg/m³ must know

Pressure P = F / A; 1 pascal (Pa) = 1 N/m² must know

Fluid pressure P = hρg — deeper or denser fluid → more pressure

1 atm (sea level) = 1.01 × 10⁵ Pa ≈ 101 kPa

30 g ÷ 12 cm³ = 2.5 g/cm³ → denser than water, sinks

600 N ÷ 0.03 m² = 20,000 Pa = 20 kPa

0.002 m³ under water → 2 kg displaced → buoyant force 19.6 N

Commonly confused

	Floats	Sinks
Average density	Less than the fluid's	Greater than the fluid's
Fully underwater, buoyant force is…	More than its weight → it rises	Less than its weight → it goes down
Buoyant force acting?	Yes	Yes — it still seems lighter
In water (1 g/cm³)	Density below 1 g/cm³	e.g. a 2.5 g/cm³ object

Sinking objects still get buoyant force — they seem lighter

Show 3 more facts

Fraction submerged = object density ÷ fluid density (floating)

Specific gravity = density ÷ water's density (no units)

Submarines change average density; helium balloons are buoyed by air

Pearl

Water's 1 g/mL is the dividing line: less dense floats, more dense sinks.

Practice this topic, then test the whole section.

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