Study guides › C/P › Physics

Fluids

Liquids and gases obey a small set of rules: pressure grows with depth, buoyancy equals displaced weight, flow conserves volume, and moving fluid trades pressure for speed. You will apply them to pipes, manometers, blood vessels, capillary tubes, and alveoli.

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What this chapter covers

Density, Pressure & Pascal's Principle. Hydrostatic pressure, gauge versus absolute pressure, and hydraulic lifts appear on nearly every MCAT fluids set.
Buoyancy & Archimedes' Principle. Floating, sinking, and apparent-weight problems all reduce to one line: the buoyant force is the weight of displaced fluid.
Fluid Flow: Continuity & Bernoulli. Narrow pipe, fast flow, low pressure: this pattern shows up in Venturi meters, airplane wings, and stenosed arteries.
Viscosity, Poiseuille Flow & Circulation. Blood is a real fluid, so flow depends on radius to the fourth power. This is why small changes in vessel diameter matter so much.
Surface Tension & Capillary Action. Surface tension explains droplets, capillary rise, and why alveoli need surfactant, all three of which the MCAT likes to test.

Key points to know

P = P₀ + ρgh; depends on depth, not container shape
10 m of water ≈ 1 atm; 1 atm = 760 mmHg ≈ 1.0 × 10⁵ Pa
F_B = ρ_fluid V_submerged g = weight of displaced fluid
Floating: fraction submerged = ρ_object/ρ_fluid
Continuity: A₁v₁ = A₂v₂ (Q = Av constant)
Bernoulli: P + ½ρv² + ρgh = constant
Poiseuille: Q = πr⁴ΔP/(8ηL); R = 8ηL/(πr⁴)
Radius to the fourth power: halving r divides Q by 16

Try a practice question

In a static fluid, the pressure at a point depends on which of the following?

The total volume of the fluid
The depth below the surface and the fluid density
The shape of the container
The width of the container at that level

Show the answer

B. P = P₀ + ρgh, so only the depth, density, and surface pressure matter. Wide, narrow, and oddly shaped containers give the same pressure at equal depth.

Common questions

What does this chapter cover?

Fluids covers Density, Pressure & Pascal's Principle, Buoyancy & Archimedes' Principle, Fluid Flow: Continuity & Bernoulli, Viscosity, Poiseuille Flow & Circulation, Surface Tension & Capillary Action.

What should I know first?

P = P₀ + ρgh; depends on depth, not container shape 10 m of water ≈ 1 atm; 1 atm = 760 mmHg ≈ 1.0 × 10⁵ Pa F_B = ρ_fluid V_submerged g = weight of displaced fluid

24 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.