MCAT Physics · Lesson 4
Fluids
3 min read4 sectionsUpdated
4 sections
4.1 Characteristics of Fluids and Solids
Describes differences between solids and fluids, defines density, pressure, and introduces concepts like buoyancy and specific gravity.
- Fluids vs Solids
- Fluids (liquids & gases) → flow, conform to container
- Solids → rigid, resist deformation, can withstand shear
- Both exert forces perpendicular to surfaces
- Density
- , units: kg/m³ or g/mL
- 1 mL = 1 cm³, 1000 L = 1 m³
- Water density: 1 g/cm³
- Weight:
- Specific gravity:
- Pressure
- , SI unit: Pascal (Pa = N/m²)
- 1 atm = 1.013×10⁵ Pa = 760 mmHg = 760 Torr
- Absolute (hydrostatic) pressure:
- Gauge pressure:
- Example: Diver 20 m underwater, ρ = 1025 kg/m³ → Pa
4.2 Hydrostatics
Examines fluids at rest, Pascal's principle, Archimedes' principle, and buoyancy.
- Pascal's Principle
- Incompressible fluids transmit pressure uniformly
- Hydraulic systems: ,
- Work:
- Example: Piston r1 = 5 cm, weight 50 kg, r2 = 20 cm → N
- Archimedes' Principle
- Buoyant force:
- Object floats if average density < fluid
- Specific gravity indicates submerged fraction
- Example: Ice ρ = 0.92 g/cm³ → 92% submerged
- Molecular Forces in Liquids
- Surface tension → cohesion among molecules
- Adhesion → attraction between liquid & other substances
- Meniscus forms depending on adhesion vs cohesion
- Convex (e.g., mercury) if cohesive > adhesive
4.3 Fluid Dynamics
Covers fluid motion, viscosity, laminar vs turbulent flow, Poiseuille's law, continuity, and Bernoulli's principle.
- Viscosity
- Resistance to flow, SI unit: Pa·s = N·s/m²
- High viscosity → more energy loss
- Laminar Flow
- Smooth, orderly, parallel layers, speed varies by distance from wall
- Poiseuille's Law
- Laminar flow rate through a cylindrical tube
- Flow rate:
- Turbulence
- Disorderly, eddies, occurs above critical speed:
- Reynolds number depends on size, shape, surface
- Streamlines
- Tangential to velocity, never intersect, represent flow pathways
- Continuity equation:
- Narrow passages → higher linear speed
- Bernoulli's Equation
- 1/2 ρv² → dynamic pressure, ρgh → static pressure
- Energy per volume perspective
- Applications: airplane lift, Pitot tubes, Venturi meters
- Example: Bathroom 40 m above ground, v1 = 2 m/s, v2 = 8 m/s → A2 = 3.14×10⁻⁴ m², P_ground = 7.3×10⁵ Pa
4.4 Fluids in Physiology
Applies fluid principles to circulatory and respiratory systems, noting deviations from idealized equations due to biological complexities.
- Circulatory System
- Closed loop, nonconstant flow
- Valves, gravity, vessel properties, heart mechanics affect flow
- Use Poiseuille's law for isolated segments
- Pulse → measurable flow
- Resistance higher in capillaries, parallel capillaries → lower equivalent resistance
- Venous system contains 3x arterial blood
- Turbulent flow → heart murmurs
- Respiratory System
- Air moves due to pressure gradients
- Inspiration → negative pressure gradient → air in
- Exhalation → gradient reverses
- Air at alveoli has negligible speed
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