Chapter 4: Fluids
🎯 Diagnostic: Test Your Starting Level 24 questions (~2 per section). No prior reading required, see what you already know.
Aim to answer every question before checking. Missed questions point you to the sections you need most.
1. (4.1) Density is defined as:
B. Water ≈ 1000 kg/m³ (or 1 g/mL) at 4 °C.
2. (4.1) Specific gravity is:
A. SG > 1 = sinks in water; SG < 1 = floats.
3. (4.2) Pressure is defined as:
D. Unlike force, pressure is a scalar in hydrostatic settings.
4. (4.2) Atmospheric pressure at sea level is approximately:
C. Pressure falls with altitude; MCAT problems often use 10⁵ Pa as a clean approximation.
5. (4.3) Hydrostatic pressure at depth h in a fluid of density ρ is:
B. Pressure increases by ρg per meter of depth. For water, that is about 9810 Pa/m, or ~1 atm per 10 m.
6. (4.3) Pascal's principle states:
A. Hydraulic brakes multiply force by operating two pistons of different area with the same pressure.
7. (4.4) Archimedes' principle states:
D. Heavier fluids give larger buoyant forces (mercury floats iron).
8. (4.4) An object floats in a fluid when:
C. Submerged fraction = ρ_object / ρ_fluid.
9. (4.5) Young's modulus relates:
B. Steel has Y ≈ 2 × 10¹¹ Pa; rubber Y is much smaller, which is why rubber stretches easily.
10. (4.5) Stress is defined as:
A. Strain is the dimensionless fractional deformation (ΔL / L).
11. (4.6) Viscosity is:
D. Temperature strongly affects viscosity: liquids thin when heated.
12. (4.6) Laminar vs turbulent flow depends on:
C. Blood flow in most vessels is laminar but can become turbulent in diseased arteries (audible with a stethoscope).
13. (4.7) The continuity equation for incompressible flow:
B. Mass must go somewhere; narrowing the pipe speeds flow to compensate.
14. (4.7) A pipe narrows from 4 cm² to 2 cm². If flow in the wide section is 1 m/s, the narrow section flow is:
A. 4 × 1 = 2 × v ⇒ v = 2 m/s.
15. (4.8) Bernoulli's equation expresses:
D. Assumes incompressible, inviscid, steady, streamline flow.
16. (4.8) Along a horizontal pipe that narrows:
C. Fast flow = low pressure. This underlies lift on airplane wings and the Venturi effect.
17. (4.9) The Venturi effect describes:
B. Direct consequence of continuity + Bernoulli.
18. (4.9) A common application of the Venturi effect is:
A. Also underlies how perfume atomizers or paint sprayers work.
19. (4.10) Surface tension arises because:
D. Hydrogen bonding gives water unusually high surface tension, which allows insects to walk on water.
20. (4.10) Capillary action occurs because:
C. Water rises in glass (adhesive > cohesive); mercury falls (cohesive > adhesive).
21. (4.11) Blood flow follows Poiseuille's law (approximately):
B. Halving the radius reduces flow by a factor of 16 at constant pressure. Vasoconstriction is a powerful control mechanism.
22. (4.11) Systolic blood pressure represents:
A. Diastolic is the minimum (~80 mmHg); pulse pressure is systolic minus diastolic.
23. (4.1) Which is densest?
D. Of the four, water is densest. Mercury (~13,600 kg/m³) would be the winner if listed.
24. (4.3) A diver 10 m below the surface of water experiences an additional pressure of about:
C. This is why divers experience doubled pressure at 10 m depth.
You step into a swimming pool and immediately feel lighter. You put your thumb over a garden hose and the water shoots twice as far. You sip a drink through a straw without thinking about what’s actually pulling the liquid up.
Fluids surround you constantly, and the physics that describes them is — once you see the patterns — both elegant and intuitive. It’s also one of the highest-yield physics topics on the MCAT, because fluid concepts bridge physics and biology so naturally:
- Poiseuille’s law explains why a 50% arterial blockage cuts blood flow to ~6% of normal.
- Bernoulli’s equation explains why aneurysms get worse over time and why airways collapse during asthma attacks.
- The continuity equation explains why blood crawls through capillaries even though the heart pumps it forcefully — and why that slow flow is biologically essential for gas exchange.
Master this chapter and you can reason through an enormous slice of MCAT passages, including most of the cardiovascular and respiratory problems.
In This Chapter
- 4.1 Density and Specific Gravity
- 4.2 Pressure
- 4.3 Hydrostatic Pressure and Pascal’s Law
- 4.4 Buoyancy and Archimedes’ Principle
- 4.5 Elastic Properties of Solids
- 4.6 Fluid Dynamics and Viscosity
- 4.7 The Continuity Equation
- 4.8 Bernoulli’s Equation
- 4.9 The Venturi Effect
- 4.10 Surface Tension and Capillary Action
- 4.11 Circulatory System Applications
- Section Test