Medium MCAT Fluid Mechanics Practice Questions

Mastering fluid mechanics is essential for scoring high on the Chemical and Physical Foundations of Biological Systems section of the MCAT. This topic covers the behavior of liquids and gases at rest and in motion, which is directly applicable to physiological systems like blood flow in the circulatory system and air movement in the lungs. By working through Medium MCAT Fluid Mechanics Practice Questions, you can bridge the gap between basic definitions and the complex application scenarios found on the actual exam.

Concept Explanation

Fluid mechanics is the study of how fluids—substances that flow and conform to the shape of their containers—behave under various physical forces and conditions. In the context of the MCAT, this field is divided into hydrostatics (fluids at rest) and hydrodynamics (fluids in motion). Key concepts include density $ho = \frac{m}{V}$, pressure $P = \frac{F}{A}$, and buoyancy, which is governed by Archimedes' principle. Hydrodynamics introduces the continuity equation $A_1v_1 = A_2v_2$, asserting that for an incompressible fluid, the flow rate remains constant regardless of the cross-sectional area. Bernoulli’s equation further relates pressure, velocity, and height, demonstrating that as a fluid's speed increases, its pressure decreases. Understanding these relationships is vital for interpreting biological phenomena, such as how Bernoulli's principle affects blood pressure in narrowed arteries or how the Venturi effect operates in respiratory equipment.

Solved Examples

1. Hydrostatic Pressure Calculation: A diver is 20 meters below the surface of a lake. If the density of water is $1000 \text{ kg/m}^3$ and atmospheric pressure is $1 \times 10^5 \text{ Pa}$, what is the total pressure experienced by the diver?
   1. Identify the formula for absolute pressure: $P = P_{atm} + ho gh$.
   2. Substitute the values: $P = (1 \times 10^5 \text{ Pa}) + (1000 \text{ kg/m}^3)(10 \text{ m/s}^2)(20 \text{ m})$.
   3. Calculate the gauge pressure: $1000 \times 10 \times 20 = 2 \times 10^5 \text{ Pa}$.
   4. Sum the pressures: $1 \times 10^5 + 2 \times 10^5 = 3 \times 10^5 \text{ Pa}$.
2. Continuity Equation in Physiology: Blood flows through an artery with a cross-sectional area of $4 \text{ cm}^2$ at a velocity of $5 \text{ cm/s}$. If the artery narrows to an area of $1 \text{ cm}^2$, what is the new velocity of the blood?
   1. Use the continuity equation: $A_1v_1 = A_2v_2$.
   2. Rearrange for the unknown: $v_2 = \frac{A_1v_1}{A_2}$.
   3. Plug in the values: $v_2 = \frac{4 \text{ cm}^2 \times 5 \text{ cm/s}}{1 \text{ cm}^2}$.
   4. Solve: $v_2 = 20 \text{ cm/s}$.
3. Buoyancy and Floating: An object with a volume of $0.05 \text{ m}^3$ and a density of $800 \text{ kg/m}^3$ is placed in water ($ho = 1000 \text{ kg/m}^3$). What volume of the object is submerged?
   1. For a floating object, the buoyant force equals the weight: $ho_{fluid}V_{sub}g = ho_{obj}V_{obj}g$.
   2. Cancel $g$ and rearrange: $V_{sub} = \frac{ ho_{obj}V_{obj}}{ ho_{fluid}}$.
   3. Calculate: $V_{sub} = \frac{800}{1000} \times 0.05$.
   4. Final answer: $V_{sub} = 0.8 \times 0.05 = 0.04 \text{ m}^3$.

Practice Questions

1. A horizontal pipe carrying water narrows from a radius of $2 \text{ cm}$ to $1 \text{ cm}$. If the velocity in the wider section is $2 \text{ m/s}$, what is the velocity in the narrow section?

2. An unknown fluid exerts a gauge pressure of $15 \text{ kPa}$ at a depth of $1.5 \text{ m}$. What is the density of this fluid? (Assume $g = 10 \text{ m/s}^2$)

3. A block of wood with a density of $600 \text{ kg/m}^3$ floats in an oil with a density of $900 \text{ kg/m}^3$. What fraction of the wood's volume is submerged?

4. Water flows through a pipe with a pressure of $50 \text{ kPa}$ and a velocity of $4 \text{ m/s}$. If the pipe rises $2 \text{ m}$ and the velocity increases to $6 \text{ m/s}$, what is the new pressure? (Density of water = $1000 \text{ kg/m}^3$)

5. A hydraulic lift has two pistons. Piston A has an area of $0.01 \text{ m}^2$ and Piston B has an area of $0.25 \text{ m}^2$. If a force of $50 \text{ N}$ is applied to Piston A, how much mass can Piston B lift?

6. An object weighs $100 \text{ N}$ in air and $80 \text{ N}$ when fully submerged in a liquid. What is the buoyant force acting on the object?

7. According to Poiseuille's Law, if the radius of a blood vessel is reduced by half, by what factor does the resistance to flow increase?

8. A tank filled with water has a small hole $5 \text{ m}$ below the surface. At what speed does the water exit the hole? (Use Bernoulli’s principle/Torricelli’s Law)

9. A cube of side length $0.1 \text{ m}$ is submerged in water. What is the difference in pressure between the top and bottom surfaces of the cube?

10. If the density of an object is $3000 \text{ kg/m}^3$, what is its specific gravity?

Answers & Explanations

1. Answer: 8 m/s. Using the continuity equation $A_1v_1 = A_2v_2$. Since $A = \pi r^2$, the ratio of areas is proportional to the square of the radii. $(2)^2 \times 2 = (1)^2 \times v_2$, so $4 \times 2 = v_2 = 8 \text{ m/s}$.
2. Answer: 1000 kg/m³. Gauge pressure $P = ho gh$. Rearranging for density: $ho = \frac{P}{gh} = \frac{15,000}{10 \times 1.5} = \frac{15,000}{15} = 1000 \text{ kg/m}^3$.
3. Answer: 2/3 (or 66.7%). The fraction submerged is equal to the ratio of the densities: $\frac{ ho_{obj}}{ ho_{fluid}} = \frac{600}{900} = \frac{2}{3}$.
4. Answer: 20,000 Pa (or 20 kPa). Bernoulli’s equation: $P_1 + \frac{1}{2} ho v_1^2 + ho gh_1 = P_2 + \frac{1}{2} ho v_2^2 + ho gh_2$. Let $h_1 = 0$. $50,000 + \frac{1}{2}(1000)(4^2) = P_2 + \frac{1}{2}(1000)(6^2) + (1000)(10)(2)$. This simplifies to $58,000 = P_2 + 18,000 + 20,000$. Therefore, $P_2 = 58,000 - 38,000 = 20,000 \text{ Pa}$.
5. Answer: 125 kg. Pascal’s Principle: $\frac{F_1}{A_1} = \frac{F_2}{A_2}$. $\frac{50}{0.01} = \frac{F_2}{0.25}$. $F_2 = 50 \times 25 = 1250 \text{ N}$. Since $F = mg$, $m = \frac{1250}{10} = 125 \text{ kg}$.
6. Answer: 20 N. The buoyant force is the difference between the weight in air and the apparent weight in the fluid: $100 \text{ N} - 80 \text{ N} = 20 \text{ N}$.
7. Answer: 16. Poiseuille’s Law states resistance $R$ is proportional to $\frac{1}{r^4}$. If $r$ becomes $\frac{1}{2}r$, then $R$ becomes $\frac{1}{(1/2)^4} = 16$ times larger.
8. Answer: 10 m/s. Torricelli’s Law: $v = \sqrt{2gh}$. $v = \sqrt{2 \times 10 \times 5} = \sqrt{100} = 10 \text{ m/s}$.
9. Answer: 1000 Pa. The pressure difference is $\Delta P = ho g \Delta h$. Here, $\Delta h$ is the side length of the cube ($0.1 \text{ m}$). $\Delta P = 1000 \times 10 \times 0.1 = 1000 \text{ Pa}$.
10. Answer: 3. Specific gravity is the ratio of the density of the substance to the density of water ($1000 \text{ kg/m}^3$). $\frac{3000}{1000} = 3$.

1. Which of the following best describes the relationship between fluid velocity and pressure in a horizontal pipe according to Bernoulli’s principle?

• AAs velocity increases, pressure increases.
• BAs velocity increases, pressure decreases.
• CVelocity and pressure are independent of each other.
• DPressure is only dependent on the depth of the fluid.

Frequently Asked Questions

What is the difference between gauge pressure and absolute pressure?

Gauge pressure is the pressure relative to atmospheric pressure, whereas absolute pressure is the total pressure including atmospheric effects. You calculate absolute pressure by adding the local atmospheric pressure to the measured gauge pressure.

How does the continuity equation apply to the human circulatory system?

The continuity equation ensures that the volume flow rate remains constant throughout the closed loop of the circulatory system. This means that while blood slows down in the tiny capillaries due to their massive total cross-sectional area, the total volume passing through them per second equals the volume leaving the heart.

What is the significance of the Venturi effect on the MCAT?

The Venturi effect occurs when a fluid's pressure drops as it passes through a constricted section of a pipe, which is a specific application of Bernoulli's principle. On the MCAT, this is often used to explain how oxygen masks or atomizers work by drawing in a secondary fluid through a low-pressure zone.

When should I use Poiseuille’s Law instead of Bernoulli’s Equation?

Use Poiseuille’s Law when dealing with real, viscous fluids in laminar flow where resistance and pressure drops are significant, such as in small blood vessels. Bernoulli’s Equation is reserved for ideal, non-viscous fluids where energy conservation is the primary focus.

Does buoyancy change with depth?

For an incompressible object submerged in an incompressible fluid, the buoyant force does not change with depth because it depends only on the weight of the displaced fluid. However, if the object is compressible (like a balloon), increasing depth and pressure will decrease its volume, thereby reducing the buoyant force.

For more practice with related physical and chemical concepts, check out our Medium MCAT General Chemistry Practice Questions or review Medium MCAT Electrochemistry Practice Questions to strengthen your foundational knowledge. If you are moving into organic chemistry, our Medium MCAT Organic Chemistry Practice Questions offer excellent preparation.