Fluid Mechanics for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about fluid mechanics, plus practice questions and answers
(NOTE: THIS GUIDE IS PART OF OUR MCAT PHYSICS SERIES.)
Table of Contents
Part 1: Introduction to fluids
Part 2: Definition and applications
Part 3: Density, buoyancy, and applications of pressure
a) Density and specific gravity
b) Buoyancy
c) Hydrostatic pressure
d) Pressure and depth
Part 4: Fundamental laws
a) Pascal’s law
b) Poiseuille flow
c) Continuity equation
d) Bernoulli’s equation
Part 5: High-yield terms and equations
Part 6: Passage-based questions and answers
Part 7: Standalone questions and answers
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Part 1: Introduction to fluids
As a study of substances including liquids, gases, and plasmas, the study of fluids is extremely important for a lot of real-world applications of physics, such as blood flow in the circulatory system.
This guide won’t necessarily follow a self-contained narrative from start to finish. Instead, we’ll start with a basic definition of fluids, which we’ll then combine with other physics intuitions to think about other implications. This unit should give you a comprehensive understanding of the fluid-related concepts you’ll see on the MCAT, as well as showing how concepts from other topics have implications everywhere!
On the MCAT, fluid mechanics is a medium-yield topic. Below, the most important terms are in bold font. When you see one, try to define it in your own words, and use it to create your own examples. This is a great way to check your understanding, and phrasing things in a way that makes the most sense to you will make studying much easier (and much more effective!) in the long run.
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Part 2: Definition and applications
What is a fluid? You might know that fluids don’t only refer to liquids—other states of matter, like gases, are fluids as well. Let’s try to find a better definition of what a fluid is.
A fluid is a substance that will move under any shear stress. A shear stress is the result of a force that acts on the material from away from its center of mass.
When we place a fluid into a container (like water into a cup), the shear forces will continue to act on the fluid and shape it until it reaches an equilibrium.
Figure: The liquid doesn’t settle until its surface is flat, because there is no longer a horizontal component of gravity exerting a shear stress here.
We can classify the flow of fluids as laminar or turbulent. Think of the difference between a calm stream and a river with rapids. The former is slow and smooth; the latter is fast-moving and full of violent disturbances. This is generally the case with slow-moving and fast-moving fluids. Low-velocity fluids create laminar flow, and high-velocity fluids create turbulent flow.
In laminar flow, a slight disturbance is quickly corrected in an orderly manner, and in turbulent flow, a disturbance forms eddies and causes a much greater interruption.
Figure: Laminar (left) vs. Turbulent (right) flow caused by a circular disturbance.
You may also have noted the distinct spherical shapes that water droplets form, or the ability of water to “creep” up sheets of paper. Surface tension is a result of cohesion between the same type of molecules on the surface of the fluid. It allows the surface of a liquid to support light things, such as a leaf or an insect, which would otherwise sink.
Surface tension is also the reason water droplets form on a surface rather than covering the entire surface with an infinitesimal layer of liquid.
Figure: Among other effects, surface tension allows for liquid droplets to form.
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