Physics Series 4: Fluid Dynamics on the MCAT

Session 92

In our last week of the physics series, we’re going to cover some fluid dynamic discrete questions that seem to trip up a lot of premed students. Listen now!

The Premed Playbook: Guide to the MCAT is now available for preorder or available to buy on Amazon and Kindle. The eBook is now available while the paperback is coming soon. This book is basically The MCAT Podcast boiled down into a book.

As always, I’m joined by Bryan Schnedeker from Blueprint MCAT (formerly Next Step Test Prep) and we go over almost everything you need to know for the MCAT.

[03:35] Absolute Pressure

Question 14: A billiard ball is submerged in a small tank of water at a depth of exactly 60 cm. However, the tank is located in Denver, Colorado, where the ambient pressure is roughly 0.83 atmospheres. What is the absolute pressure experienced by the ball?

(A) 6,000 Pascal’s
(B) 6,830 Pascal’s
(C) 84,245 Pascal’s
(D) 90, 245 Pascal’s

Bryan’s Insights:

When asked about the absolute or total pressure, you have to add atmospheric or gauge pressure, which is the pressure you get from being underwater or submerged in any fluid. There’s some amount of gauge pressure here.

One of the things you need to walk into the test knowing is that an atmosphere is around 101,000 Pascal’s. Here, 0.83 atmospheres x 101,000 is already going to give you about 84,000 Pascal’s. And if you had paper and pencil or laminated board and marker. You could do it all by hand. Looking at the answer choices, it’s going to be either (C) or (D).

If 0.83 atmospheres is already going to give you about 84,000 Pascal’s but you also have to add the gauge pressure, then the answer is (D). You actually don’t have to do any more math.

[07:20] Some Techniques When Answering Math Questions

Pretty much, a lot of students would really want to make sure they’re getting the specific answers, especially for a lot of the Type A premed students. But Bryan says the MCAT is very much about answering with 100% confidence when you’re only 70% on the way there.

Additionally, the test allows you to “flag” where you click a little button to flag a question. So if you’re unsure about a particular answer, just flag it then move on. And if you have the time, go back and do the flagged question.

[07:55] Cohesion and Adhesion

Question: 20 milliliters of an unknown fluid is added to an empty test tube resulting in the situation picture provided. (The picture shows a test tube with your typical meniscus where the fluid is crawling up the sides of the test tube. The lowest point of the meniscus is in the middle of the fluid and on the edge crawls up the sides of the tube.)

(A) Adhesive forces between fluid particles are greater than cohesive forces between the fluid and the glass.
(B) Adhesive forces between fluid and glass are greater than cohesive forces between fluid particles.
(C) Cohesive forces between fluid particles are greater than adhesive forces between the fluid and the glass.
(D) Cohesive forces between fluid and glass are greater than adhesive forces between fluid particles.

Bryan’s Insights:

The word “cohesion” already gives you a clue that like things are being together. And an adhesive is like glue where you stick two separate things together. In this case, the fluid is crawling up the sides of the test tube. It’s clearly the attractive force between the fluid and the test tube is the stronger thing.

[10:45] Pressure on Vessel Walls

Question 18: A salt solution with a density of 1075 kg/cubic meter is moving through two consecutive closed vessels. Both vessels are positioned at the same height but the solution travels twice as quickly through one vessel A as the next vessel B. If a pressure exerted by the fluid on the walls of vessel A is 6,000 Pascal’s, the pressure exerted on Vessel B is:

(A) 3,000 Pascal’s
(B) 6,000 Pascal’s
(C) 12,000 Pascal’s
(D) A value that cannot be determined

Bryan’s Insights:

Bryan stresses that so much of what students are scared of on Physics is they’ll have to do all this math. And yet so often, it’s not the math. It’s just the conceptual use of the equation. You have to know what the equation is but then you don’t actually have to do any calculation. In this case, with density showing up and height, speed, pressure, you know you’re using Bernoulli’s equation. It says that the pressure of Point A plus the height at Point A. Pressure + height + velocity is 1/2*rho*v2. And this equation at Point A has to equal the same thing at Point B.

The tricky bit is those plus signs. If all stuff were being multiplied together or divided, then it would be so simple. Like it travels with twice the velocity so it’s half or double the pressure. Or if it’s a squared relationship, then quadruple the pressure. But because you’re adding these various terms together, it doesn’t translate into a perfect direct relationship.

You can drop out the Height since the question says it’s at the same height. The density was given as well as the ratios of the velocity. And then you’re still left with an equation where it’s:

6000 + 1/2*rho*v2 = x + 1/2 * rho *2v2

But you still have the unknown which are two and then the velocity. The way you handle it when you have multiple unknowns in a problem is just divide the x over to the other side of the equation. Then the variables will cancel each other out and then you can solve it.

The + sign here in the Bernoulli’s equation, it just throws the monkey wrench into the works there. If you try to divide velocity over, you’d end up with multiple wacky ratios separated by + signs.

The important point is don’t be afraid if the answer choice cannot be determined because maybe it can’t be really determined.

[17:37] Is Water Wet?

As to whether water is really wet, Bryan would have to look at the definition of wet to see what exactly that’s meant.

Water is a molecule and water itself is just a bunch of water molecules and isn’t wet. But water on a surface is then wet. So while water itself is not wet, but when it’s on a surface then it’s wet.

[19:00] About Blueprint MCAT (formerly Next Step Test Prep)

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