Physics Series 2: Water, Buoyancy and the MCAT

Session 89

Water and fluid dynamics and physics go hand in hand on the MCAT. This week we’re cover this topic to help you improve your physics MCAT section score.

As always, we’re joined by Bryan Schnedeker from Blueprint MCAT (formerly Next Step Test Prep). The questions are taken from the QBank, which costs $50 if you purchased it separately. And if you purchase the ten packs of full lengths for $49, then you’re going to get the QBank for free. It’s great deal. So for $250, you’re getting ten full-length tests, so it’s $25 per test. It’s literally the best deal you can buy anywhere. The QBank has the equivalent of essentially two more full length MCAT worth with 127 passages and a thousand discrete questions and more!

[02:25] Life Raft and Density

Question 11: A foam life raft has a density of 491 kg/m3. When placed in a pool of pure water, what percentage of the raft’s volume will be above the surface of liquid?

(A) 4.91%
(B) 49.1%
(C) 50.9%
(D) None. The raft will be entirely submerged.

Bryan’s Insights:

You always have to follow the math where it leads you, but at the same time, the MCAT is grounded in reality, real science, the real world. So you’re pretty sure, it’s going to float. Hence, it’s going to be a, b, or c.

When they’re giving you 4.91 and 49.1, this is a strong clue that it’s not D. They’re expecting you to mess up by a factor of 10.

So there’s outside knowledge that you need here. You have to walk into the test knowing the density of water. And water’s density is 1000kg/m3. It sounds odd but people don’t realize how big a cubic meter is and it’s an enormous box. It’s very heavy. And this life raft only has a density of 491kg/m3. So a little less than half is dense. So if you put it in water, it’s going to float.

The simple rule of thumb is if you take the density of the object and divide it by the density of the fluid, that ratio is going to give you the percentage that will sink into the fluid.

Think of like a pool noodle that barely sinks into the water at all. And if you said it has a density that’s 5% of water, a density of 50kg/m3 and water has a density of 1000 kg/m3. So it’s 5% the density of water and only 5% of it sinks down into the water.

And if you think of an actual pool toy, it really does seem to rest right on the top of the water.Whereas it’s something at a density of 97% of water, it would almost entirely sink. IN this case, 491/1000 is 49.1. So this is how much it would sink. And the question asks for that which is floating above the surface, so the correct answer is (C) 50.9% that’s emerging above the water.

Tip: Always read the questions carefully. Answer the question they actually asked you.

[08:15] The Mystery of the Ideal Fluid

Question: A variety of experiments are being conducted in a large tank containing an ideal fluid. A spherical object with a volume of 0.84 m3 and a specific gravity of exactly 1.05, which is true?

(A) The object will rapidly sink beneath the surface of the fluid until it reaches the bottom of the tank.
(B) The object will sink beneath the surface but will not hit the bottom.
(C) At least part of the object will project above the surface of the fluid.
(D) Not enough information available to answer.

Bryan’s Insights:

The rule for floating is simple. If you have a density greater than the fluid, you sink. If you have a density less than the fluid, you float. And if you have a density equal to the fluid, you just stay wherever you are right now, or you keep moving with whatever your current velocity is.

This tells us the object has a specific gravity of 1.05 but it doesn’t tell us the specific gravity of the fluid. In this case, the correct answer is (D).

[10:15] Density of Ionized Water

Question 13: A sample of deionized water is kept in a cylindrical beaker with a radius of 4cm and a height of 10cm. The density of the water is closest to:

(A) 5.024 x 10-4 kg/m3
(B) 1 kg/m3
(C) 502.4 g/m3
(D) 1000 g/L

Bryan’s Insights:

The reason Bryan picked this question here is to make this general point. You want to remember that on the MCAT, on the passage and even in the questions, there can be extra information.

In this case, what’s the density of water. Who cares what’s in or if it’s a beaker or it in a cup or in the ocean. Regardless, the density of water is what it is wherever it is.

As mentioned a while ago, the density of water is 1000 kg/m3. However, you need to know the other way of expressing this same notion which is 1g/cm3, 1 gm/ml, or 1kg/L. The density of water is often described as 1 because it is in three different unit analysis.

The correct answer here is (D) 1000 g/L which is just 1 kg. So that’s it.

Note that the question only required one thing and that’s you should know the density of water.

[13:00] Blueprint MCAT (formerly Next Step Test Prep)

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