Discrete Physics Questions – Breaking them Down

Introduction

Dr. Ryan Gray: The MCAT Podcast is part of the Med Ed Media network at www.MedEdMedia.com.

This is The MCAT Podcast, session number 23.

A collaboration between the Medical School Headquarters and Blueprint MCAT (formerly Next Step Test Prep), The MCAT Podcast is here to make sure that you have the information you need to succeed on your MCAT test day. We all know that the MCAT is one of the biggest hurdles for a premed, and this podcast will give you the motivation and information that you need to know to help you get the score you deserve so you can one day call yourself a physician.

Alright we are back again at The MCAT Podcast. If this is your first time joining us, welcome, my name is Dr. Ryan Gray. I host several podcasts, The MCAT Podcast being one of them. I also host The Premed Years, the Old Premeds Podcast, and Specialty Stories. You can find everything that I do over at www.MedEdMedia.com. This podcast is one that I do with Blueprint MCAT (formerly Next Step Test Prep). If you haven’t heard of Blueprint MCAT (formerly Next Step Test Prep), they are the premier MCAT test prep company specifically for one-on-one tutoring. And now they are breaking into the course game with an online course all about the MCAT. It’s a course that you can take at your own pace, but they have amazing content, a ton of content, more than the other big box companies out there and for a cheaper price. Go check them out, www.NextStepTestPrep.com. Use the promo code MCATPOD, that’s MCATPOD all capital letters, MCATPOD to save some money on their class or their one-on-one tutoring. One of the biggest things I forgot to mention with their class, they have amazing live office hours with the people that actually made the course, and it’s not just some random teachers, it’s the people that actually put the course together there to help you maximize your score. Again www.NextStepTestPrep.com.

Before we jump in this week I want to remind you this is another episode where we cover content and go over questions, so to look at the questions if you’re not driving and you have time to actually open up a website, go to www.TheMCATPodcast.com/23 and download the question for this episode that we’re going to cover. We’re going to jump into this one right now.

Alright Bryan we’re into week number two of diving into actual questions and talking through questions, and answers, and passages, and trying to figure out how to break these down. So let’s dive into this new one here.

Question Number One

Bryan Schnedeker: Sure yeah so we’re going to do a series of short physics discrete questions, so each one of these is going to be tested one particular concept, or one particular application of a concept, it’s how a lot of these questions will end up working on the MCAT. You know you and I have talked a lot in the past about how the MCAT is a mile wide but an inch deep, so when you look at any individual question just remember this is only an inch deep, I only have to figure out one or two kind of real hooks to open up this question for us. So we’ll start with the question at the top, and again if you want to visit the show page you can download an actual printout of these questions, make it a little easier to follow along. But I’ll go ahead and read them out loud. So the first question says, ‘The flow rate of stomach content emptying is 100 cubic centimeters a second. Patients who undergo gastric bypass surgery will increase this rate to almost 1,600 cubic centimeters a second.’ Okay so 100 cubic centimeters per second up to 1,600 cubic centimeters per second. ‘Assuming the flow of stomach contents approximates Poiseuille’s Law, what change to their GI connection would explain this provided no other changes occur in the conditions of the stomach content flow?’ And our choices are the connection is two times longer, the connection is four times longer, the connection radius is two times larger, or the connection radius is four times larger. So I emphasized though while I was reading, of course on test day you would be reading this question really quickly, then you would check out the answer choices, go back to the question again and just refresh it. Okay I went from 100 to 1,600 for the flow of this kind fluid going from your stomach to your intestines. So we had a 1,600 times increase in the flow of this fluid, and we have to decide whether the connection is two or four times longer, or the radius is two or four times larger. And the question itself actually gives you the clue, it’s Poiseuille’s Law, and for the MCAT we know of course there are tons and tons of equations we have to memorize, and the flow rate for a fluid is something we should at least be familiar with. There’s a bunch of constants in there that we don’t particularly worry about but we do need to know that flow rate is proportional to pressure. Right? The harder you push the more the fluid flows. It is proportional to the fourth power of radium, okay radius to the fourth. The MCAT really likes to test whether you know whether a relationship is exponential, or inverse, or square rooted, so flow is proportional to pressure, proportional to the fourth power of radius, and inversely proportional to the length of the pipe. So going back to this, to get a sixteen times increase in flow from 100 to 1,600, you would only need to increase the radius, make a bigger, fatter pipe, increase the radius that lets more fluid through, but you actually only need to increase the radius two times. If you double the radius two to the fourth power is sixteen times more. And this is actually a question, Ryan, that we use on our diagnostic test, and most students actually get this wrong. They pick that the radius has to go four times larger because they think it is a squared relationship. So this question emphasizes knowing that exponent is going to be real key when it comes to working through equations. What this question also demonstrates for us is that you don’t actually have to do the calculation, and in some sense you don’t even have to have the whole formal equation memorized, you just need to know the underlying relationships that it represents.

Question Number Two

Let’s take a look at number thirteen here, the next one we’re going to try. ‘Which of the following explains why the pitch of a person’s resident voice harmonics rises when they inhale helium instead of air?’ One more time. That’s ‘Which of the following explains why the pitch of a person’s voice rises when they inhale helium instead of air’ and then we have a bunch of answer choices that are going to try to explain that. So we want to take just a second and remember our science. Okay pitch going up, the MCAT is going to expect us to know that what is subjectively perceived as pitch is the physical property of frequency. So higher pitch means higher frequency. In other words, this question now that we’ve kind of thought it through and rephrased it, is why does the frequency go up when you inhale helium? Now let’s check the answer choices. A) The wave length increases due to the change in temperature from the helium atoms. Okay, maybe, it’s mentioning wave length. We’ll hold onto that for a second. The change in timber results in a higher perceived frequency of the voice. Okay this is bringing in this outside idea of the timber or timber of the voice. But it does say higher frequency so again let’s leave that in because that sounds like it fits what we thought the question said. Choice C) The frequency decreases due to the change in air density from the helium atoms. Okay well now that says the frequency decreases, and we can more or less stop reading right there. Right? The rest of the answer choice talks about density and so on, but because it said frequency decreases it’s already wrong. Then we go to D) The speed of sound wave remains constant regardless of changes to the medium through which the sound wave propagates. And we would know from our outside knowledge that- well first of all the question was asking about frequency, not speed, so D doesn’t directly get on point. But more importantly the speed of the sound wave does change, it doesn’t remain constant. When a sound wave goes from air into solid it goes faster. So D is just wrong on the science. So we go back to answer choice A and B, and A said the wave length increases and B said the per higher frequency. And again we were looking for an increase in frequency so answer choice B is the right answer here. Now the interesting thing about this question, and part of the reason, Ryan, that I pulled it out for us to look at during today’s podcast is because this is given on one of our practice tests and the majority of students, 45% actually picked answer choice C, the frequency decreases due to the change in air density from the helium atoms. And my guess here is that students are just reading so quickly, they’re rushing and they feel so pressed for time that they read an answer choice that they see the word ‘frequency,’ they see the word ‘density,’ they see the word ‘helium,’ and they immediately start thinking that answer choice looks really good. And what that really reflects for good test takers is you have to take a minute, read the original question carefully to figure out what it’s actually asking you. Really understand the scenario being presented before you start just barreling through the answer choices, otherwise you’ll end up getting fooled by a question that talks about density when the whole answer choice was just flat out wrong.

Dr. Ryan Gray: I like it, and I like sucking helium from balloons too, so that’s a great question.

Bryan Schnedeker: Yeah and actually I mean B is the truth. If you put a person within oscilloscope, you know like to measure the frequency, the actual frequency, their voice doesn’t change when you suck in helium atoms. It just changes the- how do I put this? The texture of the sound, you know how we perceive it. And so we perceive it as being high frequency, whereas if you ask like a trained professional singer to say, ‘Sing a middle C and then inhale a bunch of helium, sing a middle C,’ and you put them up to an oscilloscope it would show the exact same frequency. We would just perceive it as higher frequency.

Dr. Ryan Gray: Interesting.

Bryan Schnedeker: Yeah. I think so being a science nerd.

Dr. Ryan Gray: Science is cool.

Question Number Three

Bryan Schnedeker: Yeah. Okay so we have one more that we want to look at before we wrap things up for today. So we’re going to look at question number 29 now, a little bit more of a straightforward physics question that most students on our test got correctly. ‘Modern MRI machines use electricity to generate their magnetic fields inside a circular chamber instead of a permanent magnet. Which of the following would not increase the strength of the MRI field?’ Okay that we’re using electricity to make a magnetic field, we want to not increase the magnetic field. In other words leave it the same or decrease the strength of the magnetic field. Okay A says increase the radius of the MRI chamber. B says increase the power supplied to the MRI machine. C says decreased resistance of the MRI machine. And then D, increased current through the MRI machine. Now this question had the word ‘not’ in it, and when questions have ‘not’ or ‘except’ in them; you know ‘blah, blah, blah, except,’ or ‘all of the following would not,’ or so on, it’s usually easier to do process of elimination and this is one where it’s actually easier to do process of elimination. Because again if we look at answer choice C, decreased resistance, and answer choice D, increased current, we can actually eliminate both of them right away because they say the same thing as each other. If we remember- if we remember Ohm’s Law, V=IR, if current goes up, resistance goes down and vice versa. So answer choice C and D can’t both be right, right? If C was right then D would also be right. So decreased resistance, increased current, those mean the same thing, we can eliminate both of them. So then we have A and B. A, increased radius, and B, increased power. Now we already eliminated D which said increased current, and if you increase power, right supply more wattage, supply more joules per second, that’s going to increase the current running through the machine. So again, doing answer choice B, increasing the power would do answer choice D, increase the current, and so we can eliminate B as well as essentially B, C, and D all say the same thing as each other. And so we’re left with answer choice A, increasing the radius of the MRI chamber. And you know, we can answer this question correctly without even knowing the big complicated equation for solving for a magnetic field where there’s a whole bunch of magnetism equations and if you couldn’t remember it you can still get your way there through good old trusty process of elimination.

Final Thoughts

Dr. Ryan Gray: Alright so another one where the MCAT at its core is a reasoning and thinking test, just think through the question, think through the answers, and get to where you need to go without truly understanding really every equation behind the question.

Bryan Schnedeker: Absolutely I always say it’s full credit for partial knowledge. Right? If you only remembered one equation, V=IR, that partial knowledge would get you full credit on this question.

Dr. Ryan Gray: I like it, full credit, partial knowledge. Alright there you go. Don’t forget to- if you listened to this and you want to re-listen to it with the actual instructions, go to www.TheMCATPodcast.com and download the actual question for this episode. Go find episode 23 and you’ll download it there. You can go to www.TheMCATPodcast.com/23 to download it.

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