Let’s Talk MCAT Physics Discrete Questions

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 39.

A collaboration between the Medical School Headquarters and Blueprint MCAT (formerly Next Step Test Prep), The MCAT Podcast is here to make sure 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, and this podcast will give you the motivation and information that you need to know to help get you the score you deserve so you can one day call yourself a med student.

Welcome back to The MCAT Podcast. My name is Dr. Ryan Gray, your host here every week along with my pal, Bryan from Blueprint MCAT (formerly Next Step Test Prep). Bryan, last week we ended the podcast talking about polarity with chemical structures, which is all physics. This week we’re going to talk physics.

Bryan Schnedeker: Yeah, absolutely. This new MCAT is so cross-disciplinary. We haven’t seen it for a little while because we’ve been doing a lot of just science discrete questions really to hammer home the content knowledge that the MCAT requires. But when you start working those passages, and even some of the discrete questions, they will expect you to cross disciplinary lines when trying to answer a lot of problems that you would see.

Dr. Ryan Gray: Which makes sense, because that’s what you do in the real world.

Bryan Schnedeker: Yeah, absolutely. These artificial distinctions between- I mean the worst would be organic and biochem. Where is the line? Well, who knows?

Dr. Ryan Gray: Yeah. It’s whatever the author said.

Bryan Schnedeker: Right, exactly.

Dr. Ryan Gray: Alright so go ahead and dig into this first one here.

Question #1

Bryan Schnedeker: Yeah so I’m going to apologize in advance. We’re going to look at a couple of calculation questions here. I tried to pick ones where there’s really only kind of one equation to apply just for the audio format of the podcast. But as always, very, very helpful, go, go, go download the show notes handout, and you can follow along with the actual question right in front of you.

Dr. Ryan Gray: Yeah www.TheMCATPodcast.com/39.

Bryan Schnedeker: There you go. Okay, so first question. ‘A 12-volt battery is used to charge a 20-microfarad capacitor in a defibrillator. How much charge is stored on the plates of the capacitor? And we have 0.24 millicoulombs, 0.6 millicoulombs. 24 coulombs. 60 coulombs. So first, let’s say you can’t remember what the heck you’re supposed to do here. Like, ‘Oh capacitance, it’s farads, there’s something coulombs involved. I just don’t remember this relationship.’ You could at the very least say- the question said a 12-volt battery and a 20-microfarad capacitor. So looking at the answer choices, chances are that ten to the minus sixth, that micro is going to give us an answer that ends up being milli; millicoulombs instead of regular coulombs. That alone would narrow you down to choices A and B. And sometimes on the MCAT, you know we get this question all the time, ‘Oh what if I can’t do calculations quickly? What if I really am just uncomfortable doing calculations with paper and pencil without a calculator?’ I always say get as far as you can. There’s almost always one or two you can eliminate, and then don’t actually do the calculation. Just do your best process of elimination you can, make your best guess, and move on. In this case hopefully you remember your units. Very important, much more important to remember units than it is to remember equations for the MCAT. So here the unit of farad, we want to remember is a coulomb per volt. So farad, F, equals coulomb, C, divided by volt, V. So then you can set up the equation by multiplying volts up to the other side to solve for coulombs. So a coulomb is a farad times a volt, so in this case that’s 12 times 20. And again, forget where the exponent goes, forget where the decimal goes. Just look at 12 times 20 and say, ‘Well that’s really 12 times 2, so my answer has to be 24 times 10 to the something,’ right? Don’t worry about the something. As soon as you know your answer is 24, you know the right answer is choice A, 0.24. And in this case 0.24 millicoulombs.

Dr. Ryan Gray: And now without going into the math behind it, you said okay you know that that microfarad is going to be a millicoulomb. But I don’t trust myself. How can you say that confidently and let the listener know that it’s true?

Bryan Schnedeker: So the likelihood- the equations that you see on the MCAT definitely have co-efficients in them where you have to do this relationship between the variables, but then multiply it by some other number. I’m thinking of say like the gas law. The energy and the gas is three halves MRT, so there’s that three halves out front. Or think of the kinetic energy equation in physics; 1/2mv^2 so there’s that times one half out front. But if you have microfarads, micro is 10^-6. It’s unlikely that you’re going to have an equation on the MCAT that is takes the farads, multiply it by a million, and then work in the volts somehow, right? And that’s what you would have to do to cancel out micro, you’d have to multiply it by a million in order to get answer choice C or D, 24 coulombs or 60 coulombs. So it’s a little bit of kind of gut instinct, right? Like, ‘I’ve studied these equations, I don’t remember all of them,’ but none of them have times a million at the beginning of the equation.

Question #2

Dr. Ryan Gray: Okay. Alrighty moving onto question 55. ‘An artificial leg designed for use by runners is spring-based to mimic the compression required of a muscle during hard running. For safety reasons it was determined that the leg should be able to absorb as much as 125 joules of kinetic energy without compressing more than ten centimeters, or the runner would be likely to stumble. What should the spring constant be?

Bryan Schnedeker: Stumble over the words.

Dr. Ryan Gray: Yes.

Bryan Schnedeker: So you think about an artificial limb, an artificial leg here that you’re just pounding on the ground over, and over, and over again, and it only compresses ten centimeters, or 0.1 meters. So it’s not compressing very much which means if you think of it like a spring, it’s going to be really stiff, you know this is a really strong spring so it’s more like the shocks in your car and less like a Slinky, right? It’s not moving very much, it’s not like loosey goosey like a Slinky. And so conceptually before you start plugging and chugging with equations, always make sure you’ve got the concept right in your head. I want a really stiff spring and so you want to walk into the MCAT knowing that a higher K, a higher spring constant means a stiffer spring. So when you look at these answer choices; 250, 2,500, 12,500, 25,000. Again if you’re just kind of guessing on a conceptual level, probably C or D is a better guess. Like guess one of the bigger numbers because that would suggest that it was a stiffer spring.

Dr. Ryan Gray: Okay.

Bryan Schnedeker: Okay so now the actual equation that you need to know, they gave us energy, so you need to know the equation for the potential energy that goes into a spring, and that’s energy equals one half KX squared. So 1/2 times K, the spring constant, that’s what we’re solving for, times X squared, and X is how much the spring crunches up when we store the energy in it. So for springs, two equations to know overall, one of them for this problem, one is what we call Hooke’s Law, F=KX, and one for the potential energy, 1/2KX squared. At that point for energy you would plug in 125, and for X you would plug in 0.1 because you have to express it in meters. So either 0.1 or 10^-1, and then solve for the K. And remember it was energy=1/2KX^2 so energy, 125, the very first thing you’re doing to that 125 is you’re multiplying it by 2 to cancel out the one half on the other side of the equation. Right? Multiplying those sides by 2 so that 125 becomes 250, and then you’re dividing it by 10^-2. Remember 10^-1 but it got squared, so 10^-2 now, and 250 divided by 10^-2 means just smack on two more zeros so 250 becomes 25,000 just like we said the big number ended up being the right answer, answer choice D, 25,000.

Dr. Ryan Gray: Alright so the take home message is go big or go home on the MCAT.

Bryan Schnedeker: Absolutely. Yeah you know we spend so much time in the verbal section, in the CARS part of the test, telling students to avoid extreme answers, and then along comes the science, especially physics, and it’s like, ‘Go for the big one! Go for the extreme! Biggest or smallest!’ Yup.

Dr. Ryan Gray: Man, you wonder why premeds are crazy.

Bryan Schnedeker: Yeah right, it’s all a massive contradictory, ‘This is the rule, except when it’s not, except except when it is.’

Dr. Ryan Gray: Yeah our anatomy professor in medical school said, ‘This is the way it always happens except when it doesn’t.’

Bryan Schnedeker: Right, yeah.

Dr. Ryan Gray: Last one here.

Bryan Schnedeker: Okay you want to read it first?

Dr. Ryan Gray: I’ll let you.

Question #3

Bryan Schnedeker: Okay so, ‘Colliding cells meet on a frictionless surface near the vascular membranes. If cell one collides into a stationary cell two on an arterial wall, which of the following describes what happens to cell two after the collision? Cell two continuously accelerates? Cell two moves to its decreasing velocity? Cell two moves with constant speed? Cell two moves with constant velocity?’ And for the purposes of this problem, let’s just assume that things are as simple as possible. The cells are spheres, after the collision they don’t kind of continue to wiggle around and deform. When it comes to physics on the MCAT, assume things are as simple as possible unless they give you a specific reason to make it some sort of complicated real world thing. And the question itself said that these cells are colliding in a frictionless way. So let’s just keep it as simple as possible and imagine two billiard balls on a table smacking into each other. So after one ball smacks into the second ball, what happens to that second ball? Well a certain amount of energy has been given to it, so it’s going to start moving, and if it’s moving across a frictionless surface, Ryan what do you think is going to happen to how it moves?

Dr. Ryan Gray: It’s going to keep moving.

Bryan Schnedeker: Yeah, forever. I guess until it gets back to the heart and hits turbulent flow in the atrium.

Dr. Ryan Gray: Or collides into cell three.

Bryan Schnedeker: Sure, yeah exactly. Or gets infected by plasmodium. Right until something biological happens to it. Right so it’s going to move at a constant speed, and as long as it’s going in that same direction, a constant speed and a constant velocity are the same thing as each other. So this is actually a Roman numeral question where both Roman numeral three and four are correct. It will move at a constant speed and just point in one direction at a constant velocity.

Dr. Ryan Gray: So Roman numeral one on here is cell two continuously accelerates, and so ‘continuously’ is what makes that one wrong? Because it obviously accelerates up to the point where it reaches its speed.

Bryan Schnedeker: Right well the questions specifically said, ‘What happens to cell two after the collision?’ So it’s only accelerating while in contact with cell one. While they’re touching, cell one is pushing on cell two. Once the collision is over cell two doesn’t get any extra energy from anywhere, now it’s just sliding along this frictionless surface.

Dr. Ryan Gray: It’s a good thing to pick out, okay.

Bryan Schnedeker: Absolutely. And Ryan that brings up- that’s a good place to kind of wrap up this physics discussion because it’s the single most important lesson in all of MCAT, the single most important lesson in all of test prep, answer the question they actually asked you.

Final Thoughts

Dr. Ryan Gray: Alright there you have it, some physics discrete questions for you to ponder over. Go check out the handouts over at www.TheMCATPodcast.com and search for session number 39, and we’ll get you squared away.

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