Today, I am once again joined by Bryan of Next Step Test Prep, as we take a grab bag of physics questions and break them down to help you on your MCAT journey! Don’t forget to share this with your classmates.
Check out all of our other podcasts on MedEd Media. And if you need help with your MCAT prep, go check out Next Step Test Prep. Use MCATPOD to save 10% off their package.
[01:43] Isotope Decay
Question 27: Following radioisotope-aided imaging, the isotope 99-TC decays to a 99-RU, and it’s cleared by the kidneys. What particle is ejected during the additional decay step?
- (A) An electron
- (B) A positron
- (C) A gamma photon
- (D) An alpha particle
This is one where you have to recall each of the answer choices what their mass and charge are. In this case, the mass didn’t change. The TC and RU both have a mass of 99. This means we can’t be ejecting anything with mass.
Basic conservation of mass says that alpha particle (answer choice D) is the wrong answer. An alpha particle has a mass of 4. And if you shoot out something that has a mass of 4, you would have to go from 99 down to 95.
Next, a photon has no mass and no charge. It means that when a radioactive element undergoes gamma decay, it doesn’t change its mass or its number of protons. You would have to go from 99-TA to 99-TA in order to eject the gamma ray. Think of all the protons and neutrons that make up the element. They’re jumbling around into a lower energy state. None of the blocks go anywhere but they release a whole lot of noise when they collapse.It’s just like a Jenga tower!
So in gamma decay, the nucleons jostle around a lot and settle into a more stable environment. Then that jostling around shoots out a gamma particle.
In this question, it went from TA to RU. So something changed. The number of protons changed. So (C) is also out. Now, it’s between an electron and a positron. But you would need a periodic table to make the final determination between the two.
For the purposes of answering it here (since you would have a periodic table on test day), TA has 43 protons and RU is element 44 so it has 44 protons.
To get an additional proton, you would have to shoot out a negative charge. Take away a negative and the guy that gets left behind is more positive as a result. So in this case, the answer is an electron or a beta minus particle.
[05:10] Unit Conversion Knowledge
Question 28: The inner mitochondrial membrane has a thickness of 5 nanometers and an average membrane potential of 150 millivolts. What’s the magnitude of the electric field across the mitochondrial membrane in these cells?
- (A) 3 times 10-2 volt per meter
- (B) 3 times 104 volt per meter
- (C) 3 times 107 volt per meter
- (D) 3 times 1010 volt per meter
This question illustrates an important principle when it comes to math on the MCAT. People tend to freak out about calculations and equations. But when in doubt, you can kind of side your way up to the right answer.
In this case, the units for every answer choice is volt per meter. And when you look at the question, it says 150 millivolts and 5 nanometers. So there was a volt and a meter. The units in the answer choices imply that you just divide the volts per the meter. Is it really that simple? Bryan says it is.
The MCAT can be subtle but it’s not actually out to trick you. So just read the exact wording on the screen. Read the exact wording on the answer choice.
So here, you have to know your exponents. Milli is 10-3 and nano is 10-9. Remember that when a negative exponent is in the denominator, you minus a negative so it’s becoming a positive. So 10-3 minus 10-9 becomes 106. So in this case, answer choice (C) is close enough so you’d already have the right answer.
[08:07] Let’s Talk About Pressure
Question 44: Turbulent flow in humans is a major risk factor for atherosclerosis, the build up of plaque on arterial endothelium. Ignoring any potential effects of turbulence, what effect would atherosclerosis have on blood flow?
- (A) Narrowing of the artery causes the velocity to increase and the hydrostatic pressure to decrease.
- (B) Narrowing of the artery causes the velocity to increase and the osmotic pressure to decrease.
- (C) Narrowing of the artery causes the velocity to decrease and the hydrostatic pressure to decrease.
- (D) Expansion of the artery causes the velocity to decrease and the hydrostatic pressure to increase.
A couple of weeks ago, we did an interview on The Premed Years Podcast with Dr. King Li, the Dean of Carle Illinois College of Medicine. It’s an engineering-based medical school. It’s interesting how he described how they’re going to teach the curriculum. They’re not adding a ton of stuff. They’re just teaching it differently.
He gave a specific example of plaque buildup. Instead of just memorizing what happens with plaque, you go into an engineering model of what exactly happens.
Bryan explains that the answer is (A). Osmotic pressure implies that the blood is getting more concentrated. Osmotic pressure relates to the number of solutes dissolved in the blood. There’s no reason to think that based on the simplified model that the question is proposing. Fluid flowing through a narrower straw doesn’t make the fluid saltier.
So you can start by eliminating (B) saying that in order to change the osmotic pressure of the blood, you’d have to sweating more or drinking more water. You have to be doing something with your fluid balance to change it.
Now we’re left with A. When fluids move faster, they exert less pressure on the walls of the container.
The classic example of this is the simplified model of a paper airplane. Where you tape a paper over a pencil and blow it across the top. Because the air is moving faster across the top, you an get the little paper wing to swing upwards. There’s less pressure on top.
Or another example is the shower curtain. The warm shower heats up the air so the air moves up and out of the shower. So it’s a combination of both movement of air passed through the curtain lowers the pressure. This cause the curtain to swing in. And it’s the physical movement of the air by convection.
[12:55] Final Thoughts
I spoke to five premed clubs the week before this has been recorded. And out of five, one had three students who knew about the podcast. And if you’re part of a premed club and you’re not emailing or posting in your Facebook group or on Twitter to your club members, you are doing them a disservice. So go let them know about this podcast.
Also, check out what Next Step Test Prep has to offer you on your MCAT prep journey. One of the biggest mistakes students make is not taking enough practice tests. And, they’re not reviewing the practice tests properly. Don’t fall into that same trap.
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