23: Discrete Physics Questions - Breaking them Down

Session 23 In today's episode, Ryan and Bryan cover MCAT content and dive into actual questions and passages as they try to help you figure out how to break discrete physics questions. Here are the highlights of the conversation between Ryan and Bryan: Look at individual questions as "an inch deep" so you only have to figure out 1-2 hooks to open up the questions. Question #1: 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 1600 cubic centimeters a 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 stomach content flow? The connections are 2x longer. The connection is 4x longer. The connection radius is 2x larger. The connection radius is 4x larger. Strategies for answering this question: Go back to the question again and refresh it. Then you notice the 16x increase in the flow of this fluid. Then you have to decide whether the connection is 2 or 4 times longer or the radius is 2 or 4 times larger. The question itself already gives you the clue - Poiseuille's law. The flow rate of fluid is something you should at least be familiar with. Know that flow rate is proportional to pressure. The harder you push, the more the fluid flows. It is proportional to the 4th power of radius. MCAT likes to test whether you know if the relationship is exponential or inverse or square rooted. Flow is proportional to pressure, proportional to the 4th power of radius, and inversely proportional to the length of the pipe. To get 16x increase in flow from 100 to 1600, you would only need to increase the radius to let more fluid through but you only need to increase 2x if you double the radius. 2 to the 4th power is 16x more. This question demonstrates that you don't have to do the calculation or even have the formal equation memorized. You just need to know the underlined relationships it represents. Question #2: Which of the following explains why the pitch of a person's resonate voice harmonics rises when they inhale helium instead of air? The wavelength increases due to the change in temperature from the helium atoms. The change in timber results in a higher perceived frequency of the voice. The frequency decreases due to the change in air density from the helium atoms. The speed of sound wave remains constant regardless of changes to the medium through which the sound wave propagates. Strategies for answering this question: The question was asking for frequency, not speed so choice (d) doesn't get on point. More importantly, the speed of the sound wave does change. (a) says the wavelength increases while (b) says higher frequency. Hence, (b) is the right answer. In the practice test, majority of students answer (c) probably because students read this so quickly and are rushing. So you have to take a minute and read the question carefully to figure out what they're asking you. In truth, the actual frequency does not change but it changes the texture of the sound (how we perceive it) so we perceive it as higher frequency. Question #3: Modern MRI machines use electricity to generate the magnetic fields inside a circular chamber instead of a permanent magnet. Which of the following would not increase the strength of the MRI field? Increase the radius of the MRI chamber Increase the power supplied to the MRI machine Decreased resistance of the MRI machine Increased current through the MRI machine Strategies for answering this question: When questions have "not" or "except" in them, it's easier to do the process of elimination. (c) and (d) are practically the same. They can't both be right so eliminate both of them. Now, it's between (a) and (b). (b) increases power and that's going to increase the current so this answer should be eliminated as well since (b), (c), and (d) say the same thing. Hence, the answer is (a). You can answer this without even knowing the complicated equation to solve for the magnetic field. Full credit for partial knowledge - If you only remembered one equation, v=ir, that partial knowledge would get you full credit on this question Links and Other Resources: www.mededmedia.com Next Step Test Prep Use the promo code MCATPOD to save some money off their classes or one-on-one tutoring.

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