MP 59: Breaking Down Physics Discrete Questions for the MCAT

Session 59

Physics on the MCAT is hard for a lot of students. Today, we'll go over some physics discrete questions and break down each of them to give you a head start.

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[01:03] Diving into Physics

Physics was my favorite subject as a premed. And it's Bryan's favorite to teach but as an MCAT student himself, he was indifferent to it. Nevertheless, what he loves about physics is being able to do problems that are grounded in the “real world” so to speak.

[02:55] The Doppler Effect

Question 16: A scientist uses and ultrasound device mounted to a vehicle to measure fluid flow underground. The device makes use of the Doppler effect to track fluid movement in the water table. Which of the following scenarios is most likely to produce a readable Doppler shift?

  • (I) The fluid is flowing at a velocity twice that of the sound-emitting device in the same direction as the device is moving.
  • (II) The fluid is flowing at the same velocity and in the same direction as the sound-emitting device.
  • (III) The fluid is not moving at all.

Thinking points:

The example usually uses the ambulance where it's high-pitched coming towards you. Then as soon as it moves past you, it becomes low-pitched. The sound is a wave and when the vehicle is moving towards you, the high points of the wave or the low points gets all crunched up. With a shorter wavelength, the frequency is higher which the human brain perceives it as a higher pitch.

If we relate this to the question here, the sound is going in the same direction the ambulance is going and coming at you. Bryan says, imagine two cars driving in the same direction at 40 miles an hour. Relative to each other, they're not moving at all. So there's not going to be a Doppler shift if everybody is flowing in the same direction at the same speed. So (II) is out which lets us eliminate choices (C) and (D).

That said, (III) doesn't make sense since you need that movement to have that Doppler effect so the correct answer here is (1).

[06:12] A Sensation Question

Question 18: Presbyopia is diagnosed when the lens of the eye focuses incoming light rays to a position between the retina and the choroid. Which type of lens should be placed in front of the eye to focus light on the retina and correct this condition.

  • (A) Flat
  • (B) Spherical
  • (C) Diverging
  • (D) Converging

Thinking points:

Answer choice (A) is out since it's just a window. It wouldn't do anything to where the focal point is.

Both (C) and (D) could be spherical lenses so (B) doesn't directly answer the question.

In the case of myopia, the light rays are getting focused together soon before you even make it to the retina. So you fix that with a diverging lens. So the right answer here is (D).

Note: You actually want to walk in with this fact already in your head. Myopia is nearsightedness corrected with a diverging lens and presbyopia or farsightedness is corrected with a converging lens.

[10:16] Total Internal Reflection

Question 19: Light inside the thin glass tube of a laparoscopic surgical device strikes the edge of the glass tube and is entirely reflected back into the tube with none of the light exiting to the surrounding medium. Which of the following must be true?

  • (A) Theta incident is 90 degrees.
  • (B) Theta incident is zero degrees.
  • (C) Theta incident is greater than or equal to the theta critical value.
  • (D) Theta refracted value is equal to theta incident.

Thinking points:

The phenomenon here is called Total Internal Reflection – light striking the inside of the glass tube and reflecting inside the tube itself. This is the entire basis of fiber optics. In order to get total internal reflection, the key issue is that every medium (glass, water, plastic, etc.) has a certain critical angle.

The nice thing about the MCAT is you can get full credit for partial knowledge. If critical angle was the only thing you remembered about fiber optics, then you'd probably choose answer choice (C). And this is the right answer. The rest of the answers do not give any relationship relative to the critical angle. So just remembering that fact is enough to get you the right answer.

[12:30] Radiation Question

Question 31: A certain type of tissue is sensitive to radiation with damage the tissue receives directly proportional to charge on the radiating particle. Which of the following radiation types will cost the least damage?

  • (A) Gamma
  • (B) Positron
  • (C) Beta
  • (D) Alpha

Thinking points:

This question has a recall element to it where you have to know the charges of all the physics particles. Specifically, since you want the least damaged, you want the least charged. So you should obviously walk into the test knowing that an alpha particle has a charge of +2, a beta particle has a charge of either +/- 1 depending on the type of beta particle. A position is just a beta plus charge or it's +!. And finally, the right answer of the question is (A) gamma because it's a very high energy photon so it has no charge at all.

[14:14] Final Thoughts

Even on physics, it seems to be the most independent of the sciences. It doesn't tie into biochem or chem. But there is still this kind of clinical, biological lab-based lens. So you know that even if it's as abstract as physics, you've got to be able to think about it through the lens of the biological sciences.

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