## Diagnostic C/P #4

Snazia
Posts: 9
Joined: Fri Mar 09, 2018 9:57 pm

### Diagnostic C/P #4

How do we know what formula to use to answer this question? I thought we should use Q=Av, where Q is flow rate and A is cross-sectional area, which gives me answer C.
NS_Tutor_Andrew
Posts: 520
Joined: Mon May 23, 2016 1:47 pm

### Re: Diagnostic C/P #4

Hi Snazia,

Excellent question! The short answer—and to be honest, the one that is probably the best guidance from the point of view of MCAT success—is that the question tells you to use Poiseuille's Law: "Assuming the flow of stomach contents approximates Poiseuille’s Law,...".

The more interesting question has to do with the difference between Poiseuille's Law and the continuity equation. Basically, the continuity equation is a highly idealized approach that assumes that you're dealing with an ideal (nonviscous, noncompressible) liquid and that kinetic energy is conserved throughout. Poiseuille's law is more realistic -- it assumes that viscosity is present (which is a good assumption for blood) and that you need a pressure differential to drive flow (although it does require assuming laminar, rather than turbulent, flow).

In terms of which approach to use, it kind of depends on the question, so it's difficult to give 100% guidance -- basically, you have to be very alert to clues in the question and/or passage about the assumptions that are being made re: fluid dynamics.

Hope this helps clarify the question!
Andrew D.
Content Manager, Next Step Test Prep.
Snazia
Posts: 9
Joined: Fri Mar 09, 2018 9:57 pm

### Re: Diagnostic C/P #4

Thanks so much! I completely missed that when I was reading the question. And the distinction btw the two formulas is super helpful! Thanks!
NS_Tutor_Andrew