## NS FL4 C/P Q8

tcbf
Posts: 1
Joined: Fri Jul 19, 2019 6:18 pm

### NS FL4 C/P Q8

Hi,

I was wondering why the answer will be 0 degrees for this question. Using the Vb=(Fd x Vs)/(2Fo x cos(theta)) and the answer choices given (0, 30, 45 and 90 degrees), wouldn't 45 degrees give the highest value to Vb?

NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

### Re: NS FL4 C/P Q8

I apologize if I'm being dense, but I'm not understanding why you assume 45 degrees would give the largest Vb: cos(45) is 0.71, while cos(0) is 1.

You can also answer this in a more intuitive sense, since we're not really trying to "maximize the measured velocity of blood" but trying to do so most accurately. This relies entirely on the doppler effect, and is therefore only possible if the object is either approaching or moving away from the observer at least in part. If the fluid is traveling perpendicular to the observer, no approaching component of velocity would be measured. Conversely, if the fluid is flowing parallel to the observer (at or away), then all it's actual velocity will contribute to the measured velocity by this doppler-shift reliant technique.

I hope this helps, but feel free to elaborate on how you came to your answer. I'm happy to elaborate on this!
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

### Re: NS FL4 C/P Q8

Follow-up:
I thought about this for a bit, and I see what you meant - the largest Vb being indeed from having the smallest possible divisor.

My "intuitive" answer was resting on the key fact that the magnitude of Vb doesn't really determine the accuracy of your measurement, and to ask to recognize this would be a fair MCAT question. I don't think the passage gives you good information to really answer this though, so I will see if this can get reviewed and whether maybe there is a key part from the source passage material that got lost here. Thank you for bringing it up!
aeque
Posts: 3
Joined: Sat Jun 27, 2020 8:26 pm

### Re: NS FL4 C/P Q8

Hi Mathias,

Just to follow up on this. I was looking for Cos 60 = 0.5 since that way formula 2 would look identical to formula 1, therefore being a "perfect" estimation if you will.
Was my thinking here logical? I ended up picking 45 for similar reasons as the OP mentioned above.

Thanks,
Brandon
NS_Tutor_Yuqi
Posts: 40
Joined: Fri May 29, 2020 11:43 pm

### Re: NS FL4 C/P Q8

Hey! I see what you were getting at by trying to match Formula 2 with Formula 1. However, the passage doesn't indicate that the first formula is necessarily the more "correct" one, and therefore the one that you want to imitate. Instead, it seems that the two formulas are serving two different purposes, with the second one allowing you to take into account the angle. I'm not quite sure why there is the difference of having 2F0 in the denominator instead of only F0 like in Formula 1, but since the question asks about the angle, it would be best to focus on Formula 2. After that, you can follow the two lines of the thinking that Mathias mentioned earlier.