Question 48

[eo]

If you are traveling away from the police car then you would expect to have less frequency. So, wouldn't you subtract Vdetector from Vsound?

[admin]

Your logic is correct but the problem is your interpretation of the question.

The distance between the source (siren) and the observer/detector (you) is decreasing by 30 m/s (the police car is catching up). The sign convention for when the distance is decreasing: use +Vdetector and -V_source (the opposite for when the distance is increasing).

NB This is a test question. Usually the doppler equation is provided on the MCAT then. of course, you need to know how to use it.

[eo5]

I checked my other books and it says that if the detector is going away from the source you subtract the Vdector from V.....

[admin]

We have said the same thing. Away is increasing, I gave the situation of "decreasing."

[student]

I thought if you are moving away from the sound, it should be minus vdetector? positive v detector means you are going toward the sound (it says this in my other books as well)

[admin]

Yes, all correct.

Moving away from the source = distance increasing = negative Vdetector

Moving towards the source = distance decreasing = positive Vdetector

The calculation was done as though the detector is moving towards the source. Just as a reminder though: this was a test question so the score did not count towards your overall score.