Exam Krackers Doppler Effect Question

[Renaissance Man]

So I was taking Gold Standard CBT7 and was given this problem:

[FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif]A police car is traveling towards you at 50 m/s on a straight highway. Its siren is emitting a single tone of 400 Hz. You are traveling towards the police car at 20 m/s. What is the frequency of the sound you hear?

(Take the velocity of sound in air as 330 m/s.)

A. 470 Hz
B. 485 Hz
C. 450 Hz
D. 500 Hz
.....[FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Verdana, Arial, Helvetica, sans-serif]
I used the ExamKrackers Doppler equation that I had memorized, because it is easy and fast (although not 100% accurate), and I got the exact answer 485 Hz. Their equation is basically: (net velocity towards or away from source/speed of wave in medium)=(change in frequency/frequency of the source).

However, the right answer is 500 Hz, and I looked at Wikipedia for the exact Doppler equation and it does indeed give me that answer, my question is: will I need to be this precise on the real MCAT? Is the EK equation no good? I am sorry for making a thread about this, but what do you think?

Thanks!


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[Rabolisk]

Yes, I could see this being a question on the MCAT. I don't know what EK's formula is, but make sure to use the one that gets you the right answer. Understand the limitations of EK's formula.

 

[zoner]

Yes, I could see this being a question on the MCAT. I don't know what EK's formula is, but make sure to use the one that gets you the right answer. Understand the limitations of EK's formula.

I suggest you use TPR or TBR equation for it.

 

[BHaus9]

When the relative velocity of the source and observer is >10% of the wave velocity, the EK shortcut gets a little iffy. However, I'd be surprised if the real MCAT had 4 choices that were all so close to the correct answer.

 

[NYR56]

I HIGHLY doubt you will see a question on the real MCAT where the choices are that close. The EK formula should always work for the MCAT. Don't waste your time learning the complex equations.