Doppler Effect Q

[Deepa100]

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Hi,
I am not clear why D is a bad choice compared to C. Can some one explain?
Thanks!


A bus is traveling at 25 m/s, and a cyclist is traveling at 5 m/s behind the bus, in the same direction. The frequency of a sound coming from the bus is observed by the cyclist to be 1000 Hz. Approximately what is the frequency of the sound as perceived by the bus driver?
A. 950 Hz
B. 1000 Hz
Correct Answer C. 1060 Hz
D. 2100 Hz



Since the bus is traveling away from the cyclist faster than the cyclist is traveling toward it, there is a steady increase in the distance between the source and detector. Motion “away” always corresponds to a frequency decrease. Thus, if the cyclist (detector) hears a frequency of 1000 Hz, the actual source frequency must be higher than 1000 Hz; this eliminates choices A and B. No calculation is really necessary to decide between choices C and D, because for detector and source speeds less than the speed of sound, the change in frequency due to the Doppler Effect will always be less than a factor of 2.Choice D is too high a frequency to be correct here.

 

[tncekm]

df/f = v/c

This says that the change in frequency observed is proportional to the ratio of the relatively velocity difference between the observer and the source divided by the velocity of the sound wave. A sound wave is 340m/s and the relative velocity difference between the two of them is 20m/s.

There is no way that the change in the frequency could be even close to the original frequency, so there is no way that the frequency observed could be doubled.

If you did the math the df value is about 58, and considering the frequency observed will be lower than the actual frequency, it could only be 1060Hz.

 

[Vihsadas]

...There is no way that the change in the frequency could be even close to the original frequency, so there is no way that the frequency observed could be doubled.

Exactly...and this is how you guys should be thinking for your MCAT. You should (with practice!) be able to look at that problem and know right away that C is the right answer without doing any calculations. A and B are obviously wrong because the frequency must be higher. D is way too large an increase for the velocities listed. It must be C.

Play around with the doppler equation to see why when the velocity of the detector and source are smaller than the speed of sound, the frequency increase can never be a factor of two.

Try and get comfortable with learning to recognize which answers must be wrong without doing too many calculations. Then, if you're still not sure you can do some calculations. This is the perfect type of discrete for the real MCAT!

 

[Deepa100]

Exactly...and this is how you guys should be thinking for your MCAT. You should (with practice!) be able to look at that problem and know right away that C is the right answer without doing any calculations. A and B are obviously wrong because the frequency must be higher. D is way too large an increase for the velocities listed. It must be C.

Play around with the doppler equation to see why when the velocity of the detector and source are smaller than the speed of sound, the frequency increase can never be a factor of two.

Try and get comfortable with learning to recognize which answers must be wrong without doing too many calculations. Then, if you're still not sure you can do some calculations. This is the perfect type of discrete for the real MCAT!

Thanks, I did not review sound waves yet but with the info you gave me, I think I just did.

 

[tncekm]

Thanks, I did not review sound waves yet but with the info you gave me, I think I just did.

Wait a minute... Vihsadas didn't give the info 🙁 Last time I help out... pfft. J/K

 

[Vihsadas]

Wait a minute... Vihsadas didn't give the info 🙁 Last time I help out... pfft. J/K

rofl. True, true...
You are hereby fully credited with the info giving. 😛

 

[tncekm]

rofl. True, true...
You are hereby fully credited with the info giving. 😛

You should be flattered, you're obviously like MCAT God to some of our fellow SDNer's. All in the MCAT world that is good is attributed to you, regardless of whether or not its your doing

 

[Vihsadas]

You should be flattered, you're obviously like MCAT God to some of our fellow SDNer's. All in the MCAT world that is good is attributed to you, regardless of whether or not its your doing

Nah. I'm no god. I'm just a dude (particularly handsome though 😛) that just happens to be a good example for you guys to show that you don't have to be a megalomaniacal super genius to get above a 37.

 

[tncekm]

Nah. I'm no god. I'm just a dude (particularly handsome though 😛) that just happens to be a good example for you guys to show that you don't have to be a megalomaniacal super genius to get above a 37.

Peter Venkman says: "When someone asks you if you're a God, you say YES!"

So, you're saying if "I" can get it, anybody can, huh? Thanks for making me feel good about myself.... 🙁 J/K

 

[Kaustikos]

Peter Venkman says: "When someone asks you if you're a God, you say YES!"

So, you're saying if "I" can get it, anybody can, huh? Thanks for making me feel good about myself.... 🙁 J/K

I'm just curious; when are you taking your MCAT?

 

[Vihsadas]

Peter Venkman says: "When someone asks you if you're a God, you say YES!"

So, you're saying if "I" can get it, anybody can, huh? Thanks for making me feel good about myself.... 🙁 J/K

That is a fantastic Ghostbusters reference.