Doppler Effect

[loreben]

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Hi SDners i was hoping someone could please assist me with this question. I tried solving it but i did not get it right and when i looked at the way it was solved in Kaplan subject test 4 i was still not clear about how to solve this question.

Cheers🙂

Question

A sound wave that has a velocity of 10,000ft/s and a frequency of 1000hz is emitted by a source at rest. When the source is moving at a constant velocity of 2000ft/s, what is the ratio of the wavelength that would be heard by a stationary observer behind the moving source to an observer in front of the moving source?

 

[kehlsh]

Hi SDners i was hoping someone could please assist me with this question. I tried solving it but i did not get it right and when i looked at the way it was solved in Kaplan subject test 4 i was still not clear about how to solve this question.

Cheers🙂

Question

A sound wave that has a velocity of 10,000ft/s and a frequency of 1000hz is emitted by a source at rest. When the source is moving at a constant velocity of 2000ft/s, what is the ratio of the wavelength that would be heard by a stationary observer behind the moving source to an observer in front of the moving source?

hard one
moving from behind means it's coming to u
f = 1000[10000/(10000 - 2000)]

moving in front means it's going away from u
f = 1000[10000/(10000 + 2000)]

essentially it comes down to frequency of

1/8000 vs 1/12000

then the ratio of wavelength would be the reciprocal of that

8000:12000

2:3

is that right? lulz

 

[loreben]

It say's the right answer is 3:2

 

[ljc]

hard one
moving from behind means it's coming to u
f = 1000[10000/(10000 - 2000)]

moving in front means it's going away from u
f = 1000[10000/(10000 + 2000)]

essentially it comes down to frequency of

1/8000 vs 1/12000

then the ratio of wavelength would be the reciprocal of that

8000:12000

2:3

is that right? lulz

You mixed it up. You said "moving from behind means it's coming to u", but actually you aren't moving, you're standing still -- the object is moving AWAY from you. So this means a lower frequency and thus a higher wavelength. So should be 3:2, not 2:3.

 

[kehlsh]

You mixed it up. You said "moving from behind means it's coming to u", but actually you aren't moving, you're standing still -- the object is moving AWAY from you. So this means a lower frequency and thus a higher wavelength. So should be 3:2, not 2:3.

oops my bad