Doppler effect question

[dannybht]

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I don't know why B for number 757 is the right answer. I used the f (observer) = (v + v (observer)/ v) x f (sound) but I got 10/11 x wavelength of sound

 

[sps27]

Yes me too. So I am a bit puzzled myself. Did not see 10/11 in the ans choices.....

 

[dannybht]

According to the solutions in the back Examkrackers tells to use a different equation v/c = change in wavelength / wavelength of the source. Im so confused...

 

[dalamber]

If you solve for frequency the observed frequency is 11/10 of original frequency and with the inverse relation of wavelength and frequency the wavelength is 10/11 of the original wavelength. This is closest to 9/10 I think

 

[monkeyvokes]

If you solve for frequency the observed frequency is 11/10 of original frequency and with the inverse relation of wavelength and frequency the wavelength is 10/11 of the original wavelength. This is closest to 9/10 I think

Yeah I agree. Only two of the choices show the frequency decreasing, and one is a very dramatic decrease. That leaves 9/10 as the only reasonable choice.

 

[Neutral]

This is an error, as the doppler effect equation is

the source is stationary, receiver is moving, therefore it should be 10/11 rather than 9/10, though using 9/10 will get you extremely close to the correct answer.

 

[monkeyvokes]

This is an error, as the doppler effect equation is

the source is stationary, receiver is moving, therefore it should be 10/11 rather than 9/10, though using 9/10 will get you extremely close to the correct answer.

It could be intentional, especially since they give the option of 11/10. Some may incorrectly select 11/10 when they look for 10/11. I have noticed that correct answers are sometimes approximate or similar to exact answer.

 

[dannybht]

Ahh I see...I guess I was being too specific...