Doppler effect when source moves with observer

Question

I have a doubt concerning a scenario of Doppler Effect wherein the source and observer are on the same relative frame.

A car is moving towards a stationary observer with speed 10 m/s. The horn has a frequency of 640 Hz. What is the frequency of the horn as heard by the driver?

The answer given to this states that driver hears a different frequency. However, don't the centre of the waves and the driver's position coincide allowing the driver to hear the same frequency?

What happens here?

The driver will hear the same frequency unless the question wants you to calculate the frequency of sound reflected by the observer. — Hrishabh Nayal, Jul 08 '20 at 13:25
I only have this much data in the question. can it be conclusively determined (one way or the other)? — Safdar Faisal, Jul 08 '20 at 15:25

score 2 · Answer 1 · answered Jul 08 '20 at 13:58

2

Is the answer given greater or smaller than 640hz? If smaller, it's possible that the question means the horn has a frequency of 640hz in the stationary observer's POV.

answered Jul 08 '20 at 13:58

Dedados

443

660 is what you get with respect to the observer and that is correct. So, the horn is at 640 Hz. – Safdar Faisal Jul 08 '20 at 15:23
This would make sense. The question is poorly written though – AnOrAn Jul 08 '20 at 16:02

score 0 · Answer 2 · answered Jul 08 '20 at 10:01

0

The driver hears the same frequency. The stationary observer does not.

answered Jul 08 '20 at 10:01

FGSUZ

8,293

Are you sure about this? The answer given says it wont be the same – Safdar Faisal Jul 08 '20 at 11:59
I'm sure about y ansewr, which is true. The thing is whether they want you to do something much more convoluted... – FGSUZ Jul 08 '20 at 13:36
That is my worry as well. Is there anything else you can do in this scenario? – Safdar Faisal Jul 08 '20 at 15:23

score 0 · Accepted Answer · answered Jul 08 '20 at 16:49

0

In this case, the horn is the source of a wave and is moving away from the driver who is moving toward the source. The observed frequency is $f_o$ (v + $u_d$)/(v + $u_s$) where v is the speed of sound. The driver hears the frequency of the horn.

answered Jul 08 '20 at 16:49

R.W. Bird

12,139

but for such a doppler shift to take place doesn't the observer have to be at a distance from the source. According to what you say(correct me if i am wrong) we can never hear the actual frequency of moving horn irrespective of our position. – Safdar Faisal Jul 08 '20 at 17:05
I said that the driver does hear the actual frequency of the horn. The driver and the horn have the same velocity. As far as know, the observed frequency does not depend on distance. The simple formula does assume that all motion is along one line. – R.W. Bird Jul 08 '20 at 19:54

Doppler effect when source moves with observer

3 Answers3