EK FL 1: Q38 & Q41 (Physics)

[remedy23]

Can anyone help me with these questions? I don't get the explanations at all.

It states that the echolating source is considered the sound source & the receiver, but what about the person running away? In this case, are they not considered the receiver. I didn't make that connection at all.

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[raca1111]

hmm is number 38 a passage question?

for 41, this is how I like to think of it:
detected frequency is higher as you move toward the source and lower as you move away from the source. In the first situation, you are not moving while the source is moving away, while in the second example the source is still moving away from you but you are also moving away from the source in the opposite direction, so there is increased distance between you (the detector) and the source. Thus, the frequency detected would be even lower than in the first situation and therefore the difference between the source frequency and the detected frequency would be greater. Since they already told you that the difference between source and detected frequency in the first situation is 10 Hz, the difference in the second situation must be larger than that and choice A is the only option that shows that.