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Tamil Nadu Board of Secondary EducationHSC Science Class 11

Discuss the following case- Observer in motion and Source at rest. a. Observer moves towards Source b. Observer resides away from the Source - Physics

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Question

Discuss the following case-

Observer in motion and Source at rest.

  1. Observer moves towards Source
  2. Observer resides away from the Source
Long Answer
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Solution

(a) Observer moves towards Source:


Observer moves towards Source

We can assume that the observer O moves towards the source S with velocity vo. The source S is at rest and the velocity of sound waves (with respect to the medium) produced by the source is v.

From the Figure, It is observed that both vo and v are in opposite direction. Then, their relative velocity is vr = v + vo. The wavelength of the sound wave is λ = `"v"/"f"`, which means the frequency observed by the observer O is f ‘ = `"v"_"r"/lambda`. Then

`"f"' = "v"_"r"/lambda = (("v" + "v"_0)/"v")"f"`

`= "f"(1 + "v"_0/"v")`   ...(7)

(b) Observer recedes away from the Source:

If the observer O is moving away (receding away) from the source S, then velocity v0 and v move in the same direction. Hence, their relative velocity is vr = v – v0. Hence, the frequency observed by the observer O is

`"f"' = "v"_"r"/lambda = (("v" + "v"_0)/"v")"f"`

`= "f"(1 - "v"_0/"v")`   ...(8)

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Chapter 11: Waves - Evaluation [Page 278]

APPEARS IN

Samacheer Kalvi Physics - Volume 1 and 2 [English] Class 11 TN Board
Chapter 11 Waves
Evaluation | Q III. 16. (2) | Page 278

RELATED QUESTIONS

In discussing Doppler effect, we use the word "apparent frequency". Does it mean that the frequency of the sound is still that of the source and it is some physiological phenomenon in the listener's ear that gives rise to Doppler effect? Think for the observer approaching the source and for the source approaching the observer.


Answer briefly.

State the expression for apparent frequency when source of sound and listener are

  1. moving towards each other
  2. moving away from each other

What is meant by the Doppler effect?


Discuss the following case-

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  2. Source and Observer resides from each other
  3. Source chases Observer
  4. Observer chases Source

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