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A source of sound S emitting waves of frequency 100 Hz and an observor O are located at some distance from each other. The source is moving with a speed of 19.4 ms–1 at an angle of 60° with the source observer line as shown in the figure. The observor is at rest. The apparent frequency observed by the observer is (velocity of sound in air 330 ms–1)
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- 103 Hz
- 106 Hz
- 97 Hz
- 100 Hz
Correct Option: A
Here, original frequency of sound, f0 = 100 Hz
Speed of source Vs = 19.4 cos 60° = 9.7 
From Doppler's formula
f1 = f0
| f1 = f0 |  |  | |||
| v - vs |
| f1 = 100 | | | |||
| v - (19.7) |
| f1 = 100 | = | ||||||||||||||
| V | | 1 - | | | 1 - | | |||||||||
| V | 330 | ||||||||||||||
= 103Hz
Apparent frequency f1 = 103 Hz
