First slide

Doppler

Question

A person is observing two trains, one is approaching him with a velocity of 4 m/s while the other is receding from him with the same velocity. If both the taring blow their respective whistles of frequency 240 hearts, the beat frequency heard by the observer will be (speed of sound in air = 320 m/s)

Easy

A

6
B

3
C

zero
D

12

Solution

$n_{1} = (\frac{v - v_{o}}{v - v_{e}}) n = (\frac{320 - 0}{320 - 4}) \times 240 or n_{1} = 243 \cdot 04 Hz n_{2} = (\frac{320 - 0}{320 - (- 4)}) \times 240 = 237 \cdot 04 Hz Beat frequency = n_{1} - n_{2} = 243 \cdot 04 - 237 \cdot 04 = 6$

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