First slide

Doppler's effect

Question

Two persons A and B each carrying a source of sound of frequency $γ$ , are standing a few meters a part in a quiet field, ‘A’ starts moving towards ‘B’ with a speed of $u$ .If V be the speed of sound then the number of beats heard per second by A will be

Moderate

A

$\frac{γ u}{V}$
B

$\frac{2 γ u}{V}$
C

$\frac{γ u}{V + u}$
D

$\frac{γ u}{V - u}$

Solution

Person A hears the sound of his own source whose frequency $γ$ . He also hears the sound of source carried by person B $γ^{1} = γ (1 + u / v)$
Hence, No. of beats heard $γ^{1} - γ = \frac{γ u}{v}$
APPARENT FREQUENCY of source with B received by the person A will be given by $n^{'} = \frac{n (v - v_{o})}{(v - v_{s})}$
here $v_{o} = - u a n d v_{s} = 0$

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