Two persons A and B, each carrying a source of sound of frequency f, are standing a few metres apart in a quiet field. A starts moving towards B with a speed u. If v is the speed of sound, the number of beats heard per second by A will be
A
fuv
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B
2fuv
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C
fu(v+u)
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D
fu(v−u)
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Solution
The correct option is Afuv Person A hears the sound of his own source whose frequency in f. He also hears the sound of the source carried by person B, towards whom he is moving with a speed u.
The apparent frequency of this sound, f′=f(1+uv) ⇒f′−f=fuv ∴ Beat frequency fb=f′−f=fuv
Hence, the correct choice is (a).