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

Person A hears the sound of his own source whose frequency γ . He also hears the sound of source carried by person B γ1=γ1+u/v                 Hence,  No. of beats heard γ1−γ=γuvAPPARENT FREQUENCY  of source with B received by the person A will be given by n' =nv-vov-vshere vo=-u and vs=0