Two cars are approaching each other at an equal speed of 7.2 km/hr. When they see each other, both blow horns having frequency of 676 Hz. The beat frequency heard by each driver will be ……… Hz. (velocity of sound in air is 340 m/s)

Original frequency, f0=676HzSpeed of car=7.2×518=2 m/sApparent frequency , f'=f0v+uv−u=676340+2340−2=684HzDriver will hear  frequency (676Hz) of the horn of his own car & apparent frequency (684Hz) of other car. Beat frequency =f'−f=684-676=8 Hz