Question

A car is moving with a constant velocity `10 m//s` towards the stationary wall. The car blows a horn of frequency `320 Hz`. The velocity of sound in air is `330 m//s`. The beats frequency received by the observer is

A. `2Hz`
B. `4 Hz`
C. `6 Hz`
D. `8 Hz`

Answer 1

Correct Answer - B
`f_(1) = ((v - v_(D))/(v - v_(s))) f` and the freqency of sound received by the observer after reflection from the stationary wall is `f_(2) = ((v + v_(D))/(v - v_(s)))f`
`:.` Beats frequency received by the observe, `f_(b) = f_(2) - f_(1) = ((v + v_(D))/(v - v_(s))) f - ((v - v_(D))/(v - v_(s))) f`
`:. f_(b) = ((2v_(D))f)/((v - v_(s))) = (2 xx 2)/((330 - 10)) xx 320 = 4Hz`.