A boat is travelling in a river with a speed `2m//s`. From this boat, a sound transmitter is lowered into the river through a rigid support. The wavelength of the sound emitted from the transmitter inside the water is `14.45mm`. Assume that attenuation of sound in water and air is negligible.
(a) What will be the frequency detected by a receiver kept inside the river downstream?
(b) The transmitter and the receiver are now pulled up into air. the air is blowing with a speed `5m//s` in the direction opposite the river stream. Determine the frequency of the sound detected by the receiver.
Temperature of the air and water = `20^(@)C`, Density of river water `= 10^(3) kg//m^(3)`,
Bulk modulus of the water `= 2.088 xx 10^(9)`Pa, gas constant `R = 8.31 J//mol-K`,
Mean molecular mass of air `= 28.8 xx 10^(-3) kg//mol`, `C_(P)//C_(V)` for air `= 1.4`)

Answer

A, B, C

Explanation

KEY CONCEPT : The question is based on Doppler's effect where the medium through which the sound is travelling is also in motion.
By Doppler's formula
`v' = v[(c + v_(m) +- v_(0))/(c + v_(m) +- v_(s))]` …(i)