An observer is standing 500 m away from a vertical hill. Starting between the observer and the hill, a police van sounding a siren of frequency 1000 Hz moves towards the hill with a uniform speed. If the frequency of the sound heard directly from the siren is 970 Hz, the frequency of the sound heard after reflection from the hill (in Hz) is about,(velocity of sound = `330ms^-1`

Options

A. 1042
B. 1032
C. 1022
D. 1012

Answer

B

Explanation

Frequency of sound heard directly is given by,
`V_1=V_0(V/(V+V_s))`
`970=1000[330/(330+V_s)]implies V_s=10.2 ms^-1`
The frequency of sound reflected is given by,
`V_2=V_0(V/(V+V_s))`
`=1000[330/(330+10.2)]implies V_2=1032 "Hz"`