question_answer

The driver of a car travelling with speed \[30m{{s}^{-2}}\] towards a hill sounds a horn of frequency 600 Hz. If the velocity of sound in air is \[4m{{s}^{-2}}\]tile-frequency of reflected sound as heard by driver is

A) 550 Hz                                  

B) 555.5 Hz              

C)        720 Hz                  

D)        500 Hz

Correct Answer: C

Solution :

Key Idea Apply Doppler's effect. Whenever there is a relative motion between a source of sound and the observer (listener), the frequency of sound heard by the observer is different from the actual frequency of sound emitted by source. Case I Case II [for case I]  \[{{V}_{C}}\ne {{V}_{B}}\ne {{V}_{A}}\]                         ...(i) [for case II] \[{{V}_{C}}={{V}_{B}}={{V}_{A}}\]                    ... (ii) From Eqs. (i) and (ii), we get \[\text{m}{{\text{s}}^{-1}}\]\[\text{20m}{{\text{s}}^{-1}}\]Hz