question_answer

A motor car blowing a horn of frequency 124vib/sec moves with a velocity 72 km/hr towards a tall wall. The frequency of the reflected sound heard by the driver will be (velocity of sound in air is 330 m/s)                      [MP PET 1997]

A)            109 vib/sec                            

B)            132 vib/sec

C)            140 vib/sec                            

D)            248 vib/sec

Correct Answer: C

Solution :

           Similar to previous question The frequency of reflected sound heard by the driver                    \[{n}'=n\,\left( \frac{v-(-{{v}_{0}})}{v-{{v}_{s}}} \right)=n\,\left( \frac{v+{{v}_{0}}}{v-{{v}_{s}}} \right)\]            \[=124\,\left[ \frac{330+(72\times 5/18)}{330-(72\times 5/18)} \right]\]= 140 vibration/sec.