Question

A motor car blowing a horn of frequency $124\, vib/ \sec$ 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 $330\, m /s$)

• A. 109 vib/sec
• B. 132 vib/sec
• C. 140 vib/sec
• D. 248 vib/sec

Answer 1

Answer: (C)

Similar to previous question

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