Question

An engine is moving towards a wall with a velocity $50 \,ms^{-1}$ emits a note of $1.2\, kHz$. Speed of sound in air = $350 \,ms^{-1}$. The frequency of the note after reflection from the wall as heard by the driver of the engine is

• A. 1.2 kHz
• B. 1.6 kHz
• C. 0.24 kHz
• D. 2.4 kHz

Answer 1

Answer: (B)

The reflected sound appears to propagate in a direction opposite to that of moving engine.
Thus, the source and the observer can be presumed to approach each other with same
velocity.
$v' = \frac{v\left(v+v_{0}\right)}{\left(v-v_{s}\right)} $
$= v \left(\frac{v+v_{s}}{v-v_{s}}\right) \left[\because v_{0} = v_{s}\right] $
$ \Rightarrow v' = 1.2 \left(\frac{350 + 50}{350-50}\right)$
$ = \frac{1.2 \times400}{300}$
$ = 1.6 \,kHz $