Question

A train whistling at constant frequency ‘$n$’ is moving towards a station at a constant speed $V$. The train goes past a stationary observer on the station. The frequency ‘$n$’ of the sound as heard by the observer is plotted as a function of time ‘$t$’. Identify the correct curve

• A.
• B.
• C.
• D.

Answer 1

Answer: (D)

Whistling train is the source of sound, $v_s=B$. Before crossing a stationary observer on station, frequency heard is $n^{\prime }=\frac{vn}{\left(v-v_s\right)}=\frac{vn}{v-V}=$ constant, and $n^{\prime }>n^2$. Here , $v$ is velocity of sound in air and $n$ is actual frequency of whistle. After crossing the stationary observer, frequency heard is $n^{\prime }=\frac{v}{\left(v+v_s\right)}=\frac{v}{v+V}=$ constant and $n^{\prime }<n$