Question

A source and an observer move away from each other, each with a velocity of 10 m/s, with respect to ground. If the observer finds the frequency of sound coming from the source as 1950 Hz, the original frequency of the source is : (assume, velocity of sound in air = 340 m/s)

• A. 1950 Hz
• B. 2068 Hz
• C. 1832 Hz
• D. 2186 Hz

Answer 1

Answer: (B)

Applying Dopplers effect, if source and observer both are moving away from each other. Then, the apparent frequency heard by observer is given by $n=\left(\frac{v-v_o}{v+v_s}\right)n$ $\left(\begin{array}{rl}& Given:n=1950Hz\\ & v_o=v_s=10m/s\\ & v=340m/s\end{array}\right)$ $1950=\left(\frac{340-10}{340+10}\right)n$ $\Rightarrow$ $n=\frac{1950}{\frac{340-10}{340+10}}=\frac{1950\times 350}{330}$ $=2068Hz$