Question

A toy-car, blowing its horn, is moving with a steady speed of 5m/s, away from a wall. an observer towards whom the toy car is moving, is able to hear 5 beats per second. If the velocity of sound in air is 340m/s, the frequency of the horn of the toy car is close to:

• A. 170 Hz
• B. 640 Hz
• C. 722.5 Hz
• D. Both (1) & (2)

Answer 1

The correct option is A 170 Hz

$\begin{array}{c}{f}_{dir}=\left[\frac{340}{340-5}\right]f\\ f_{ind}=\left[\frac{340}{340+5}\right]f\\ f_{ind}-f_{dir}=5\\ \left\{\left[\frac{340}{340+5}\right]\left[\frac{340}{340-5}\right]\right\}f=5\\ 340\left\{\frac{10}{\left(340+5\right)\left(340-5\right)}\right\}f=5\\ f=\frac{340\times 5}{10}\\ f=170Hz\end{array}$

Hence,

option $\left(A\right)$ is correct answer.