Question

A train moves towards a stationary observer with speed $34m/s$ . The train sounds a whistle and its frequency registered by the observer is $f_1$. If the train's speed is reduced to $17m/s$ , the frequency registered is $f_2$ . If the speed of sound of $340m/s$ , then the ratio $f_1/f_2$ is

• A. $18/19$
• B. $1/2$
• C. $2$
• D. $19/18$

Answer 1

The correct option is D $19/18$

The frequency in first case is:

$f_1=\left(\frac{340}{340-34}\right)f=\frac{10}{9}f$
The frequency in the second case is:
$f_2=\left(\frac{340}{340-17}\right)f=\frac{20}{19}f$
$\therefore \frac{f_1}{f_2}=\frac{\frac{10}{9}}{\frac{20}{19}}=\frac{19}{18}$