Question

A toy-car, blowing its horn, is moving with a steady speed of $5\, m/s$, 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 $340 \,m/s$, the frequency of the horn of the toy car is close to :

• A. 680 Hz
• B. 510 Hz
• C. 340 Hz
• D. 170 Hz

Answer 1

Answer: (D)

170 Hz

Answer 2

$F _{\text {dir }}=\left(\frac{340}{340-5}\right) f$
$f _{\text {ind }}=\left(\frac{340}{340+5}\right) f$
$f _{\text {ind }}- f _{\text {dir }}=5$
\left\\{\left(\frac{340}{340+5}\right)\left(\frac{340}{340+5}\right)\right\\} f =5
340\left\\{\frac{10}{(300-5)(340+5)}\right\\}=5
$f =\frac{340 \times 5}{10}$
$f =170\, Hz$