Question

A man see a car is traveling at speed 20 m/s away. If the car's horn emits a frequency of 600 Hz, what frequency will man hear? Assume v = 340 m/s for the speed of sound.

• A. (34/36)(600 Hz)
• B. (34/32)(600 Hz)
• C. (36/34)(600 Hz)
• D. (32/34)(600 Hz)
• E. (32/36)(600 Hz)

Answer 1

The correct option is A (34/36)(600 Hz)

Speed of car $v_{source}=20m/s$

Speed of sound $v_{sound}=340m/s$

Original frequency $\nu =600$ Hz

Using Doppler effect when the source of sound moves away from the stationary source :

Apparent frequency ${\nu }^{\prime }=\nu [\frac{v_{sound}}{v_{sound}+v_{source}}]$

$\therefore$ ${\nu }^{\prime }=600[\frac{340}{340+20}]=\frac{34}{36}\times 600$ Hz