question_answer

Two cars A and B are moving away from each other in opposite directions. Both the cars are moving with a speed of \[20\,m{{s}^{-1}}\]with respect to the ground. If an observer in car A detects a frequency 2000 Hz of the sound coming from car B, what is the natural frequency of the sound source in car B? (speed of sound in air \[=340\,m{{s}^{-1}}\]) :- [JEE Main 9-4-2019 Afternoon]

A) 2250 Hz

B) 2060 Hz

C) 2150 Hz                      

D) 2300 Hz

Correct Answer: A

Solution :

20m/s Applying Doppler effect for sound \[f=\frac{\text{v}+\text{v}}{\text{v}-{{\text{v}}_{s}}}{{f}_{0}}({{\text{v}}_{0}}\And {{v}_{s}}\] is taken \[\oplus \]when approaching each other) \[2000=\frac{340+(-20)}{340-(-20)}{{f}_{0}}\] \[{{f}_{0}}=2250Hz.\]