question_answer

A source of sound S is moving with a velocity of 50 m/s towards a stationary observer. The observer measures the frequency of the source as 1000 Hz. What will be the apparent frequency of the source when it is moving away from the observer after crossing him? (Take velocity of sound in air is 350 m/s)                    [JEE Main 10-4-2019 Afternoon]

A)  857 Hz                        

B)  807 Hz

C)  750 Hz                        

D)  1143 Hz

Correct Answer: C

Solution :

(c)

\[{{f}_{app}}=\left( \frac{V-0}{V-50} \right){{f}_{source}}\]

\[1000=\left( \frac{350}{300} \right){{f}_{source}}\]

\[{{f}_{source}}=\frac{1000\times 300}{350}\]

\[{{f}_{app}}=\left( \frac{V}{V+50} \right).{{f}_{source}}\]

\[=\frac{350}{400}\times 1000\times \frac{300}{350}\]\[=750Hz\]