JEE Mains · Physics · STD 11 - 14. waves and sound
A person driving car at a constant speed of \(15\,m / s\) is approaching a vertical wall. The person notices a change of \(40\,Hz\) in the frequency of his car's horn upon reflection from the wall. The frequency of horn is ............ \(Hz\). (Given : Speed of sound : \(330\,m / s\) )
- A \(456\)
- B \(489\)
- C \(400\)
- D \(420\)
Answer & Solution
Correct Answer
(D) \(420\)
Step-by-step Solution
Detailed explanation
\(\text { Frequency of reflected sound }=\left(\frac{ v + v _{ c }}{ v - v _{ c }}\right) f _0\) \(f =\left(\frac{330+15}{330-15}\right) \times f _0\) \(=\frac{345}{315} f _0\) \(\frac{345}{315} f _0- f _0=40\) \(\frac{30}{315} f _0=40\) \(f _0=\frac{4 \times 315}{3}=420\,Hz\)
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