JEE Mains · Physics · STD 11 - Waves and sound
- JEE Mains 2021
- Easy
- Numerical
The frequency of a car horn encountered a change from \(400\, {Hz}\) to \(500\, {Hz}\), when the car approaches a vertical wall. If the speed of sound is \(330\, {m} / {s}\). Then the speed of car is \(.....\,{km} / {h} .\)
Pick an option to check your answer.
Answer & Solution
Correct Answer
(B) \(132\)
Step-by-step Solution
Detailed explanation
Wall as an observer
Frequency received by wall
\({f}_{1}={f}_{0}\left(\frac{{C}}{{C}-{V}}\right)\)
Again wall as a source
Frequency received by observer on car
\({f}_{2}={f}_{1}\left(\frac{{C}+{V}}{{C}}\right)\)
\(500=400\left(\frac{{C}+{v}}{{C}-{V}}\right)\)
\(\frac{5}{4}=\frac{{C}+{V}}{{C}-{V}}\)
\({C}=9 {V}\)
\({V}=\frac{{C}}{9}=\frac{330}{9}\,{m} / {s}\)
\({V}=\frac{330}{9} \times \frac{18}{5}=132\, {km} / {hr}\)
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