TS EAMCET · Physics · Waves and Sound
A source of sound of frequency \(640 \mathrm{~Hz}\) is moving at a velocity of \(\frac{100}{3} \mathrm{~m} / \mathrm{s}\) along a road, and is at an instant \(30 \mathrm{~m}\) away from a point \(A\) on the road (as shown in figure). A person standing at \(O, 40 \mathrm{~m}\) away from the road hears sound of apparent frequency \(v^{\prime}\). The value of \(v^{\prime}\) is (velocity of sound \(=340 \mathrm{~m} / \mathrm{s}\) ) 
- A \(620 \mathrm{~Hz}\)
- B \(680 \mathrm{~Hz}\)
- C \(720 \mathrm{~Hz}\)
- D \(840 \mathrm{~Hz}\)
Answer & Solution
Correct Answer
(B) \(680 \mathrm{~Hz}\)
Step-by-step Solution
Detailed explanation
We know that, \[ n^{\prime}=n\left[\frac{v}{v-v_s \cos \theta}\right] \] Hence, \[ \begin{aligned} n^{\prime} & =640\left[\frac{340}{340-\frac{100}{5}}\right] \\ n^{\prime} & =640 \times \frac{340}{320}=2 \times 340 \\ & =680 \mathrm{~Hz} \end{aligned} \]
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