Sound WavesHardBloom L3

Question

Source and observer both start moving simultaneously from origin, one along x-axis and the other along y-axis with speed of source = twice the speed of observer. The graph between the apparent frequency observed by observer f and time t would approximately be :

Options

A.
B.
C.
D.

Solution

Sol.

tanθ = $\frac{vt}{2vt}$ = $\frac{1}{2}$

⇒ cosθ = $\frac{2}{\sqrt{5}}$ , sinθ = $\frac{1}{\sqrt{5}}$

So f = $\left( \frac{v_{s} - v\cos(90 - \theta)}{v_{s} + (2v)\cos\theta} \right)$f0 (where vs is speed of sound)

= $\left( \frac{v_{s} - \frac{v}{\sqrt{5}}}{v_{s} + \frac{4\nu}{\sqrt{5}}} \right)$f0

Hence f < f0 and constant

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