A source of sound is travelling at 100 3 ,m/ along a road, toward… - Vidyadip

MCQ

A source of sound is travelling at $\frac{{100}}{3}\,m/\sec $ along a road, towards a point $A$, when the source is $3\, m$ away from $A$, a person standing at a point $O$ on a road at perpendicular position. The distance of $O$ from $A$ at that time is $4\,m$. If the original frequency is $640\, Hz$,then the value of apperent frequency listen by person is ...... $Hz$ (speed of sound is $340\, m/sec$)

A
$620$
✓
$680$
C
$720$
D
$840$

✓

Answer

Correct option: B.

$680$

b
Effective value of velocity of source

$\mathrm{V}_{\mathrm{s}}=\frac{100}{3} \cos \theta$

$\mathrm{V}_{\mathrm{s}}=\frac{100}{3} \times \frac{3}{5} \quad \mathrm{V}_{\mathrm{s}}=20 \mathrm{\,m} / \mathrm{sec}$

apperent frequency

$\mathrm{n}^{\prime}=\left(\frac{\mathrm{v}}{\mathrm{v}-\mathrm{v}_{\mathrm{s}}}\right) \mathrm{n}=\frac{340}{340-20} \times 640=680 \mathrm{\,Hz}$

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