The driver of bus approaching a big wall notices that the frequency of his bus's horn changes from 420Hz to 490Hz when he hears it after it gets reflected from the wall. Find the speed of the bus if speed of the sound

Correct answer: A. 91 kmh ^ - 1 Frequency appeared at wall f _ w = 330 330-v .f . ..... (1) f ^ ' = (330 + v)/(330) · f _ w = (330 + v)/(330 - v)f 490 = (330 + v)/(330 - v) · 420 v = (330 × 7)/(91) 25 . 38m / s = 91Km / s

JEE Main 20204 SeptemberEvening Shift

Exam: JEE Main
Year: 2020
Date: 4 September
Shift: Evening Shift
Subject: Physics
Chapter: Waves and Sound

JEE Main 2020 · 4 September · Evening Shift · Physics · Waves and Sound

The driver of bus approaching a big wall notices that the frequency...

The driver of bus approaching a big wall notices that the frequency of his bus's horn changes from

420Hz

490Hz

when he hears it after it gets reflected from the wall. Find the speed of the bus if speed of the sound is

330{ms}^{– 1}

Options

A $91{kmh}^{- 1}$
B $81{kmh}^{- 1}$
C $61{kmh}^{- 1}$
D $71{kmh}^{- 1}$

Correct answer

A. $91{kmh}^{- 1}$

Step-by-step solution

Frequency appeared at wall

${\text{f}}_{\text{w}}\text{=}\dfrac{\text{330}}{\text{330-v}}\text{.f} . ..... \text{(1)}$

${f}^{'} = \dfrac{330 + v}{330} \cdot {f}_{w} = \dfrac{330 + v}{330 - v}f$

$490 = \dfrac{330 + v}{330 - v} \cdot 420$

$v = \dfrac{330 \times 7}{91} \approx 25 . 38m / s = 91Km / s$

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