Question

A person in topless jeep is travelling with uniform velocity $20m{s}^{-1}$ away from a vertical mountain. The person blows horn of the jeep and hears its echo from the mountain. If actual frequency of horn is $630Hz,$ the apparent decrease in the frequency of echo with respect to actual frequency of the horn is $($velocity of sound in air $=340m{s}^{-1})$

• A. $35Hz$
• B. $45Hz$
• C. $70Hz$
• D. $90Hz$

Answer 1

The correct option is C $70Hz$

Apparent frequency $f^{{}^{\prime }}=\left(\frac{v}{v+v_s}\right)f.$ for person moving away.

$f^{{}^{\prime }}$ acts as source.

$\therefore$ frequency of echo is $f_1=\left(\frac{v-v_o}{v+v_s}\right)f_1$

$v_o=v_s=20m{s}^{-1}$

$f=630Hz$

$\Rightarrow f^{{}^{\prime }}=595Hz$

$\therefore f^{{}^{\prime }}=560Hz$

$\therefore$ apparent change $=630-560$

$=70Hz.$