Test Index

Waves Part 1

Section: Physics

© examsnet.com

Question : 31 of 100

Marks: +1, -0

A driver in a car, approaching a vertical wall notices that the frequency of his car horn, has changed from

440 \text{ Hz}

480 \text{ Hz},

when it gets reflected from the wall. If the speed of sound in air is

345 \text{ m/s},

then the speed of the car is

[5 Sep 2020 Shift 2]

$36 \text{ km/hr}$
$24 \text{ km/hr}$
$18 \text{ km/hr}$
$54 \text{ km/hr}$

Solution:

f_1=

frequency heard by wall

= f_s = \left(\frac{v_s}{v_s - v_c}\right)

f_2=

frequency heard by driver after reflection from wall

f_2 = \left(\frac{v_s + v_c}{v_s}\right) f_1

= \left(\frac{v_s + v_c}{v_s - v_c}\right) f_0

\frac{f_2}{f_0} = \frac{v_s - v_c}{v_s + v_c}

\frac{48}{44} = \frac{v_s - v_c}{v_s + v_c}

12(v_s + v_c) = 11(v_s - v_c)

23 v_c = v_s

v_c = \frac{v_s}{23} = \frac{345}{23} = 15 \text{ m/s}

= \frac{15 \times 18}{5} = 54 \text{ km/hr}

© examsnet.com

Go to Question:

Prev Question

Next Question