Question

A car moves with a speed of 54 km $h^{-1}$ towards a cliff. The horn of the car emits sound of frequency 400 Hz at a speed of 335 m $s^{-1}$. (a) Find the wavelength of the sound emitted by the horn in front of the car. (b) Find the wavelength of the wave reflected from the cliff. (c) What frequency does a person sitting in the car hear for the reflected sound wave ? (d) How many beats does he hear in 10 seconds between the sound coming directly from the horn and that coming after the reflection ?

Answer 1

Here, $V_{car}=54km/h=15m/sec$

$\therefore$ Wavelength of sound in front of car

$=\frac{v_{air}-v_{car}}{\text{frequeny}}$

$=\frac{335-15}{400}=80cm$

(b)Since frequency at lift

$n=\frac{V_{air}+V_{lift}}{v_{air}-v_{car}}\times n_0$

$=\frac{335}{335+5}\times 400=\frac{335\times 400}{320}Hz$

So, Required wavelength

$=\frac{v}{n}=\frac{335}{\frac{335}{320}\times 400}=\frac{320}{400}=80cm$

(c)Frequency of the reflected sound wave heard by the person sitting in the car

$n=\frac{v+v_0}{v}\times n_0$

$=\frac{335+15}{335}\times \frac{335}{320}\times 400$

=437 Hz

(d)No Heat.