A car moves with a speed of `54 kmh^-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

A, B, C, D

Explanation

a. Here `V_(car)=54km/h=15m/sec`
`:. Wavelength of sound in front of car
`=(v_(air)-v_(car))/frequency
`=(335-15_/400=80 cm`
b. since frequency at lift
`n=(V_(air)+v_(lift))/(v_(air)-v_(car))xxn_0`
`=335/(335+5)xx400=(335x400)/320Hz`
So required wavelength
`=v/n= 335/((335/320)xx400)=320/400=80cm`
c. Frequency of the reflected sound wave heard by the person sitting in the car
`(v+v_0)/vxxn_0`
`=(335+15)/335xx335/320xx400`
`=437 Hz`
d. No heat.