An operator sitting in his base camp sends a sound signal of frequency 400 Hz. The signal is reflected back from a car moving towards him The frequency of the reflected sound is found to be 410 Hz. Find the speed of the car. Speed of sound in air` = 324 m s^-1`.

Answer

A, D

Explanation

Let the speed of the car =xm/sec
So, frequency heard at car n
`v=(v+v_(car))/vxxn_0=(324+x)/324xx400`
and frequency heard by operator for reflected sound wave
`n_1=324/(324-x)xxn`
`rarr 410=324/(324-x)xx(324+x)/324xx400`
`rarr 324(410-400)=810xrarrx=4m/sec`