Question

Beats: A policeman in a stationary car measures the speed of approaching cars by means of an ultrasonic device that emits a sound with a frequency of 39.6 kHz. A car is approaching him at a speed of 35.0 m/s. The wave is reflected by the car and interferes with the emitted sound producing beats. What is the frequency of the beats when the speed of sound in air is 343 m/s?

Answer 1

Answer:

The frequency of the beats is 43.6408 kHz

Explanation:

Given:

f = frequency = 39.6 kHz

vc = speed of the car = 35 m/s

vs = speed of the sound = 343 m/s

Question: What is the frequency of the beats, f' = ?

As the car moves towards the source, the frequency of the beats

$f'=f*(1+\frac{v_{c} }{v_{s} } )=39.6*(1+\frac{35}{343} )=43.6408kHz$