Apoliceman in a stationary car measures the speed of approaching cars by means of an ultrasonic device that emits a sound with a frequency of 41.2 khz. a car is approaching him at a speed of 33.0 m/s. the wave is reflected by the car and interferes with the emitted sound producing beats. what is the frequency of the beats? the speed of sound in air is 330 m/s.

4.4 kHz

Explanation:

The frequency of the beats is given by the frequency of the original ultrasound, , and the frequency of the ultrasound reflected back from the car, :

(1)

The frequency of the reflected wave can be found by using the Doppler effect formula:

where

v = 340 m/s is the speed of sound

is the speed of the car

is the frequency of the original sound

Substituting,

So, the beat frequency (1) is

double the distance = divide strength by 4