Chapter 15.1, Problem 12SSC

To determine

To Explain: Whether a trooper using a radar detector at the side of the road can determine the speed of a car at the instant the car passes the trooper.

Explanation of Solution

Introduction:

The frequency of wave varies when the source and observer move towards or away from each other. This effect on frequency is known as Doppler Effect.

For the trooper to determine the speed of the car, the car has to be approaching or moving away from the trooper according to Doppler Effect. Sound travels as longitudinal waves in air so, the transverse motion at the instant of the car crossing the trooper does not produce Doppler effect.

Hence, the trooper would not be able to detect the speed of the car.

Conclusion:

The speed of the car cannot be determined at the instant when the car passes the trooper.