A minivan moving at 38 m/s passes an unmarked state police carmoving at 24 m/s.

Question

A minivan moving at 38 m/s passes an unmarked state police carmoving at 24 m/s. The electromagnetic waves produced by the radargun in the police car have a frequency of 8.25 ×109 Hz. What is the magnitude of the difference infrequency between the waves emitted by the gun and those that arereflected back from the speeding minivan?

a. 2100 Hz
b. 390 Hz
c. 1440 Hz
d. 180 Hz
e. 770 Hz A minivan moving at 38 m/s passes an unmarked state police carmoving at 24 m/s. The electromagnetic waves produced by the radargun in the police car have a frequency of 8.25 ×109 Hz. What is the magnitude of the difference infrequency between the waves emitted by the gun and those that arereflected back from the speeding minivan? 2100 Hz 390 Hz 1440 Hz 180 Hz 770 Hz

Explanation / Answer

There are two Doppler frequency changes in the emitted wave inthis case. First, the mini van observes the wavefrequency coming from the radar gun to have a frequency thatis different from the emitted (source) frequencyfs. The wave then reflects and returns to the policecar, where it is observed to have a frequency that is differentthan its frequency at the instant of reflection. Although thepolice car is now moving, the relative motion of the two vehiclesis one of approach. , it is shown that, when the source andthe observer of the radar are approaching each other, the magnitudeof the difference between frequency of the emitted wave and thewave that returns to the police car after reflecting fromthe minivan is

           f01 - fs =2fs (vrel/c)

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