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MCAT Physics – Doppler Effect

Radar guns used to detect the speed of motorists relies on the principles of sound and Doppler effect. All of the following measurements may be utilized to calculate the relative velocity of a motor vehicle EXCEPT:

A.    Speed of sound in air
B.    Period of the wave emitted
C.    Density of the air
D.    Frequency of the waved detected

 

Explanation

A.    Speed of sound in air
B.    Period of the wave emitted
C.    Density of the air
D.    Frequency of the waved detected

This question is testing your understanding of the Doppler effect as it pertains to a practical example involving the use of radar guns to determine the speed of motorists. To answer this question, you must know the Doppler equation:

doppler effect explanation image

Note, the density of air will have an effect on the velocity of sound in air. On a hot day, the velocity of sound is greater than on a cold day, but if you know v, you don’t need to now the density of air.

 

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