Physics: Multiple Choice: what happens to the wavelengths of the siren as measured by the observer while the police car approaches to when he goes away?

Question

A police car is driving past an observer sitting by the side of the road, what happens to the wavelengths of the siren as measured by the observer while the police car approaches to when he goes away?

 

a. They get longer as he drives away

b. They get shorter as he drives away

c. They stay the same

Answer 1

The answer is a. The observed frequency (fo, you could call it "pitch") decreases. The velocity of sound is its (frequency*wavelength), (i.e. #waves per sec*length of a wave = distance per sec = velocity), so if f decreases, wavelength increases, because the velocity of sound is constant, even if the source is moving.

 

The formula for the frequency of sound emitted by an object moving away from a stationary observer, is observed (fo) to be:

fo = fs[1/(1 + Vs/V) where fs is the frequency at the source, Vs is velocity of moving object, and V is the velocity of sound.