1) A person listening to a siren from a stationary police car observes the frequ

Question

1) A person listening to a siren from a stationary police car observes the frequency and wavelength of that sound. The car now drives rapidly AWAY from the person:

A) The wavelength of the sound the person observes will be shorter than it was, but the frequency will not be changed.

B) The frequency of the sound the person observes will be higher than it was, but the wavelength will not be changed.

C) The wavelength of the sound the person observes will be longer than it was, and the frequency will also be lower than it was.

D) The wavelength of the sound the person observes will be shorter than it was, and the frequency will also be higher than it was.

2) A segment A of wire stretched tightly between two posts a distance L apart vibrates in its fundamental mode with frequency f. A segment B of an identical wire is stretched with the same tension, but between two different posts. You observe that the frequency of the third harmonic of wire B is the same as the fundamental frequency of wire A. The length of wire B must be:

A) L

B) 3L

C) 9L

Explanation / Answer

From Doppler ffect

fL = fo *(V-Vs)/(V+Vs)

so

when the Source moves away, Vs increases and the factor (V -Vs)(V +Vs) decrases

thus FL also Decreases

option C it is

------------------------------------

Tension T in a string = V^2 /u

where u is mass per unit length

so

f = (n/2L) * sqrt(T/u)

f ^2 = n^2/4L^2 * T/u

f^2 proortional to 1/u = m/L

so

f^2 is Directly porportioanl to l

f2^2/f1^2 = l2/l1

l2 = L * 3^2/2

l2 = 9l

option C it is

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