Question

A police car is driving down the street with it's siren on. You are standing still on the sidewalk beside the street. If the frequency of the siren is 1500 Hz and the car is moving towards you at 15 m/s, what is the frequency you would hear from the siren? (v of sound in air = 343 m/s) (1 point)
1568.60 Hz
1321.43 Hz
2019.11 Hz
1676.92 Hz


If the same police car in question 1 has passed you and is now moving away from you at the same speed, what is the frequency you will hear from the siren? (1 point)
1198.67 Hz
1390.41 Hz
1437.15 Hz
1781.55 Hz

Answer 1

Answer:

A) 1568.60 Hz

B) 1437.15 Hz

Explanation:

This change is frequency happens due to doppler effect

The Doppler effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source

$f_(observed)=\frac{(c+-V_r)}{(C+-V_s)} *f_(emmited)$

where

C = the propagation speed of waves in the medium;

Vr= is the speed of the receiver relative to the medium,(added to C, if the receiver is moving towards the source, subtracted if the receiver is moving away from the source;

Vs= the speed of the source relative to the medium, added to C, if the source is moving away from the receiver, subtracted if the source is moving towards the receiver.

A) Here the Source is moving towards the receiver(C-Vs)

and the receiver is standing still (Vr=0) therefore the observed frequency should get higher

$f_(observed)=\frac{C}{C-V_s} *f_(emmited)\\=\frac{343}{343-15}*1500\\ =1568.60 Hz$

B)Here the Source is moving away the receiver(C+Vs)

and the receiver is still not moving (Vr=0) therefore the observed frequency should be lesser

$f_(observed)=\frac{C}{C+V_s} *f_(emmited)\\=\frac{343}{343+15}*1500\\ =1437.15 Hz$