Qn on classical doppler effect

[Wen]

I've got 2 qns
1)
Student holding a tuning fork vibrating at 440Hz walk away from a wall with a speed of 1.2m/s. What's the freq of the echo the student can hear from the wall? Speed of sound is 330m/s

Can i just use Freq= [(330-1.2)/(330+1.2)] (440Hz) to solve
2) A car traveling at 10m/s sound its horns, which has a freq of 500Hz and this is heard by another car traveling behind the 1st car in the same direction but at a speed of 20 m/s.The sound can also be heard by the 2nd car by the reflection from the bridge ahead. What frequency will the 2nd car hear? Take speed of sound be 340m/s.

Is the frequency they are asking beat freq?
How do i cal. the freq due to the doppler effect? Must i make use of relative velocity?

 

[Chi Meson]

1) No they don't want the beat frequency, but in this case the walking person is both the source AND the observer (and both source and observer are moving)

2) Same thing; think of the situation as though the observer is the reflection in a mirror attached to the bridge. Use observer direction as it appears in the reflection.

 

[quicknote]

For the first question, yes, you can use the formula Freq= [(330-1.2)/(330+1.2)] (440Hz) to solve for the frequency of the echo. This formula is derived from the classical Doppler effect equation, which takes into account the relative velocities of the source, observer, and medium in which the sound waves are traveling. The change in frequency due to the Doppler effect can be calculated using the formula F' = F (v± vr)/(v± vs), where F' is the observed frequency, F is the original frequency, v is the speed of sound, vr is the relative velocity between the source and the medium, and vs is the relative velocity between the observer and the medium. In this case, the relative velocity between the student and the wall is 1.2m/s, and the speed of sound is 330m/s. Therefore, the frequency of the echo heard by the student will be slightly lower than the original frequency of the tuning fork (440Hz).

For the second question, the frequency of the sound heard by the second car will be affected by both the relative velocity between the two cars and the reflection from the bridge. To calculate the frequency due to the Doppler effect, you will need to take into account both of these factors. The formula for the change in frequency due to the Doppler effect in this case will be F' = F (v± vr)/(v± vs), where F' is the observed frequency, F is the original frequency, v is the speed of sound, vr is the relative velocity between the source and the medium, and vs is the relative velocity between the observer and the medium. You will also need to consider the change in frequency due to the reflection from the bridge, which can be calculated using the formula F' = F (v± vr)/(v± vs), where F' is the observed frequency, F is the original frequency, v is the speed of sound, vr is the relative velocity between the source and the medium, and vs is the relative velocity between the observer and the medium. Once you have calculated the individual frequency changes, you can add them together to get the total frequency heard by the second car. This frequency will not be a beat frequency, but rather the combined effect of the Doppler effect and the reflection from the bridge.