Qn on classical doppler effect

1. Oct 16, 2005

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 travelling at 10m/s sound its horns, which has a freq of 500Hz and this is heard by another car travelling 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?

2. Oct 16, 2005

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.