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Doppler Effect Problem



I have a question regarding the Doppler Effect. The following is the
problem I'm trying to 'do':

A car is chasing a UFO through still air. The car is traveling at 30
m/s toward the UFO, while the UFO is traveling at 100 m/s away from the
car. The aliens are playing a note at 330 Hz. What frequency do you,
in the car, hear?

Now, I worked this as f_heard = f_emit (v_air + 30)/(v_air + 100) and I
get an answer, assuming v_air = 343.

However, I _thougth_ that I could also work this as the car being
stationary and the UFO traveling away at 70 m/s using

f_heard = f_emit (v_air) /( v_air + 70), again using v_air = 343. But,
I don't get the same answer.

What am I doing wrong; or more likely, what assumption have I made in
the second case that is wrong?

Thank you,
Peter Schoch
SCCC