Posted by Anonymous on .
.Two automobiles are equipped with the same singlefrequency horn. When one is at rest and the other is moving toward an observer at 14 m/s, a beat frequency of 5.1 Hz is heard. What is the frequency the horns emit? Assume T = 20°C.

physics 
drwls,
The frequency difference is 5.1 Hz
(1 + V/a)*f  f = (V/a)*f = 5.1
a is the sound speed (340 m/s), and
V = 14 m/s
Solve for f.
f = 124 Hz
I did not use the exact formula for a dopplershifted moving source, but this should be close enough.