Homework Help: physics--please help

Posted by sara on Sunday, April 1, 2012 at 10:53am.

You are traveling in a car toward a hill at a speed of 40.8 mph. The car's horn emits sound waves of frequency 243 Hz, which move with a speed of 340 m/s.The frequency with which the waves strike the hill is 254.7Hz and the frequency of the reflected sound waves you hear 271.2 Hz.
What is the beat frequency produced by the direct and the reflected sounds at your ears?

271.2-243 is not the correct answer

physics--please help - Elena, Sunday, April 1, 2012 at 6:08pm
40.8 mph = 18.24 m/s

(a) Frequency at hill (approaching source) = 243• ( 1/(1 - 18.24 /340) ) Hz = 271.2 Hz
(b) Reflected frequency at car (effective approaching source) = 271.2• ( 1/(1 - 18.24 /340) Hz = 271.2 Hz

(c) Beat frequency is equal to the absolute value of the difference in frequency of the two waves.
The beat frequency = 271.2 Hz received - 243 Hz transmitted = 28.2 Hz

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