# Physics

posted by Jon

1)As a sound source moves away from a stationary observer, the number of waves will.

A)increase
B)decrease
C)remain the same
D)need to know the speed of the source

(answer:decrease.) As the detector recedes from the source, the relative velocity is smaller, resulting in a decrease in the wave crests reaching the detector each second. (thats from the book)

2)How fast should a car move toward you for the car's horn to sound 2.88% higher in frequency than when the car is stationary? The speed of sound is 343 m/s

1. bobpursley

I agree with the book, the number of waves per second (which is frequency) decreases. I am still not certain of the poor wording of the question.

2. I will be happy to critique your calcs.

2. Jon

2.88% times 343
= 9.8784 I don't understand this one b/c the doppler equation doesnt work

3. bobpursley

actually, it does. And, you just used the approximate solution in your argument.

F/fo= 343/(343-v)= 1/(1-v/343) or

1-v/343=1/1.0288
solve for v. As I recall, this is one of your choices.

4. Jon

drwls already worked it out for me,I found it on another page. answer is 9.6 m/s

5. bobpursley

good. I hope you understand how it is worked.

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