# 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.

3)A car moving at 16.0 m/s, passes an observer while its horn is pressed. Find the difference between the frequencies of sound heard when the car approaches and when it recedes from the stationary observer. The velocity of sound is 343 m/s and the frequency of the sound of the car's horn is 583 Hz.

4)A boy is blowing a whistle of frequency 536 Hz and walking toward a wall with a speed of 1.64 m/s. What frequency of the reflected sound will the boy hear if the speed of sound is 343 m/s?

I really don't have a clue on the last 3, I've been trying for 2 days but I don't understand, can I get a formula or SOMETHING ANYTHING

1. Damon

OK, here are Doppler effect recipes for wave speed v, frequency Fs:

Moving listener:
Listener L moving toward Stationary source S with speed Vl (Vl negative if moving away)
Fl = (v+Vl)/lambda = (v+Vl)/(v/Fs)
or
Fl = [ (v+Vl)/v ] Fs

Moving source
If the source is also moving:
Vs is speed of source, positive if moving AWAY from listener.
Fl = { (v+Vl)/(v+Vs) ] Fs

in the final problem, first do the source whistle approaching the reflector listener (Vl = 0, Vs = -16m/s). Get the frequency felt by the wall, which is what it will send back. then do the listener moving toward a stationary source.

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