Doppler effect, moving car and whistle

[cheff3r]

Homework Statement

Sally is a police officer who is standing in an intersection. A car driven by David approaches the intersection at a speed of 75 km/h. Sally blows her whistle (at a frequency of 900 hz) and signals the driver to stop. What does frequency does David hear? (speed of sound is 343 m/s). David does not stop and passes through the intersection and accelerates to 90 km/h, sally blows her whistle again, what frequency does David hear?

Homework Equations

f'=(v-(v(D)-v(M)))/(v-(v(S)-v(M))*f

The Attempt at a Solution

So the velocity of the median is zero, v is speed of sound =343 m/s , v(D) is David and at 75 km/h = 20.83 m/s and v(S) is sally = 0 and i also make it negative since traveling at sally
so i go
f'=(343-(-20.83))/343*900 = 954.65 Hz

next I do the same as above except i make the v(D) = 90 km/h = 25 and is positive
hence
f'=(343-25)/343*900 = 834.40Hz this is my problem shouldn't the traveling away Doppler affect give a greater frequency than started with?

 

[Matterwave]

No, traveling away should lower the frequency, and traveling towards should increase the frequency. When you move towards someone each successive wavefront is closer together than expected, when you move away from someone each successive wavefront is farther apart than expected (hence, lower frequency).

 

[cheff3r]

So my method is right? If anyone has spare time on their hands to double check my answers that will be much appreciated (i'm a little foggy in the air and would love to know if I'm getting the right answer)

 

[Matterwave]

I don't know about the figures since I'm too lazy to plug in numbers, but the concept is right. For a detector moving away from a source, the frequency should be lower than at the source, and for a detector moving toward a source the frequency should be higher.