A police car moves at 50m/s in the same direction as a truck that has a speed of 25m/s. The police siren has a frequency of 1200 Hz. What is the frequency heard by the truck driver when the police car is behind the truck? take the speed of sound to be 350m/s
2006-08-17 04:31:20 · 6 answers · asked by jonessunrunner1 2 in Science & Mathematics ➔ Physics
Since both objects are going away from each other, you can assume one is standing still and the other is going at 75m/s away. Hence, the speed of sound is slowed (obviously), and travels at 275m/s towards the truck or police car, same difference. Fine the new wavelength, and use your formula for Hz to figure it out from there.
2006-08-17 04:47:14 · answer #1 · answered by Krzysztof_98 2 · 0⤊ 1⤋
I'm so embarrassed. I rated spongyform's answer as bad, read the next answer, and started wondering, "Why the heck do these guys think the frequency would increase, when the two vehicles are getting further away from each other?" Doh! (They should have listed the speed of the first vehicle first! Or, alternatively, I guess I could have read the question better.) None the less, both spongyform and doug tried to take a short cut that's just not there. You have to take the motion of both objects into account, not just the relative motion.
The complete formula should be:
f=f' [ (v-vo) / (v-vs)]
where v = speed of sound (350 m/s in air at 20 deg C),
vo = speed of the observor (he's moving away from the source),
vs = speed of the source (it's moving towards the observer).
1200 ( 325/300 ) = 1200 (1.08) = 1300 Hz
I like Doppler shift. Everytime the traffic light turns yellow, I stomp on the gas to try to make the light green again.
(Of course, there's more than a couple problems with that idea, one being I'm the only one that sees the light as green. I've come to the conclusion that one shouldn't do physics and drive.)
2006-08-17 12:17:42 · answer #2 · answered by Bob G 6 · 0⤊ 0⤋
Since the relative velocity between the truck and the police car is 25 m/s. The formula for Doppler shift is
f = f0(1-v0/v) where v0 is the velocity of the source with respect to the observer (and is taken as negative for decreasing distances), v is the velocity of the wave in the medium (in this case ari), f0 is the original frequency, and f is the 'perceived frequency'. So...
f = 1200(1+25/350) = 1285.714 Hz.
Doug
2006-08-17 11:50:25 · answer #3 · answered by doug_donaghue 7 · 0⤊ 0⤋
DOPPLER EFFECT
apparent frequency=original frequency{(c+v)/(c-u)}
v=velocity of observer
u=veloity of source
when they r moving towords each other
HERE
app. freq= 1200(325/300)=1300 hz
2006-08-17 12:24:58 · answer #4 · answered by PIKACHU™ 3 · 1⤊ 0⤋
1292 Hz = 1200 Hz * (350 m/s) / (350 m/s - 25m/s)
2006-08-17 11:48:22 · answer #5 · answered by spongyform 2 · 0⤊ 1⤋
What do you mean? An African or European swallow?
2006-08-17 14:24:32 · answer #6 · answered by Anonymous · 0⤊ 1⤋