***Doppler effect…
What is the Doppler effect?
NEXRAD
Stationary observers hear the
frequency of the source.
Can you estimate how fast this thing is moving
through the air?
Source moving at subsonic
speeds (Mach 0.7 )
Moving firetruck (source)
Moving source, stationary observer
 ' =  - vsT
Moving observer, stationary source
Doppler effect can involve a
moving source, a moving
receiver, or both.
***Doppler effect can involve a
moving source, a moving
receiver, or both.
Why do these curves behave differently?
A high-speed train is traveling at a speed of 44.7 m/s (100 mi/h) when
the engineer sounds the 415-Hz warning horn. The speed of sound is
343 m/s. What are the frequency and wavelength of the sound, as
perceived by a person standing at a crossing, when the train is (a)
approaching and (b) leaving the crossing?
A train sounds its whistle as it approaches a tunnel in a cliff. The
whistle produces a tone of 650 Hz. If it travels at a speed of
21.2 m/s, what is the frequency heard in the tunnel and in the
reflected sound?
A bat flies toward a moth at speed 6.7 m/s while the moth is flying
toward the bat at speed 5.5 m/s. The bat emits a sound wave of
51.99 kHz. What is the frequency of the wave detected by the bat
after it reflects off the moth? (Assume the speed of sound is 343
m/s.)
Mission accomplished!
A siren, mounted on a tower, emits a sound whose frequency is
2140 Hz. A person is driving a car away from the tower at a speed
of 27 m/s. The sound reaches the person by two paths: the sound
reflected from a building in front of the car, and the sound coming
directly from the siren. What frequency does the person hear for
the (a) reflected and (b) direct sounds?
fo = fo·(1 + v0/v) = 2140·(1 + 27/343) = 2310 Hz
fo = fo·(1 – v0/v) = 2140·(1 – 27/343) = 1970 Hz
Two loudspeakers are mounted on a merry-go-round whose radius is 9
m. When stationary, the speakers both play a tone whose frequency is
100 Hz. As the drawing illustrates, they are situated at opposite ends
of a diameter. If the merry-go-round revolves once every 20 s, what
is the beat frequency that is detected by the listener when the
merry-go-round is near the position shown?
Suppose a police radar gun emits an electromagnetic wave with a frequency
of fs = 8.0×109 Hz. The car in question is approaching a police car parked on
the side of the road in an essentially head-on fashion. The wave from the
radar gun reflects from the speeding car and returns to the police car,
where on-board equipment measures its frequency to be greater than that
of the emitted wave by 2100 Hz. Find the speed of the car with respect to
the highway.
***Moving medium (wind)…
Two trains are traveling toward each other at 16 m/s relative to
the ground. One train is blowing a whistle at 1320 Hz.
(a) What frequency will be heard on the other train in still air?
(Assume the speed of sound in air is 343 m/s.)
(b) What frequency will be heard on the other train if the wind
is blowing at 16 m/s toward the whistle and away from the
listener?
(c) What frequency will be heard if the wind direction is
reversed?
Two train whistles, A and B, both have a frequency of 386 Hz. A
is stationary and B is moving toward the right (away from A) at a
speed of vB = 33.0 m/s. A listener is between the two whistles
and is moving toward the right with a speed of 14.1 m/s. Assume
that the speed of sound in still air is 343 m/s and that no wind is
blowing. What beat frequency is detected by the listener?
A girl is sitting near the open window of a train that is moving at a
velocity of 23 m/s to the east. The girl's uncle stands near the
tracks and watches the train move away. The locomotive whistle
emits sound at a frequency of 580 Hz. Assume that the speed of
sound in still air is 343 m/s. (a) What frequencies do the girl and
uncle hear? (b) How would your answers change if a wind now blows
from the east at 9.2 m/s.
End