***Doppler effect …

***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) = 1970 Hz

fo = fo·(1 + v0/v) = 2140·(1 + 27/343) = 2310 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